frozen bread dough: effects of freezing storage and dough improvers

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0733-5210/$ - se

doi:10.1016/j.jc

Abbreviations

GMP, glutenin

bromate; MDG

electron micros�CorrespondE-mail addr

Journal of Cereal Science 45 (2007) 1–17

www.elsevier.com/locate/yjcrs

Frozen bread dough: Effects of freezing storage and dough improvers

Vania Octaviani Selomulyo, Weibiao Zhou�

Food Science and Technology Programme, Department of Chemistry, National University of Singapore, Science Drive 4, Singapore 117543

Received 23 April 2006; received in revised form 23 September 2006; accepted 4 October 2006

Abstract

This review focuses on the effects of freezing storage on the microstructure and baking performance of frozen doughs, and provides an

overview of the activities of dough improvers, including emulsifiers, hydrocolloids and other improvers used in frozen dough

applications. The overall quality of bread baked from frozen dough deteriorates as the storage of the dough at sub-zero temperatures

increases due to several factors which are discussed. Lipid-related emulsifiers such as diacetyl tartaric acid esters of mono and diglycerides

and sucrose esters employed as anti-staling agents, dough modifiers, shortening sparing agents, and as improvers for the production of

high-protein bread have also been employed in frozen doughs. Hydrocolloids are gaining importance in the baking industry as dough

improvers due to their ability to induce structural changes in the main components of wheat flour systems during breadmaking steps and

bread storage Their effects in frozen doughs is discussed. Other dough improvers, such as ascorbic acid, honey and green tea extract, are

also reviewed in the context of frozen doughs.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Bread; Frozen dough; Dough improver; Frozen storage

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Effects of frozen storage on bread dough and bread quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1. Dough strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2. Dough structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3. Yeast survival and gassing power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.4. Bread quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.5. Bread storage quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. Improvers used in frozen dough and their effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1. Emulsifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1.1. DATEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1.2. Sucrose esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2. Hydrocolloids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2.1. Xanthan gum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.2. Guar gum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.3. Hydroxypropylmethylcellulose (HPMC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.4. k-Carrageenan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

e front matter r 2006 Elsevier Ltd. All rights reserved.

s.2006.10.003

: AA, ascorbic acid; CO2, carbon dioxide; CMC, carboxymethylcellulose; DATEM, diacetyl tartaric acid esters of mono and diglycerides;

macropolymer; GTE, green tea extract; HLB, hydrophilic–lipophilic balance; HPMC, hydroxypropylmethylcellulose; KB, potassium

, monodiglyceride; R5cm, resistance to extension at 5 cm; R/E, resistance to extension; SDS, sodium dodecylsulphate; SEM, scanning

copy; SSL, sodium stearoyl lactylate; T 0g, glass transition temperature; WHO, World Health Organization

ing author. Tel.: +656516 3501; fax: +65 6775 7895.

ess: [email protected] (W. Zhou).

www.elsevier.com/locate/yjcrs

dx.doi.org/10.1016/j.jcs.2006.10.003

mailto:[email protected]

ARTICLE IN PRESSV.O. Selomulyo, W. Zhou / Journal of Cereal Science 45 (2007) 1–172

3.2.5. Other hydrocolloids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3. Other dough improvers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3.1. Ascorbic acid (AA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3.2. Sodium stearoyl lactylate (SSL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4. Non-conventional dough improvers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.4.1. Honey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.4.2. Green tea extract (GTE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4. Future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction

Bread is one of the most widely consumed food productsin the world and breadmaking technology is probably oneof the oldest technologies known. This technology hasevolved continuously over the years as new materials,ingredients and equipment have been introduced toproduce better quality bread while research has generatedsteady and impressive progress in breadmaking.

The major ingredients for breadmaking are flour, water,salt, fat and sugars. Leavening agents (micro-organisms orchemical), improvers, a generic term for a wide range ofadditives used in bread formulations that include stabili-zers, emulsifiers, oxidants, gums and supplementaryenzymes (e.g. exogenous a-amylases, proteases, hydrolasesfor non-cellulosic polysaccharides, lipases, lipoxygenases)are also frequently added (Gujral and Singh, 1999). Freshbread typically presents an appealing golden brown crust, apleasant roasted aroma, fine slicing characteristics, a softand elastic crumb texture and a moist mouthfeel.

Fresh bakery products, however, have a relatively shortshelf life since during their storage, a number of physicaland chemical changes occur, a process known as staling.The loss of freshness is paralleled by an increase in crumbhardness and a decrease in flavour and aroma, leading toloss of consumer acceptance. Loss of moisture and starchretrogradation are accepted as two of the basic mechanismsin the firming of the crumb. These preservation problems,coupled with the complex processes involved in conven-tional breadmaking and increasing market demands, haveled to the continuous search for efficient methods toproduce superior bakery products while preventing un-desirable changes and extending the shelf life.

The bakery industry has increasingly exploited theapplications of freezing technology and there are numerouspapers on the storage of frozen doughs and the influence ofsuch processes on the quality of the final product. Thegrowing interest of the market toward frozen bakery goodshas been driven mainly by the economic advantage of acentralized manufacturing and distribution process as wellas the standardization of product quality. These productsdo not demand specialized workers and raise the possibilitymaking ‘‘fresh’’ bread available at any time of the day(Matuda et al., 2005).

The quality of bread made from frozen dough isinfluenced by dough formulation, as well as processparameters such as dough mixing time (Rouille et al.,2000), freezing rate, storage duration, and thawing rate(Inoue and Bushuk, 1991; Le Bail et al., 1998; Lu andGrant, 1999 Mazur and Schmidt, 1968; Neyreneuf andDelpuech, 1993; Varriano-Marston et al., 1980). Thesefactors may either act independently or synergistically toreduce yeast activity which results in reduced carbondioxide (CO2) production or to damage the gluten networkwhich in turn results in poor CO2 retention and poorbaking performance (Lucas et al., 2005). The inclusion ofimprovers in bread formulations may overcome theseproblems associated with frozen dough.The aim of this review is to provide a comprehensive

summary of the effects of dough storage at sub-zerotemperatures, the various improvers used in frozen doughsand their effects on the final bread quality. Potentialresearch for further improving the current frozen breaddough technology is also discussed.

2. Effects of frozen storage on bread dough and bread

quality

Several problems arising from the production of breadmade from frozen dough have been described. Theseinclude gradual loss of the dough strength; decrease in theretention capacity of CO2 and longer fermentation time;reduced yeast activity; lowering of loaf volume; anddeterioration in the texture of the final product.

2.1. Dough strength

Dough that has been stored frozen has a reducedstrength, which in turn causes a decrease in loaf volume.Such loss of dough strength has been attributed to variousfactors, such as the release of reducing substances fromyeast. The reduction of gluten cross-linking caused by icerecrystallization and the water redistribution provoked bya modification in the water binding capacity of doughconstituents may also contribute to the loss of doughstrength.Kline and Sugihara (1968) and Hsu et al. (1979)

suggested that dough weakening can be attributed to the

ARTICLE IN PRESSV.O. Selomulyo, W. Zhou / Journal of Cereal Science 45 (2007) 1–17 3

release of reducing substances, such as glutathione fromyeast during freezing. Glutathione weakens the dough bycleaving disulphide bonds in the gluten proteins, animportant factor in determining the rheology of gluten.

