Food Structure Food Structure

Volume 10 Number 1 Article 4

1991

Microstructure and Firmness of Labneh (High Solids Yoghurt) Microstructure and Firmness of Labneh (High Solids Yoghurt)

Made From Cow's, Goat's and Sheep's Milks by a Traditional Made From Cow's, Goat's and Sheep's Milks by a Traditional

Method or by Ultrafiltration Method or by Ultrafiltration

A. Y. Tamime

M. Kalab

G. Davies

H. A. Mahdi

Follow this and additional works at: https://digitalcommons.usu.edu/foodmicrostructure

Part of the Food Science Commons

Recommended Citation Recommended Citation Tamime, A. Y.; Kalab, M.; Davies, G.; and Mahdi, H. A. (1991) "Microstructure and Firmness of Labneh (High Solids Yoghurt) Made From Cow's, Goat's and Sheep's Milks by a Traditional Method or by Ultrafiltration," Food Structure: Vol. 10 : No. 1 , Article 4. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol10/iss1/4

This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Food Structure by an authorized administrator of DigitalCommons@USU. For more information, please contact digitalcommons@usu.edu.

FOOD STRUCTURE, Vol. 10 (1991) , pp. 37-44 l046-705X/9l$3.00+ .00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA

MICROSTRUCTURE AND FIRMNESS OF LABNEH (HIGH SOLIDS YOGHURT) MADE FROM COW'S, GOAT'S AND SHEEP'S MILKS

BY A TRADITIONAL METHOD OR BY ULTRAFILTRATION

A. Y. Tamime, M. Kal<ib 1, G. Davies and H.A. Mahdi

The Scottish Agricultural College- Auchincruive, Food Science and Technology Department, Ayr, KA6 5HW, Scotland, UK.

1Food Research Centre, Research Branch , Agriculture Canada, Ottawa, Ontario, Canada, KIA OC6

Abstract

Two types of Labneh (high solids yoghurt) were made from goat's and sheep ' s milk: (a) traditional style Labneh was produced by straining yoghurt in a cloth bag, and (b) UF Labneh was made by ultrafiltration (UF) of warm yoghurt. A portion of all products were smoothened by a passage through a lactic curd homoge­nizer. Irrespective of their origin , the Labneh samples contained 20.5-22.5% total solids (including minerals and un-metabolized lactose), 6.7-8.2% protein, and 7.8-8.9% fat.

Homogenization markedly decreased firmness of the goat's and sheep's Labnehs and is therefore not re~ commended in industrial production . Firmness of Lab­neb made from cow's milk was affected by homogeniza­tion to a lesser degree.

Structures of goat ' s and sheep's Labneh, as re ~

vealed by electron microscopy, were similar to each other and less uniform than Labneh made from cow's milk. Homogenization increased the pore size of goat's and sheep ' s Labneh. There was an apparent relationship between pore size (as observed by scanning electron microscopy) and firmness.

Initial paper received December 5, 1990 Manuscript received March 8, 1991 Direct inquiries to A. Y. Tamime Telephone number: 44 292 520331 Telefax number: 44 292 521119

Key Words: Goat's milk, Labneh, Milk, Scanning elec­tron microscopy, Sheep's milk, Strained yoghurt, Trans­mission electron microscopy, Ultrafiltration of yoghurt.

37

Introduction

Strained yoghurt (Labneh) is a cultured milk prod­uct which is popular in the Middle East. Compared to yoghurt made in Europe and North America, Labneh is made from whole milk. In Arab countries, Labneh is garnished with dried herbs and olive oil and is served with bread. A similar, plain or fruit-flavored product, has recently been marketed under the name of "Greek Yoghurt" in the United Kingdom.

Traditionally, cow's, sheep's, and goat's milks have been used for the manufacture of Labneh in the Middle East. Although cow's milk is widely used, sheep's and goat's milks are very important for the prod­uction of Labneh in countries such as Lebanon, Syria and Turkey where the climatic conditions are not favora­ble for the keeping of cattle (Juarez and Ramos , 1984, 1986; Ramos and Juarez , 1981).

The traditional method of Labneh production con ~

sists of straining yoghurt using a cloth bag . The strain­ing procedure has recently been replaced by the use of mechanical separators for yoghurt made in the traditional way , by culturing retentates obtained by the ultrafiltra­tion (UF) of milk, or by concentrating warm yoghurt by UF (Tamime et al., 1989a, b).