Conversely, other workers (Autio and Sinda, 1992;Varriano-Marston et al., 1980; Wolt and D’Appolonia,1984) have suggested that the structural changes infreeze–thawed dough are not associated with the releaseof reducing substances from yeast cells, but with a lack ofgluten cross-linking. In particular, Varriano-Marston et al.(1980) hypothesized that ice crystallization contributed tothe weakening of the gluten protein network, therebyweakening the dough and increasing the proofing time. Itwas acknowledged that the longer proofing time was alsodue to the destruction of yeast during freezing, resulting indecreased gas production. They hypothesized further thatthe structural components of the dough, in particularprotein, may also be drastically altered by the recrystalliza-tion process.

2.2. Dough structure

Berglund et al. (1991) observed that the formation of icecrystals in non-fermented dough stored for 24 weeks led toa disruption of the gluten matrix resulting a network thatwas less continuous, more ruptured and separated fromstarch granules. Such a heterogeneous gluten matrix wouldretain gas poorly and hence these structural features mighthelp explain the decreased loaf volume and increased prooftime of frozen dough. The starch granules were alsodamaged by the formation of large ice masses duringrecrystallization, further contributing to the decreasedability of the gluten to retain gas during proofing. Thiswas supported by the findings of Gelinas et al. (1995),which suggested that ice crystallization particularly af-fected proteins, lowering the gas retention properties offrozen dough.

Damaged starch causes a linear increase in the waterabsorption capacity of flour (Tipples, 1969) and with moredamaged starch in the dough, it is possible that water isdrawn away from the gluten matrix by the starch granules.This was further supported by the findings of Lu and Grant(1999) which showed that the amount of freezable water(the fraction of free water that does not bind to glutenduring dough formation) in frozen dough increased withstorage time in frozen conditions. Furthermore, theincrease in the amount of freezable water was higher at astorage temperature of �15 1C and lower at �25 1C (Bot,2003). Together, the results clearly indicate that there is aredistribution of the total water present in the systemduring frozen storage.

The damage to the dough structure caused by frozenstorage can be illustrated through the use of low-temperature scanning electron microscopy (SEM) (Zouniset al., 2002). Dough can be likened to a foam in which gasbubbles are entrapped in the starch/gluten matrix. In theelectron micrographs of frozen dough, these bubbles are

shown as spherical voids. However, the presence of icecrystals formed during freezing, represented by angularvoids in the micrographs, can disrupt the foam structure.Unfrozen dough examined immediately after mixingshowed a very dense structure with few spherical voidsand with the spherical starch granules firmly embedded inthe gluten matrix, but no ice crystals. In comparison,dough mixed and stored for 24 h at �201C had a porousstructure with more uniformly sized spherical and angularvoids created by yeast fermentation during the rest periodbefore freezing, and ice crystals that formed during blastfreezing and storage. The gluten network in the 24 h samplewas more stretched than the control and was continuouswith starch granules firmly attached to the visible glutenstrands, an observation also made by Berglund et al. (1991)in dough frozen for 24 h. When stored for 10 weeks at�20 1C, the size of the voids increased and became lessuniform, with some very large voids present. The angularvoids might represent the formation of ice crystals. Thelong gluten strands visible in the 24 h sample were absent inthe 10-week sample. Further disruption was observed whenthe dough was stored for 27 weeks at �20 1C, with agreater number of large angular voids present and starchgranules detached from the gluten. These studies showedclearly that storage of dough at freezing temperature forseveral weeks resulted in structural damage caused bywater migration and ice crystal growth.There is some evidence from SDS gel electrophoresis

measurements that the weakening of the protein structureis due to denaturation of the glutenin proteins. Analysis ofthe protein structure of frozen-thawed dough shows aconsiderable increase in the number of lower molecularweight oligomers which presumably arise from depolymer-ization of glutenin. The changes are particularly noticeableafter several freeze–thaw cycles (Kennedy, 2000).Ribotta et al. (2001) studied the effects of freezing and

frozen storage on the aggregative behaviour of glutenins. Itis worth noting that omega-gliadins are unable to formdisulphide bonds, while a-, b- and g-gliadins can only formintra-chain, but not inter-chain disulphide bonds (Popi-neau and Pineau, 1988; Skerritt et al., 1990). Gluteninsubunits, on the other hand, are able to establish bothintra- and inter-molecular disulphide bonds. This differ-ence allows the formation of a glutenin macropolymer(GMP) which plays a special role in the maintenance ofgluten structure. Utilizing electrophoretic analysis of theSDS-soluble protein aggregates extracted from frozendough, Ribotta et al. (2001) found that there was adecrease in the amount of glutenin subunits of highmolecular mass between 88,700 and 129,100. This sug-gested that the protein matrix of dough underwentdepolymerization during storage at �18 1C. Furthermore,the depolymerization was found to be enhanced as theperiod in which the dough was kept in frozen conditionswas extended. Water redistribution, ice recrystallization,and an increase in the amount of freezable water may affectgluten structure and may be one of the reasons for the


depolymerization of glutenin subunits of high molecularmass. These phenomena may cause a loss in the gasretention capacity during fermentation, reflected by lowerbread volume and an increase in the fermentation time.

In comparison, Sharadanant and Khan (2006) usingSEM and electrophoretic studies to determine the effect ofgums on starch and protein characteristics of frozendoughs supplemented with three levels of gum arabic,carboxymethylcellulose (CMC), k-carrageenan, and locustbean gum after day 1 and after 4, 8, 12, and 16 weeks offrozen storage. Similar to the findings reported by Zouniset al. (2002), electron micrographs of unfrozen doughshowed starch granules securely embedded in the glutenmatrix. After 8 and 16 weeks of frozen storage, however,the frozen control dough without the gum additives clearlyshowed damage to the gluten network, and the starchgranules appeared to be separated from the gluten. Theaddition of locust bean gum and gum arabic produceddough with better capability to retain the gluten networkcompared with the frozen control evaluated after differentperiods of storage. The SDS-soluble protein contentincreased while residue protein content decreased as thefrozen storage time increased. This observation supportsthe theory of depolymerization of the higher molecularweight glutenin polymers into lower molecular weightpolymers also reported by Ribotta et al. (2001). After eachfrozen storage period, the control dough without the gumadditive had the highest amount of SDS-soluble proteins,while dough with k-carrageenan and locust bean gum hadthe lowest amount. The control dough had the lowestamount of residue proteins when compared with the doughtreated with gums, while k-carrageenan treated dough hadthe highest amount of residue proteins, followed by doughswith locust bean gum, gum arabic and CMC.

2.3. Yeast survival and gassing power

In frozen dough manufacturing, yeast survival and gasretention are major problems (Hino et al., 1987). Freezingand frozen storage of dough caused significant losses in thenumber of viable yeast cells, with about half of the originalcells being rendered unviable after 90 d frozen storage(Ribotta et al., 2003a). These results are in agreement withthose of Lorenz and Kulp (1995), which suggested thatfreezing yeast in a dough system increased the susceptibilityto cell damage compared with direct freezing of yeastbecause the yeast in the dough system was under osmoticstress. Organic compounds are concentrated by freezing ofthe aqueous phase, possibly leading to autolysis of yeastcells (Stauffer, 1993).

During dough fermentation, yeast produces CO2 andflavour compounds. The gas-forming ability of yeastdepends on the strain, the number of yeast cells, the cellactivity, and the amount of fermentable sugars. The 1%fermentable sugars in wheat flour are insufficient tosupport yeast growth and need to be enhanced by theadditional sugars produced from starch by a- and b-

amylase action. The gassing power of yeast is also affectedby frozen storage. In particular, it is affected by thefreezing and thawing rate, frozen storage temperature andduration, and freeze–thaw cycles. Fast freezing reduces thegassing power (Autio and Sinda, 1992; El-Hady et al.,1996) and the number of viable yeast cells (Lorenz, 1974).Rapid freezing also results in a much higher sensitivity tostorage duration than slow freezing and the maximumyeast activity is obtained with a slow freezing rate of�0.19 1C/min (Le Bail et al., 1996).

2.4. Bread quality

Breadmaking properties such as bread height andspecific volume are also strongly influenced by the amountof liquid that is released from the frozen dough duringthawing. Seguchi et al. (2003) separated the liquid fromthawed bread dough by centrifugation and found that theamount of liquid oozed from the dough was increased byfreezing-and-thawing cycle and there was a strong inversecorrelation between the amount of centrifuged liquid andbreadmaking properties. This phenomenon is also knownas dough syruping and is caused by the decrease in thewater binding ability of the dough due to freezing and thesubsequent thawing. With consequent deterioration of thebreadmaking properties of frozen-and-thawed breaddough compared with non-frozen dough. Gys et al.(2003) showed that the loss of water holding capacity ofthe dough and the subsequent increase in dough syrupingwas caused by the degradation of arabinoxylan, a cell wallpolysaccharide by endogenous xylanases. Dough prepara-tions using flour from partially debranned grain reducedthe apparent xylanase activity levels by 60% comparedwith flours from whole kernels and in turn retarded thesolubilization and degradation of arabinoxylans on refrig-eration at 6 1C for up to 34 days. At the same time,dough syruping was effectively suppressed. The onset ofsyruping was delayed from 3 days to more than 16 days,and the rate of syrup development was slowed. In addition,the dough showed better retention of consistency (Gyset al., 2004).The temperature of frozen storage of dough also affects

the extent of deterioration of bread quality. As mentionedpreviously, a possible explanation for the quality lossinvolves the changes in dough rheology as a result of watertransport during storage from the hydrated gluten to theice phase (Bot and de Bruijne, 2003). During baking, thegluten does not rehydrate and excess water may migrate tothe starch paste, thus affecting the yield stress of the starchpaste and compromizing the baking performance of thedough. This is especially true at temperatures not far belowthe glass transition temperature (T 0g) of the dough. Glasstransition is a time-dependent change in physical state froma glassy solid to a rubbery viscous liquid and it occurs overa temperature range (Laaksonen and Roos, 2000). If afrozen dough is stored well below T 0g, it is expected to berelatively stable over storage time (Slade et al., 1989).


Unfortunately, T 0g is usually not very much higher than thecommercially relevant freezer temperature of �18 1C(Bot, 2003).

Rasanen et al. (1998) measured the subzero properties offrozen dough by dynamic mechanical thermal analysis(DMTA) and showed that the T 0g’s measured by DMTAmoved to higher temperatures during frozen storage whenat the optimal water content of dough. A reduction inwater content eliminated this phenomenon. Frozen storageincreased the liquid phase in dough with optimal watercontent, determined by the Farinograph absorption at500BU. The growth of ice crystals during frozen storageresulted in the concentration of polymers and a higher T 0gas observed by DMTA. The increase in the liquid phaseduring storage was substantially lower when the watercontent of dough was decreased and thus ice crystal growthwas minimized.

2.5. Bread storage quality

Bread made from frozen dough exhibited detrimentaleffects on its properties and texture upon ageing. Theageing of bakery products typically involves an increase incrumb firmness, loss of flavour and aroma, and loss ofcrispiness (Cauvain, 1998). Moisture loss and starchretrogradation are two of the basic mechanisms responsiblefor the firming of bread crumb, since starch is the majorconstituent in bread crumb. Zobel and Kulp (1996)suggested that bread firming is caused by recrystallizationof the starch fraction involving amylopectin chains.However, Martin et al. (1991) suggest that the main reasonfor bread firming is the formation of hydrogen bondsbetween gluten and starch granules. Bread dough isrubbery and a fraction of the water present is free andavailable to act as a plasticizer, but during baking part ofthe water is lost and the rest is linked to the biopolymerspresent in the system. However, during staling, theformation of hydrogen bonds between the continuousprotein matrix and the discontinuous remnants of starchgranules through the displacement of the intermediatewater molecules is favoured. The water molecules thendiffuse toward the neighbouring sites, resulting in redis-tribution of water. Hence, water mobility contributes toamylopectin recrystallization and the formation of hydro-gen bonds between gluten and starch, which are respon-sible for bread staling (Davidou et al., 1996).

The redistribution of water and ice recrystallization indough during frozen storage can induce changes in thestructure and arrangement of amylose and amylopectinmolecules and such changes will be reflected during starchgelatinization and retrogradation. The longer doughremains in frozen conditions, the more pronounced thedegree of starch retrogradation. Bread made from frozendough also exhibits faster starch retrogradation on lowtemperature (4 1C) storage in comparison to bread madefrom non-frozen dough, causing an increase in breadfirmness (Ribotta et al., 2001, 2003b).

Table 1 summarizes the effects of frozen storage on theproperties of dough and the quality of the resultant bread.Thus, to obtain a product from frozen dough with a

quality comparable to freshly made bread is a complexproblem since the final structure is modified by severalparameters. To improve frozen dough, several technicalmodifications have been introduced in recent years. Theseinclude (1) the isolation of freeze-resistant yeasts (VanDijck et al., 2000); (2) addition of improvers such asemulsifiers and water-binding agents, e.g. hydrocolloids tostabilize the dough network; (3) addition of wheat proteinsto increase shelf life (Benjamin et al., 1989); (4) modifica-tion of dough composition (Wada and Tsukuda, 1997); (5)use of heat stable enzymes to shorten fermentation time(Larsen and Pedersen, 1996); (6) optimization of mixing,freezing and freeze–thaw cycles (Nemeth et al., 1996). Tolimit the scope of discussion in this review, only the use ofimprovers will be discussed in the following sections.

3. Improvers used in frozen dough and their effects

Numerous researches have been focused on the devel-opment and application of different additives for improv-ing the baking quality and extending the shelf life of breadproducts produced from frozen dough by retarding thestaling process in stored bread. Different emulsifiers havebeen tested for use as anti-staling agents and breadimprovers in wheat bread. When improvers/doughstrengthening agents are used, the dough matrix isstrengthened and hence a higher gas pressure (gassingpower of yeast) is necessary to produce an increment in theloaf volume. In this review, the improvers are classified intothree categories: emulsifiers, hydrocolloids, and otherdough improvers.

3.1. Emulsifiers

Studies involving the use of lipid-related emulsifiers inbaking have been well documented. Such emulsifiers, alsoknown as surfactants, have been employed as anti-stalingagents, dough modifiers, shortening sparing agents, and asimprovers for the production of high-protein breads (Addoet al., 1995). The effect of surfactants on retardation ofbread staling has been proposed to result from theirinteraction with starch, retarding the retrogradationprocess, and their blocking of moisture migration betweengluten and starch which prevents starch from taking upwater (Rao et al., 1992). Emulsifiers can also interact withadded lipids to reduce the surface tension in gas bubblesresulting in a larger number of smaller bubbles. Two of themost-studied emulsifiers, i.e. diacetyl tartaric acid esters ofmono and diglycerides (DATEM) and sucrose esters, arediscussed in the following sections.

3.1.1. DATEM

DATEM are anionic oil-in-water emulsifiers that areused as dough strengtheners to improve bread quality.