The objectives of this study were to produce Lab­neb from sheep's and goat's milks by the traditional method and by UF of fresh yoghurts; and to assess the effects of the origin of milk and the manufacturing pro­cedures on the composition , firmness and microstructure of the products compared to Labneh made from cow's milk.

Materials and Methods

Preparation of Labneh

Sheep's and goat's milks were obtained from a group of small farmers in the vicinity of the Scottish Agricultural College - Auchincruive. The milk was de­livered in sterilized churns and refrigerated overnight at ?"C before use.

The total solids content of the goat 's milk was II%, which is below the national average of 13%

A. Y. Tamime , M. Kalab, G. Davies and H.A. Mahdi

(Robinson and Vlahopoulou , 1988). The milk contained 3.2 % fat and 2.54% protein. Goat's milk powder (pur­chased from Tregaron Foods Ltd., Dyfed, UK) was , therefore, used to fortify the fresh goat ' s milk to 16 % total solids (Table 1).

The sheep's milk contained 17.2- 17 .5% total solids, 6.1 -6.2% fat and 5.4% protein.

Cow ' s milk Labneh reported earlier (Tamime et al., 1989a) was used for comparison.

Traditional and UF Labnehs were prepared from freshly made yoghurt as described by Tamime et al. (1984; 1989a, b). To make the yoghurt , the milk was heated to 90 °C for 5 minutes , cooled to 37 °C and was inoculated with freeze-dried lactic acid-producing starter culture (CH-1, Chr. Hansen's Laboratorium A/S, Copenhagen, Denmark) consisting of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophil us. The inoculated milk (10 g starter culture/ 100 kg milk) was incubated at 37 ' C. In the traditional method of manufacture , the resulting yoghurt was refrigerated , transferred into a polyester cloth bag , and pressed (4.5 kg weight/10-12 kg yoghurt) overnight in a refrigerated room. UF Labneh was made by ultrafiltration of freshly made yoghurt while it was still warm using an Alfa-Laval pilot scale ultrafilter , equipped with type PM2-50 series No. 6 PL 12565 membranes having a total filtering area of I. 3 m2; the cut-off molecular mass of the membranes was 50 ,000 daltons.

The Labnehs were homogenized (single stage) at 7°C by passage through an ALM homogenizer (Pierre Guerin S.A. , Mauze, France) at a pressure of 8 MPa.

Chemical Analysis

The total solids, fat , protein and ash contents in different milks , permeates and Labnehs were determined as described by Tamime et al. (1987).

Firmness Analysis

A Stevens LFRA Texture Analyzer (C. Stevens & Son Ltd. , Hertfordshire, UK) equipped with a cylindri­cal probe (type TA3-TFE I 05-504). 25 mm in diameter and 35 mm long, was used to measure the firmness of Labneh. The probe penetrated the samples into a depth of 15 mm at a speed of 0.5 mm/s and the force exerted on the probe was recorded. On an average , 3 Labneh samples at 7°C were examined in each trial and there­sults were expressed in millinewtons (mN).

Microscopic Analysis

The Labnehs were sampled using a glass tube, 7.0 mm in diameter. Sample columns, approximately 10 mm long, were fixed in a 2.8% glutaraldehyde solution and mailed to Ottawa, Canada for electron microscopy (Allan -Wojtas, 1984) . After arrival , the columns were prepared for scanning electron microscopy (SEM) and for transmission electron microscopy (TEM).

For SEM, the fixed Labneh columns were cut into prisms, lxlxlO mm, and dehydrated in a graded ethanol series (20 , 40 , 60, 80, 96 and 100%) of ethanol , defatted in chloroform, and returned into ethanol. Then the sam-

38

pies were frozen in Freon 12 at -150 °C and freeze-frac­tured under liquid nitrogen. The frozen fragments were thawed in absolute ethanol and critical-point dried from carbon dioxide. Sputter-coated fragments were exam­ined in an lSI DS- I 30 scanning electron microscope operated at 20 kV.