ARTICLE IN PRESS

Table 1

Effects of frozen storage on dough properties and bread quality

Property Effects of frozen storage References Ingredients used (flour basis)

Dough strength Loss of dough strength due to:

Release of reducing substances

from yeast

Kline and Sugihara (1968) Not reported

Hsu et al. (1979) Not reported

Reduction of gluten cross-

linking caused by ice

crystallization

Varriano-Marston et al. (1980) Not reported

Wolt and D’Appolonia (1984) Not reported

Autio and Sinda (1992) 1.78% pressed yeast, 0.17% and

0.33% cold yeast homogenate,

0.05% and 0.1% glutathione,

1.4% NaCl, 60.7% water

Dough structure Less uniform gluten matrix and

lower gas retention properties of

dough due to ice crystals

formation

Berglund et al. (1991) 4% yeast, 4% sugar, 4%

shortening, 1.5% salt, 55%

water

Gelinas et al. (1995) 0.9% yeast, 4% sugar, 3%

shortening, 2% salt, 100 ppm

ascorbic acid, 60 ppm potassium

bromate, 59% water

Sharadanant and Khan (2006) 5% yeast, 2.5% sugar, 1.5%


ascorbic acid, water (to 500BU

consistency)

Damaged starch causes a

redistribution of the total water

present in the dough system

Tipples (1969) Not reported

Lu and Grant (1999) 5% compressed yeast, 4%

shortening, 4% sugar, 1.5%

salt, 4% ice water, 100 ppm

ascorbic acid, 50 ppm potassium

bromate

Bot (2003) 2% salt, 60% demineralized

water

Decrease of high molecular

mass glutenin subunits and

protein depolymerization

Ribotta et al. (2001) 3% compressed yeast, 1.8%

salt, 0.2% sodium propionate,

0.015% ascorbic acid, 63%

water

Sharadanant and Khan (2006) 5% yeast, 2.5% sugar, 1.5%


ascorbic acid, water (to 500BU

consistency)

Yeast survival and gassing

power

Loss of viable yeast cells due to

cell damage

Stauffer (1993) Not reported

Lorenz and Kulp (1995) 4% compressed yeast, 4%

shortening, 2% sugar, 1.5%

salt, water (to 500BU

consistency)

Ribotta et al. (2003a) 3% compressed yeast, 1.8%



water

Freezing rate dependence Lorenz (1974) Not reported

Autio and Sinda (1992) 1.78% pressed yeast, 0.17% and

0.33% cold yeast homogenate,

0.05% and 0.1% glutathione,

1.4% NaCl, 60.7% water

El-Hady et al. (1996) 4% compressed yeast, 2% fat,

1.5% salt, 1% sugar, 1% skim

milk powder, water (to 500BU

consistency)

Le Bail et al. (1996) 2% compressed yeast, 2.2%

salt, 58% water

Bread quality Increase in the amount of liquid

oozed from frozen dough

during thawing, causing

Seguchi et al. (2003) 3% compressed yeast, 5%

sugar, 1% salt, water (to 500BU

consistency)

V.O. Selomulyo, W. Zhou / Journal of Cereal Science 45 (2007) 1–176

ARTICLE IN PRESS

Table 1 (continued )

Property Effects of frozen storage References Ingredients used (flour basis)

deterioration of the

breadmaking properties

Storage temperature

dependence

Slade et al. (1989) Not reported

Bread storage quality Faster retrogradation of starch,

causing an increase in bread

firmness

Ribotta et al. (2001) 3% compressed yeast, 1.8%



water

Ribotta et al. (2003b) 3% compressed yeast, 1.8%



water

V.O. Selomulyo, W. Zhou / Journal of Cereal Science 45 (2007) 1–17 7

When added to dough, they improve mixing tolerance, gasretention and resistance of the dough to collapse. They alsoimprove loaf volume (Ribotta et al., 2004) and endow thecrumb with a resilient texture, fine grain as well as goodslicing properties (Inoue et al., 1995). Haehnel et al. (1995)reported that DATEM formed hydrogen bonds with starchand glutamine.

Emulsifiers such as DATEM may promote the aggrega-tion of gluten proteins in dough by binding to the proteinhydrophobic surface. This produces a strong proteinnetwork, which in turn will produce bread with a bettertexture and increased volume. Hydrophilic emulsifiers mayalso form lamellar liquid-crystalline phases in water, whichassociate with gliadins (Ribotta et al., 2004). The formationof such structures allows the expansion of gas cells andcontributes to dough elasticity, resulting in increased breadvolume.

The inclusion of DATEM in frozen dough producesbread with lower crumb firmness and retards the rate ofstaling (Zobel, 1973). The function of DATEM as a crumb-softening agent is closely related to their interaction withstarch, particularly with the linear amylase molecules, butalso with amylopectin. The formation of these complexesinhibits bread staling either by preventing amylose oramylopectin retrogradation or by having fewer b-typeamylose nuclei that could promote amylopectin retro-gradation (Zobel, 1973). Crumb softeners may also reducewater migration from gluten to starch by forming acomplex with starch, and be absorbed into the starchsurface (Pisesookbunterng and D’appolonia, 1983).

Sahlstrøm et al. (1999) found that the addition of 0.6%DATEM into Norwegian hearth bread prepared fromfrozen dough decreased the detrimental effect of 70 daysfrozen storage as indicated by increased loaf volume andform ratio (i.e. height/width) values. This may be due to itsability to counteract the rheological changes that occur infrozen storage (Wolt and D’Appolonia, 1984). Extensi-graph measurements showed that the addition of DATEMproduced stronger dough with higher resistance to exten-sion at 5 cm (R5 cm) values when compared to the control,

and the reduction in the extensibility and R5 cm values dueto freezing and thawing was less pronounced with DATEMadded (Kenny et al., 1999).Similar results were obtained by Ribotta et al. (2001,

2004) which showed that when supplemented with 0.5%DATEM (flour based), dough pieces that were frozen andstored at �18 1C for 60 days produced bread loaves withhigher volume in comparison with those obtained with thebase formulation or with guar gum. Fig. 1 shows the effectsof DATEM in frozen dough on bread volume compared tothat of the control and guar gum. DATEM improved theability of dough to retain its form during thawing,fermentation and baking, indicating an improvement inthe quality of bread prepared with frozen dough through amechanism different from that relating to yeast survival.The improvement by DATEM, reflected by the betterretention of the gluten structure, was probably due to thestabilizing effects brought by its interaction with glutenproteins to form a glutein-DATEM-gliadin complex,thereby improving the stability of the ‘Grosskreutz’ bilayer(Stutz et al., 1973).Dough ultrastructure is highly affected by the addition

of emulsifiers, such as DATEM. Rojas et al. (1999)described unfrozen dough as a continuous matrix of glutennetwork with scattered starch granules, while Berglundet al. (1991) found that after 24 weeks of frozen storage, thegluten matrix appeared less continuous, more disruptedand separated from the starch granules. Ribotta et al.(2004) utilized SEM to image the ultrastructure of non-frozen and frozen dough with and without DATEM. In thecontrol dough, the gluten matrix was damaged after 60days of storage at �18 1C, having more porous, lessuniform and thinner gluten strands. There was a tendencyfor the frozen dough to break into small pieces, perhapsdue to the disruption of the gluten strands. In dough madewithout yeast, damage to dough structure was caused byice recrystallization substantiating the earlier findings byVarriano-Marston et al. (1980) and Inoue and Bushuk(1991). On the other hand, dough with DATEM voidsamong starch granules and the gluten network were larger,

ARTICLE IN PRESS

Loaf Specific Volume vs Frozen Storage Duration

0

50

100

150

200

250

300

350

400

450

500

Frozen Storage / days

Control

DATEM

Guar Gum

0 60

Sp

ecif

ic v

olu

me /

cm

3/1

00g

Fig. 1. Effects of storage and improvers in frozen dough on loaf volume

(data from Ribotta et al, 2004).


and the structure was less dense than dough withoutemulsifier. This suggests that the emulsifiers affected on theamount of air incorporated during mixing.

3.1.2. Sucrose esters

Sucrose ester emulsifiers consist of a hydrophilic sugarhead and one or more lipophilic fatty acid tails. The degreeof esterification and the length of the fatty acid chaindetermine the hydrophilic–lipophilic balance (HLB), i.e.whether the emulsifier has hydrophilic (high HLB) or alipophilic (low HLB) properties. A low degree of esterifica-tion and short fatty acid chain gives a high HLB value, acharacteristic desirable in bakery products. The addition ofsucrose esters in dough formulations produces bread with afine and soft crumb structure, high volume, extended shelflife, increased dough mixing tolerance, and improvedfreeze–thaw stability (Barrett et al., 2002).

Sucrose esters can interact with starch and proteins toform complexes, affecting the physical chemical propertiesof both ingredients. For starch, sucrose fatty acid estersinteract mainly with the amylose molecules to forminclusion complexes with the helical amylose moleculesduring gelatinization. These complexes inhibit starchretrogradation resulting in a baked product with longerduration freshness.

Proteins are sensitive to low pH, salts, alcohol, hightemperatures and shear; this can cause protein flocculationwhich is mostly irreversible. Sucrose esters can interactwith the protein molecules and make the proteins lesssusceptible to flocculation. A hydrophobic interaction canoccur between the non-polar side groups of amino acidsand the fatty acid chains of sucrose ester molecules. Thehydrophilic sucrose portion of the molecule will be at theprotein surface, increasing its solubility and decreasing the

sensitivity of the protein to some factors such as low pH,shear and high temperature which may cause it toflocculate.Sucrose esters increase bread compressibility (reduced

resistance to compression) after one (Pomeranz, 1994) and5 days storage (Xu et al., 1992), retarded amylopectinrecrystallization within the first 2 weeks of storage (Rao etal., 1992), and increased loaf volume (Chung et al., 1976).The crumb softening effect of the emulsifiers in bread hasbeen attributed to a number of mechanisms includinginteractions with protein that serves to modify the glutenstructure (Grosskreutz, 1961; Krog, 1981) and by complex-ing with amylose (Krog, 1981).Sensory properties of bread such as perceived firmness,

cohesiveness, chewiness, and moistness also benefit fromthe addition of sucrose esters. In Barrett et al. (2002),panellists perceived the samples containing sucrose estersto be much closer to an ‘‘ideal’’ texture compared withthose containing a regular dough conditioner. However, itis likely that only a partial substitution of the lower costdough conditioner for the higher cost sucrose esters may bepossible, due to production cost reasons.Hosomi et al. (1992) showed that a hydrophilic sugar

ester improved the baking and rheological properties offrozen dough. Addition of sucrose esters decreased yeastdamage by increasing the amount of non-frozen water inthe wheat starch, which acts as a cryoprotectant for yeastcells. Furthermore, addition of sucrose esters preventedwheat protein denaturation during freezing. As a result,damage to the baking properties of the frozen dough wasminimized.

3.2. Hydrocolloids

One group of the most extensively used additives in thefood industry is the hydrocolloids. These compounds arecapable of controlling both the rheology and texture ofaqueous systems through stabilization of emulsions,suspensions and foams. In the baking industry, hydro-colloids are of increasing importance as bread improvers asthey can induce structural changes in the main componentsof wheat flour systems along the breadmaking steps andbread storage (Appelqvist and Debet, 1997). Such struc-tural changes modify the selectivity of some enzymes andchange the technological quality of dough and bread(Armero and Collar, 1997). Hydrocolloids affect thebaking performance of dough and also the shelf life ofstored bread (Armero and Collar, 1998; Davidou et al.,1996).The presence of hydrocolloids influences melting, gela-

tinization, fragmentation and retrogradation processes ofstarch (Fanta and Christianson, 1996; Kokini et al., 1992).These effects were shown to affect the pasting propertiesand rheological behaviour of dough (Rojas et al., 1999).The synergism between hydrocolloids and starch may bedue to the formation of complexes between the starchpolymers, i.e. amylose and/or amylopectin, and the


hydrocolloids during pasting (Bahnassey and Breene,1994).

When used in small quantities (o1%w/w in flour) indough, hydrocolloids are expected to increase waterretention and loaf volume, as well as to decrease firmnessand starch retrogradation. The addition of hydrocolloidsinto frozen products can provide stability during free-ze–thaw cycles and help to minimize the negative effects offreezing and frozen storage on starch-based products(Ferrero et al., 1993; Liehr and Kulicke, 1996). It alsodecreases water activity due to the competition for water bythe hydrocolloids with the bread polymers like protein andstarch (Schiraldi et al., 1996). The overall effects on thefunctional performance of dough and the subsequent breadquality, however, depend on the nature, origin and particlesize of the principal components, dosage of the hydro-colloids incorporated into dough, as well as the formula-tion, processing condition and other ingredients.

3.2.1. Xanthan gum

Xanthan gum is an extracellular polysaccharide secretedby the bacterium Xanthomonas campestris. Itsconsists of alinear (1-4)-linked b-D-glucose backbone with trisacchar-ide side chains on every other glucose at C(O)3, containinga glucuronic acid residue linked (1-4) to a terminalmannose unit and (1-2) to a second mannose thatconnects to the backbone (Sworn, 2000). The viscosity ofxanthan gum solutions is stabile over a wide range of pHand temperature conditions and the polysaccharide isresistant to enzymatic degradation.

Xanthan gum induces cooking and cooling stability ofwheat flour-based products and improves the freeze–thawstability of starch-thickened frozen foods (Sanderson,1981). The addition of xanthan gum into a frozen doughformulation can strengthen the dough by forming a stronginteraction with the flour proteins. It also increases waterabsorption and the ability of the dough to retain gas,increasing the specific volume of the final bread and thewater activity of the crumb (Collar et al., 1999; Rosellet al., 2001). The increase in specific volume, as well as highporosity (open structure) and softer crust, however, areobtained only at low concentrations of xanthan gum(0.16% flour basis). Increased xanthan gum concentrationresulted in a decrease in specific volume compared to thatof the control samples (Mandala, 2005).

3.2.2. Guar gum

Guar gum is a polysaccharide which consists of a chainof b-D-mannopyranosyl units joined by (1-4)-linkages.On average, every second residue carries a a-D-galactopyr-anosyl residue linked to the main chain by a (1-6) linkage(Belitz and Grosch, 1999). Guar gum solutions are highlyviscous at low concentrations and useful in thickening,stabilization and water-binding applications. In bakeryproducts, guar gum is used to improve mixing and recipetolerance, to extend the shelf life of products through

moisture retention and to prevent syneresis in frozen foodsand pie fillings (Maier et al., 1993).In frozen dough, however, the addition of guar gum was

found to be disadvantageous. It yielded a product with lessdesirable properties compared with control samples as itlowered the specific volume and porosity of bread, andproduced a rubbery crust with low crust thickness(Mandala, 2005). In contrast, Ribotta et al. (2001) foundthat the addition of guar gum in frozen dough producedbread with a higher volume, a more open crumb structurewith higher percentage of gas cells than those preparedwithout it. This result was substantiated by Ribotta et al.(2004), who observed that guar gum improved the volumeand texture of bread made from frozen dough, but thenegative effect of frozen dough storage on the dynamicrheological parameters and microstructural damage wasnot avoided. Clearly, more research is needed to verifywhich of these contradictory results is true and to elucidatethe mechanism responsible. Perhaps both explanations aretrue under particular conditions. A possible reason for theconflicting results may be the low level of guar gum (0.16%and 0.65%) used by Mandala (2005), which was less thanhalf the amount incorporated into the frozen dough (1.5%)by Ribotta et al. (2001). Hence, any improving effect of thegum may have been insufficient to counteract thedetrimental effects of the sub-zero temperature storage.