For TEM, the Labneh samples were cut into 0.5 mm cubes, postfixed in a 2% osmium tetroxide solution in a 0.05 M veronal-acetate buffer, and embedded in medium hard Spurr's low-viscosity medium (J.B. EM Service, Inc., Pointe Claire-Dorval, Quebec, Canada). Sections, approximately 90 nm thick, were stained with uranyl acetate and lead citrate solutions (Reynolds, 1963), and examined in a Philips EM-300 transmission electron microscope operated at 60 kV.

Resu Its and Discussion

The Labnehs under study were made from sheep's and goat's milks in May and June 1988. Some aspects oflheir production and quality were discussed by Mahdi (1990). Labnehs made from whole cow ' s milk in January and March of the same year by Tamime et al. (1989a , b) were used for compari son.

Composition

Milks obtained from various mammalian species differ in their composition (Walstra and Jenness, 1984). The cow's and sheep's milks used in this study were within standard limits. However, the goat's milk pro­duced in the vicinity of the College in Scotland between April and June contained only 11.05% total solids com­pared to the national average of 13% (Robinson and Vlahopoulou , 1988). This means that also the casein content was reduced (Mahdi , 1990). In preliminary experiments , this milk produced a very weak coagulum compared to the cow ' s and sheep ' s milk. In order to in­crease the total solids content of the milk and improve the firmness of the resulting yoghurt, this milk was for­tified with spray-dried milk powder either to 13% or to 16.6% total solids. The former level of fortification was similar in its chemical composition to the national aver­age in the UK, and after the fermentation stage, such milk also produced a very weak coagulum. However, slightly firmer coagulum was obtained when the milk was fortified to 16.6% total solids (4.4% protein) (Table I). Sheep's milk had the highest total solids (17.25%), protein (5.4%) , and fat contents (6.1 %).

Culturing of the milks to produce yoghurt and straining of the yoghurt to obtain Labneh markedly alter­ed the total solids, protein , and fat levels (Table 1). The highest values for all three indices were found in tradi­tional cow' s milk Labneh (24 .2% , 8.2% and 11.0%, re­spectively) whereas the lowest values were found in tra­ditional goat's Labneh (20 .5%, 6.7% and 7.8% , respec­tively). This means that the total solids of cow's milk were almost doubled (194 %) by straining the yoghurt compared to the initial milk, but the increases in the total solids contents in Labnehs made from sheep's and goat's milk were only about 128%. These differences,

Labneh Made From Cow's, Goat's and Sheep's Milks

Table I. Composition (% w/w) of cow's, sheep's, and goat's milks, Labnehs, and permeates1

Product Total Solids Protein Fat Ash pH Titratable Acidity

Cow's Mile• 12.48 3.06 3.90 0.70 6.70 0.15 UF Labneh 22.07 7.43 9.79 0.71 4.32 1.28

Permeate 5.63 0.20 (-) 0.70 ND ND Traditional Labneh 24.23 8.23 11.04 0.69 4.18 1.46

Filtrate 6.21 0.30 0.01 0.76 ND ND

Sheep's milk 17.25 5.40 6.14 0.93 6.68 0.20 UF Labneh 22.48 8.22 8.95 0.89 4.10 1.63

Permeate 5.76 0.28 (-) 0.85 ND ND Traditional Labneh 22.06 8.19 8.95 0.84 4.10 1.63

Filtrate 6.53 0.57 0.07 0.97 ND ND

Goat's Milk••• 16.63 4.38 4.90 1.16 6.49 0.23 UF Labneh 21.41 7.07 9.03 1.12 4.10 1.39

Permeate 7.08 0.34 (-) 1.12 ND ND Traditional Labneh 20.54 6.67 7.80 1.12 3.95 1.56

Filtrate 8.75 0.91 0.40 1.27 ND ND

UF: Ultrafiltration; ND: Not determined ; (-): no fat loss was observed; ·: lactic acid; ••nata compiled from Tamime et at. (1989b); •••The milk was fortified with goat's milk powder. 1Results are average of two trials for cow's and sheep's Labneh and one trial for goat ' s Labneh. Each was analyzed in duplicate.