3.2.3. Hydroxypropylmethylcellulose (HPMC)

In HPMC, the etherification of hydroxyl groups of thecellulose by methoxyl and hydroxypropyl groups increasesits water solubility and also confers some affinity for thenon-polar phase in doughs. Hence, in a multiphase systemlike bread dough, this bifunctional behaviour allows thedough to retain its uniformity and to protect and maintain

the emulsion stability during breadmaking. HPMC formsinterfacial films at the boundaries of gas cells conferringsome stability to the cells against gas expansion and otherchanges in processing condition (Bell, 1990). When thetemperature rises during baking, HPMC forms gels byinteracting with the hydrocolloid chains creating atemporary network (Sarkar and Walker, 1995), thisimparts some strength to the dough during expansionand protects against volume loss. This gel also acts as abarrier against a decrease in moisture content but suchbarrier property does not remain after cooling; therefore, itprovides better texture and softness without conferring anyadverse effect on the palatability of the bread.Barcenas et al. (2004) examined the effects of HPMC as

a bread improver on partially baked frozen dough (doughwas baked at 165 1C for 7min prior to freezing and frozenstorage) and found that the specific volume of the finalbread containing HPMC was not significantly affected bythe duration of frozen storage. HPMC was also able toproduce a final bread with higher moisture content than thecontrol and to maintain an almost constant moisturecontent throughout the 42 days of frozen storage. Thisresult was supported by the findings of Collar et al. (1998)


and Dziezak (1991), which suggested that HPMC had theability to increase the water absorption and maintain themoisture content of the products containing it. It was alsoobserved that HPMC increased the softness of breadcrumb made from part-baked frozen dough compared tothe control and was not affected by the frozen storage time,thereby improving the bread texture.

HPMC retarded staling of the final bread and the rate ofcrumb hardening was independent of the frozen storagetime (Barcenas et al., 2004). Since HPMC is hydrophilic, itcould bind available water in the system, decreasing thepossibility of formation of complexes between the polymerspresent in the bread. In contrast to proteins or starchpolysaccharides, HPMC molecules do not aggregate at lowtemperatures.

Haque et al. (1993) suggested that at low temperaturesthe HPMC chains do not associate because their hydro-phobic substituents are surrounded by sheaths of struc-tured water that inhibits intermolecular associationsbetween the polymer chains. Thus, the presence of HPMCdoes not lead to redistribution of water in the dough and asa consequence the formation of bridges between gluten andstarch will not be favoured and in turn bread staling will bepartially prevented.

3.2.4. k-Carrageenan

k-Carrageenan is a sulphated polysaccharide extractedfrom certain red algae. Specifically, it is a high molecularweight linear polysaccharide comprising repeating galac-tose and 3,6-anhydrogalactose units, both sulphated andnon-sulphated, joined by alternating (1-3)-a- and (1-4)-b-glycosidic links. k-Carrageenan contains approximately25% ester sulphate and 34% 3,6-anhydrogalactose(Imeson, 2000).

When used as a dough additve, k-carrageenan has anability to improve the specific volume of the bread due toits interactions with gluten proteins (Leon et al., 2000).Sharadanant and Khan (2006) showed that the presence k-carrageenan in frozen doughs significantly lowered theamount of SDS-soluble proteins and increased the amountof residue proteins compared with control dough. How-ever, in frozen dough applications, k-carrageenan formsrigid gels that are not stable to freeze–thaw cycles. Hence,the specific volume of the final bread produced from frozendough is decreased with increasing frozen storage period.Furthermore, k-carrageenan molecules can form interac-tions with each other without competing with glutenproteins and starchy polysaccharides for the water avail-able in the system.

Leon et al. (2000) showed that in the presence of k-carrageenan, the moisture content of the final bread washigher than that of the control, although the water activitywas lower. These results agree with the ability ofhydrocolloids to increase water absorption and maintainthe moisture content of the product to which it is added,while reducing the water activity due to competition forwater with bread polymers such as proteins and starch.

However, Barcenas et al. (2004) showed that a higherhardening rate of the final bread containing k-carrageenanwas obtained at longer frozen storage time when comparedto that of the control or bread containing HPMC.Sharadanant and Khan (2003a) showed that 1% and 3%k-carrageenan decreased the amount of freezable water,increased the maximum resistance to extension (R/E), andproduced a detrimental effect on frozen dough byincreasing the proof time. Although its addition causedan increase in loaf volume, k-carrageenan gave bread withan inferior appearance as indicated by the lower L colourvalues of the chromameter and decreased the crumbquality (Sharadanant and Khan, 2003b). Hence, k-carra-geenan is not an appropriate improver for frozen dough asit promotes higher hardening rate and favours stalingcompared with dough without the additive, and does notimprove the baking performance.

3.2.5. Other hydrocolloids

In recent years, there is increasing research on the use ofother hydrophilic gums such as CMC, gum arabic andlocust bean gum to improve the quality of frozen doughand the final baked product.CMC is a cellulose derivative with carboxymethyl

groups (–CH2–COOH) bound to some of the hydroxylgroups of the glucopyranose monomers that make up thecellulose backbone. It dissolves rapidly in cold water and ismainly used for controlling viscosity without gelling.CMC solutions tend to be both highly viscous and stable,but the viscosity will drop during heating (BeMiller andWhistler, 1996).Gum arabic is an exudate of acacia trees. It is a

heterogeneous material containing two fractions; 70%polysaccharide chains with little or no nitrogenous materialand 30% protein structures. It has high solubility, lowviscosity and high compatibility with high concentrationsof sugar (BeMiller and Whistler, 1996). Gum arabic readilydissolves in water to give clear solutions ranging in colourfrom very pale yellow to orange-brown and with a pHof �4.5. The highly branched structure of the gum givesrise to compact molecules with a relatively small hydro-dynamic volume and as a consequence gum solutionsbecome viscous only at high concentrations (Williams andPhillips, 2000).Locust bean gum is the ground endosperm of seeds with

galactomannan as the main component. It has a highviscosity and is rarely used alone, but in combination withother gums such as CMC, carrageenan, xanthan and guargum (BeMiller and Whistler, 1996). The structure of locustbean gum is reported to consist of a linear chain of b-D-mannopyranosyl units linked 1, 4 with single-membereda-D-galactopyranosyl units occurring as side branches. Thegalactopyranosyl units are linked 1, 6 with the main chain(Braun and Rosen, 2000).Sharadanant and Khan (2003a, b) investigated the

effects of various levels of hydrophilllic gums such asCMC, gum arabic and locust bean gum on the quality of


dough frozen for up to 16 weeks and on the characteristicsof bread. They reported a decrease in proof time and anincrease in maximum R/E for the addition of 1% and 3%CMC and 1–3% locust bean gum compared with thecontrol, suggesting the ability of the gums to improve thequality of frozen dough by reducing freeze–thaw damage.However, addition of 3% gum arabic gave values similar tothe control while the addition of 2% CMC and 1% and 2%gum arabic gave values lower than the control. Theaddition of gums also increased the specific loaf volumeof the bread significantly, with locust bean gum producingthe highest loaf volume followed by gum arabic and CMC.External appearance of the bread and its internalcharacteristics such as texture, grain, cell wall structure,colour and softness were also improved. Bread firmnesswas significantly reduced by the addition of locust beangum, followed by gum arabic and CMC.