which reflect the compaction of the original yoghurts, may be attributed to several factors, such as the chemi­cal nature of the proteins, particularly the caseins , and their ratios in the three milks under study (Jenness, 1982) (Table 2). Although the casei ns in goat's and sheep's milks are called as 1-, as2-, {3-, and K-caseins, they are not identical and there are slight differences in their amino acid composition. For example , the bovine as 1-casein molecule consists of a chain of 199 amino acid residues; there are 5 genetic variants of this chai n and additional differences may be found in the phosphate groups present. The a 5-casei n content is lower in goat's milk than in cow's milk but the ratio of asrcasei n to as 1-casein is higher in goat's milk. In sheep's milk , most of the as-casein is in the as 1-form (Table 2). Small differences have also been found in the composition of K-caseins which have an important role in the stabiliza­tion of casein micelles in milk. However, the structure of the peptide chain which is susceptible to the action of rennet is similar in all ruminant K-caseins. The rennet­sensitive bond is Phe 10rMet 106 (Jenness, 1982). It is probable that the strength of the casei n particle Ghains in the goat's milk coagulum differs from that of cow's milk coagulum.

Syneresis in cow ' s yoghurt was found by Harwalkar and Kalab (1986) to depend on the dimen­sions of pores (void spaces) in the protein matrix and degree of denaturation of j3-lactoglobulin (Dannenberg and Kessler , 1988). This pore size depends on the final pH at which the yoghurt was cultured. Differences in susceptibility to syneresis resulting in the removal of

39

Table 2 . Range of values of casein fractions • in cow's, goat's, and sheep's milks( % of total casein).

Caseins Cow Goat Sheep

"•I 35-38 0-20 47-56

"•' 9-12 10-25 NR

{3 30-40 43-75 28-45

9-15 8-29 10-12

6.8 3.9 NR

•The above figures have been expressed to the nearest 1%. NR: data not reported. Data compiled from Anifantakis (1986), Juarez and Ramos (1986) , Remeuf and Lenoir (1986), and Richardson et al. (1978).

varying proportions of the liquid phase from the yog­hurts are the major factors which affect, in the finished Labnehs, the concentration of total solids in general and the concentration of protein in particular. It may be noticed that in the Labnehs under study , the pH values fluctuated within a very narrow range (Table I} , and were most probably not responsible for the differences in their compaction of the yoghurts.

Some protein (i.e., proteose-peptone or non-pro­tein nitrogen) was lost while the yoghurts were strained or ultrafiltered to produce the Labnehs. The losses were higher with traditional straining method than with ultra­filtration: 0.3% versus 0.2 % in cow ' s Labneh , 0.57 % versus 0.28% in sheep's Labneh , and 0.91 % versus 0.34% in goat 's Labneh. No fat losses were observed in

A. Y. Tamime, M. Kalab, G. Davies and H.A . Mahdi

Figure I. Labneh (un~smoothened) made from goat's milk by the traditional procedure had a relatively uniform structure of the protein matrix and contained void spaces (arrows) less than 10 .urn in diameter.

Figure 2. Labneh (un-smoothened) obtained by ultrafiltration of yoghurt made from goat's milk contained many void spaces larger than 20 Jlffi in diame ter (arrows). Casein particles were aggregated in the form of clusters (asterisks).

Figure 3. Labneh (un-smoothened) obtai ned by ultrafiltration of yoghurt made from sheep's milk was more uniform and contained smaller void spaces (arrows) than UF goat's Labneh.

Figure 4 Labneh (un-smoothened) obtained by ultrafiltration of yoghurt made from cow's milk had a uniform structure.

any of the UF permeates but in the cloth bag filtrates the levels were 0.01 %, 0.07 % and 0.4% with cow's, sheep's and goat's Labnehs, respectively. Heating of the milk used to make the Labnehs under study resulted in the denaturation of the whey proteins and their reten­tion in the Labnehs so that the solids of the filtrates and permeates consisted almost exclusively of lactose, organ­ic acids , and minerals (Mahdi , 1990).

Firmness

UF Labneh made from cow ' s milk , which was not

40

passed through the homogenizer , was the firmest (1470 mN) as compared to 628 mN and 476 mN in sheep's and goat's Labnehs , respectively (Table 3), despite the fact that the total solids contents were within a very narrow range of 21.4% to 22.5% (Table 1). Firmness of the traditionally made sheep's Labneh (1422 mN) was very close to UF cow's Labneh, whereas the traditionally made goat's Labneh was very soft (256 mN). Passage of all Labnehs through the homogenizer resulted in a marked decrease in firmness, for example, to 829 mN in

Labneh Made From Cow's, Goat's and Sheep's Milks

Figure 5 . Residues of fat globu le membranes (asterisks) indicate the distribution of fat globules in homogenized sheep's Labn~h; compacted protein particles (arrows) formed the walls of a small vo id space (V) in the matrix.