Asghar et al. (2005) reported similar findings with CMCand gum arabic in dough frozen up to 8 weeks. Breadcharacteristics were analyzed after every 15 days forspecific loaf volume, external and internal characteristics.Specific loaf volume increased significantly with theaddition of different levels of gums compared with thefrozen control. Although the external and internal char-acteristics of bread deteriorated with storage time, additionof gum arabic and CMC improved the characteristics ofbread as compared to the control after each storage period.

Table 2 provides a summary of the effects of thehydrocolloids discussed in this section on the final qualityof the bread made from frozen dough.

3.3. Other dough improvers

3.3.1. Ascorbic acid (AA)

Oxidants are required to improve the structure and finalloaf volume of bread as well as to increase the doughstrength. Due to the death of the yeast cells during frozenstorage, reducing substances (particularly glutathione) areformed, leading to a reduction in gluten strength as a resultof weakened disulphide bridges that are essential in thestabilization of gluten network (Kline and Sugihara, 1968;Hsu et al., 1979; Stauffer, 1993). Hence, more oxidants arerequired to compensate this reducing action in the frozendough production.

AA has been widely used as an oxidant in the bakingindustry. It is an oxidizing agent that strengthens the glutennetwork by creating disulphide bonds (Nakamura andKurata, 1997). It also gives large increases in oven rise andbread score (Yamanda and Preston, 1992). The amountused for good dough processing is 10–200 ppm, based onflour weight, and it depends on the desired effects on thequality of baked goods (El-Hady et al., 1999).

Kenny et al. (1999) demonstrated that the addition of100 ppm AA into dough frozen for 8 weeks produced breadwith significantly higher volume than the control, with loafvolume for both formulations decreasing with frozenstorage time. The crumb firmness value of bread with AA

was also lower than the control, with a gradual increase inthe firmness with frozen storage time, which was probablyrelated to the decrease in volume. The difference infirmness between the control and bread with AA and thesuperiority of AA when compared to sodium stearoyllactylate (SSL) and DATEM became more pronouncedwith increasing frozen storage time. Furthermore, extensi-graph measurements showed that stronger dough withhigher R5 cm values than the control was produced with theaddition of AA, while the reduction in R/E and extensi-bility, commonly associated with freezing and thawing,became less pronounced.When combined with potassium bromate (KB), the

ability of AA to inhibit freezing damage is morepronounced. This is shown by the higher maximumresistance ratio, i.e. the ratio of the maximum resistanceof frozen dough to the maximum resistance of non-frozendough, for the AA+KB dough compared to that of theAA dough. The decrease in loaf volume of bread withincreasing duration of dough frozen storage is also less forthe AA+KB dough than for the AA dough (Inoue andBushuk, 1991). In other words, the combination of AA andKB, compared with AA alone, strengthens the doughmore efficiently and improves the baking potential offrozen dough. These findings were further supported byEl-Hady et al. (1999).It should be noted however, that in 1992 the Joint FAO/

World Health Organization (WHO) Expert Committee onFood Additives considered the use of KB as flour improveror flour treatment agent to be not acceptable (JECFAEvaluation, 1995). This was due to its possible carcinogeniceffect on human health (International Agency for Researchon Cancer (IARC), 1986, 1999). Several states in the USAand Canada as well as many countries in Europe followedsuit based on the WHO report. The United Kingdom hadearlier prohibited the use of KB with the release of ‘‘ThePotassium Bromate (Prohibition as a Flour Improver)Regulations 1990’’. The US Food and Drug Administra-tion, however, believes that 50 ppm or less of KB as animprover in white flour and 75 ppm or less in whole wheatflour are safe (CFSAN, 2005).

3.3.2. Sodium stearoyl lactylate (SSL)

SSL is a surfactant reported to maintain volume andsoftness in fresh and frozen dough products (Varriano-Marston et al., 1980; Wolt and D’Appolonia, 1984;Armero and Collar, 1996). Kenny et al. (1999) demon-strated that the addition of SSL into dough pieces frozenfor 2, 5, and 8 weeks produced bread with significantlyhigher loaf volume than bread without the improver. Thevolume, however, was still lower than that of breadcontaining AA or DATEM, implying that both AA andDATEM are better dough improvers than SSL.Results from the extensigraph measurements indicated

that SSL produced stronger dough with higher R/E valueswhen compared to the control, although the values werestill lower than that of DATEM and AA. The reduction in

ARTICLE IN PRESS

Table 2

Comparison of the effects of hydrocolloids in bread made from frozen dough

Hydrocolloids Effects on bread made from frozen

dough

References Ingredients used (flour basis)

Xanthan gum Better retention of gluten structure Collar et al. (1999) 2% compressed yeast, 1.5% salt,

0.038–0.112% xanthan gum, water (to

500BU consistency)

Increases water activity of crumb Rosell et al. (2001) 2% compressed yeast, 2% salt,

50 ppm ascorbic acid, 0.2% sodium

propionate, 0.5% xanthan gum, water

(to 500BU consistency)

Increases specific volume of bread

Reduces specific volume of bread

when used at high concentration

(4 0.16% flour basis)

Mandala (2005) 2.07% vegetable shortening, 2.2%

sugar, 1.6% salt, 1.55% yeast, 0.16%

and 0.65% xanthan gum, 52.3% and

51.9% water

Guar gum Increases specific volume of bread Ribotta et al. (2001, 2004) 3% compressed yeast, 1.8% salt, 0.2%

sodium propionate, 0.015% ascorbic

acid, 1.5% guar gum, 63% water

Increases percentage of gas cells in

bread (open crumb structure)

Mandala (2005) 2.07% vegetable shortening, 2.2%

sugar, 1.6% salt, 1.55% yeast, 0.16%

and 0.65% guar gum, 52.3% and

51.9% water

Decreases specific volume and

porosity of bread

Produces rubbery crust with low crust

thickness

Barcenas et al. (2004) 2% compressed yeast, 2% salt, 0.5%

HPMC, water (to 500BU consistency)

HPMC Increases moisture content of bread

Increases crumb softness

Maintain constant moisture content

through duration of frozen storage

Collar et al. (1998) 2% compressed yeast, 1.8% salt, 0.2%

calcium propionate, 0.3% HPMC,

water (to 500BU consistency)

Retards staling of bread Dziezak (1991) Not reported

k-Carrageenan Increases moisture content of bread Barcenas et al. (2004) 2% compressed yeast, 2% salt, 0.5%

k-carrageenan, water (to 500BU

consistency)

Increases hardening rate and favours

staling

5% yeast, 2.5% sugar, 1.5%

shortening, 1% salt, 100 ppm ascorbic

acid, 1–3% k-carrageenan, water(to 500BU consistency)

Decreases the amount of freezable

water

Sharadanant and Khan

(2003a)

5% yeast, 2.5% sugar, 1.5%


acid, 1–3% k-carrageenan, water(to 500BU consistency)

Increases maximum resistance to

extension

Increases proof time

Increases specific loaf volume Sharadanant and Khan

(2003b)

5% yeast, 2.5% sugar, 1.5%


acid, 1–3% CMC, water (to 500BU

consistency)

Lowers the L value on the

chromameter

Decreases crumb quality

Lowers proof time

CMC Increases maximum R/E at 1% and

3% CMC, but lowers it at 2% CMC

concentration


(2003a), Sharadanant and

Khan (2003b)

Increases specific loaf volume

Improves external and internal bread

characteristics

Asghar et al. (2005) 3% active dry yeast, 4% sugar, 1%

salt, 100 ppm ascorbic acid, 5%

shortening, 1–3% CMC, water


Decreases bread firmness

Locust bean gum Increases maximum R/E at all

concentrations used


(2003a)

5% yeast, 2.5% sugar, 1.5%


acid, 1–3% locust bean gum, water


Lowers proof time Sharadanant and Khan

(2003b)Produces the highest specific loaf

volume and the most superior external

and internal bread characteristics


ARTICLE IN PRESS

Table 2 (continued )

Hydrocolloids Effects on bread made from frozen

dough

References Ingredients used (flour basis)

compared to CMC, gum arabic and k-carrageenan


Gum arabic Lowers proof time Sharadanant and Khan

(2003a), Sharadanant and

Khan (2003b)

5% yeast, 2.5% sugar, 1.5%


acid, 1–3% gum arabic, water


Lowers R/E at 1% and 2% gum

arabic concentration

Increases specific loaf volume Asghar et al. (2005) 3% active dry yeast, 4% sugar, 1%

salt, 100 ppm ascorbic acid, 5%

shortening, 1–3% gum arabic, water


Improves external and internal bread

characteristics


V.O. Selomulyo, W. Zhou / Journal of Cereal Science 45 (2007) 1–17 13

R/E and extensibility associated with freezing and thawingof dough was also less prominent in dough containing SSLthan the control (Kenny et al., 1999).

3.4. Non-conventional dough improvers

3.4.1. Honey

Addo (1997) examined the effects of honey on therheological properties of frozen wheat flour dough andfound that at 4–6% (flour basis), liquid or dry honeyimproved the rheological properties of frozen dough byincreasing the R/E values. The extensibility of frozendough containing liquid honey increased significantly overcontrol dough without additive. This effect, however, wasless pronounced when dry honey was used. Honey appearsto protect the gluten proteins from damage during freezing,possibly due to its hygroscopic properties. Liquid and dryhoney also significantly decrease staling.

Addition of honey at levels from 4% to 12% improvedthe baking quality of frozen dough. Honey also has adesirable effect on the colour development of crust andcrumb of bread from frozen dough. Sensory evaluation bya consumer panel also revealed that consumers preferredthe colour of bread made from frozen dough containing 6-8% honey and were equally satisfied with either fresh breador bread made from frozen dough to which honey wasadded (Addo, 1997).

3.4.2. Green tea extract (GTE)

GTE is a rich source of tea catechins, a group ofpolyphenols with antioxidative, anticarcinogenic and anti-arteriosclerotic activity, as well as having strong activityagainst oxidative processes in food products (McKay andBlumberg, 2002; Rietveld and Wiseman, 2003; Wang et al.,2000; Yang and Landau, 2000). GTE has been incorpo-rated into dough as an improver (Achiwa et al., 2001). Itsincorporation would make bread a functional food andhence GTE it is very attractive additive to dough and bread(Wang and Zhou, 2004).

Achiwa et al. (2001) reported the effects of GTE on thequality of dough. It was shown that the amount of thiol(SH) groups in dough increased significantly with theaddition of GTE. The gluten network is developed into across-linking structure by sulfhydryl-disulphide exchangereaction. Hence, with an increase in the amount of SHthere would be a concurrent decrease in the amount of SS,resulting in a weaker protein network. Furthermore,rheological results from the same study showed thatthe R/E ratio of the dough was greatly decreased by theaddition of GTE. However for hard wheat flour, theaddition of GTE resulted in a significant improvement tothe dough handling properties.Generally, an increase in the dough strength would lead

to an increase in the bread volume. Nevertheless this wouldbe true only up to certain level, above which the breadvolume would decrease with further increase in the doughstrength. Although initially strong dough has higher gasretention capacity, if the dough is too strong, as is the casewith flour dough made from hard wheat, the amount of gasproduced may not be sufficient to make the dough expandsufficiently and consequently the bread will have a smallervolume. In such cases, a reduction in the dough strengthmay be desirable.Consistent with these findings, Wang et al. (2006)

reported negative effects of GTE on the quality of bread.The average loaf volume of bread made from bothunfrozen and frozen dough decreased significantly withthe addition of GTE. Comparing the volume reductions inbread made by the unfrozen and frozen dough processes,frozen storage showed a more significant effect on breadvolume than the GTE once the frozen storage was fiveweeks or more. GTE also increased the firmness of thebread made from either unfrozen or frozen dough,although there seemed to be no synergistic effect betweenGTE and frozen storage. A considerable adverse effect onthe firmness of GTE containing bread was shown onstorage for 4 days at ambient temperature. In short,a higher level of GTE resulted in bread of relativelylower quality.


4. Future outlook

Further benefits to the baking industry from the rapiddevelopments in frozen dough technology may be expected.It is clear that there are still many potential lines ofresearch that should be followed to minimize the adverseeffects of dough freezing, especially with respect toimprovers that can be included in the formulation to givesuperior baking properties and longer shelf life. Theexploration of possible modifications to freezing andfrozen storage technology to improve the baking propertiesof frozen dough will be worthwhile .

In wheat bread made from non-frozen dough, the use ofemulsifiers such as monodiglyceride (MDG) and hydro-colloids such as sodium alginate have been shown toproduce significant beneficial effects. MDG was found toinhibit amylopectin retrogradation in bread significantly atall levels of usage, which in turn retarded the stalingmechanism (Rao et al., 1992), and increased oven riseduring baking (Mettler and Seibel, 1993). The effects ofsuch improvers in frozen dough, however, have yet to bestudied.

The use of alginate as bread improver has been reported(Collar et al., 1999; Davidou et al., 1996; Guarda et al.,2004; Ribotta et al., 2005; Rosell et al., 2001). However,little research has been conducted on its inclusion in frozendough formulations. In non-frozen dough, the addition of1% alginate caused a reduction in pasting temperature,implying an earlier start of starch gelatinization andsubsequently an increase in the availability of starchpolymers as the dextrinization amylase substrate for duringbaking (Collar et al., 1999). Alginate also showed thehighest level of water absorption compared to otherhydrocolloids such as HPMC, k-carrageenan and xanthangum, resulting from the extensive hydroxyl groups in itsstructure, which allow more water interactions throughhydrogen bonding. As a result, the water content of thebread crumb is augmented. Furthermore, the addition ofsodium alginate increases dough consistency, stability, andstrength (Rosell et al., 2001). The resistance of dough todeformation (tenacity), which is a predictor of the ability ofdough to retain gas, is increased with the addition ofsodium alginate. However, the dough height and specificvolume of the final bread is lowered. Such conflictingeffects would make it worthwhile to explore the effects ofsodium alginate on frozen dough.

The conflicting results found in the literature on theeffect of guar gum and GTE also indicate the need forfurther studies on the behaviour of hydrocolloids and otherimprovers in frozen dough systems. Synergistic effects ofdifferent types of improvers are also worthy of exploration.

5. Conclusions

It is clear that the use of frozen doughs allows easier andmore profitable baking, as bread can be made availablearound the clock, reducing labour and production costs

while facilitating transportation. However, the quality ofbread prepared from frozen dough is often inferior tofreshly baked breads. The specific volume of the final breadis reduced, the texture of the bread deteriorates, and starchretrogradation proceeds faster, causing an increase inbread firmness.To eliminate these problems associated with freezing and

frozen storage of dough, several additives have been usedto improve the baking quality and extend the shelf life ofbakery products made from frozen dough. All theseadditives improve the baking quality of frozen dough inone way or another, but only to a limited extent.Conflicting findings for particular improvers are reported.No one particular improver resolves all the issues pertain-ing to bread made from frozen dough. Several hydro-colloids including gum karaya, pectin and alginate have thepotential to be functional in frozen dough systems butrequire testing. Further research and analysis should beconducted to better understand the issues and mechanismsinvolved, and to explore the possibilities of using otherimprovers or multi-improvers to achieve superior qualitybread made from the convenient frozen dough.

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