Figures 6 and 7. Homogenized goat's Labneh made by the traditional (Fig. 6) and the ultrafiltration (Fig. 7) procedures. Compacted casein particle clusters (C) with fat globules embedded in them (G) were found in the tradi· tiona ! product but not in the UF Labneh (Fig. 7). Large casein micelles having smooth surfaces (arrows) were observed in both goat 's milk products. B: Bacteria.

Figure 8. Transmiss ion electron micrograph shows minute fat parti cles (small arrows) encapsulated in protein . Prote in particles aggregated on larger fat droplets (la rge arrows) and integrated them into the Labneh matrix . This micrograph shows sheep's Labneh after passage through the homogeni ze r equipped with the 0·280 head.

41

Table 3. The firmness (mN) of cow's, sheep's and goat 's Labneh • using the Stevens·LFRA texture analyzer.

Un- Smoothened** Product smoothened D- 170 D-280

Cow UF Labneh 1470 829 ND Traditional ND 1236 ND

Sheep UF Labneh 628 157 167 Traditional 1422 211 451

Goat UF Labneh 476 142 167 Traditional 256 137 162

*Results are average of three readings at 7oc in each trial and average of two trials for cow's and sheep ' s Labneh and one trial for goat ' s Labneh. **Type of homogenizer head of the ALM machine where D-280 causes the least turbulent effect. ND: not determined.

A. Y. Tamime, M. Kalab , G. Davies and H .A. Mahdi

cow's Labneh (using the D-170 head) and to 157 mN or lower in the other two Labnehs.

Compared to the D- 170 homogenizer head, the D-280 head had a sl ightl y less detrimental effect on the Lab­nebs, probably due to a different design (a smaller num­ber of res tri ctions or grooves) resulting in a less turbulent now of the Labnehs as they passed through this head. Similar results regarding the firmness of Labnehs made from various types of milk and using various thermophilic lactic ac id bacteria were reported by Mahdi ( 1990) .

Acid coagulation of the casein micelles during the manufacture of yoghurt induces structural changes of the casei n micelles. These changes could be summari zed as follows:

(a) The interaction between whey proteins and K­

casein due to the high heat-treatment of the yoghurt milk which increases the hydrophobicity of the casein micel lar surface and prevents the aggregation of micelles until the pH is reduced to 5.5.

(b) The production of acid in the milk by the start­er culture solub ili zes the colloidal calcium-phosphate from the casein micelles into the aqueous phase at pH 5.2-6.7.

(c) The ~51 ·casein starts to form loose aggregates accompanied by the sequential release of 13-caseins from the micelle.

(d) Reincorporation of the ,8-casei n in the micelles to form new particles with as-casein which are different in structu re at pH 4.8-5.2.

(e) The final protein network of acidified gel is achieved a t pH< 4.5 (Walstra etal., 1985 ; Heertje eta!., 1985; Rocfs et al., 1985; Rocfs, 1986; Visser ct al., 1986; van Hooydonk, et al., 1986; Ono and Creamer, 1986; Schmidt and Poll , 1986; Haque and Kinsella, 1988; Dalgleish and Law , 1988, 1989; van Vliet eta/. , 1989; Mottar eta/., 1989; Bringe and Kinsella , 1990).

Also , differences in the size of bovine , ovine and caprine micelles have been reported by Richardson eta/. (1974) . The ovine micelles were below 80 nm in diame­ter and smaller than bovine micelles. The caprine micel ­les were either large (- 200 nm in diameter) or small ( < 80 nm in diameter) and very similar to ovine micelles. However, the bovine micelles consisted of a wide range of sizes.

According to van Vliet et al. (1989) the main inter­ac tion force s in acid casein gels include hydrophobic and electrostatic (a ttractive via +ve or -ve force). The elec­trostatic repulsive forces are of secondary importance and the other forces, which could include conformational en­tropy , van der Waals attraction, hydrogen bonding , hy­dration repulsion and Born repu lsion, are of uncertai n im­portance. Thus if it is accepted that ~Xs-casein and the size of the micelle play a major role in the formation of acid gel , then it may be concluded, on the basis of the meas­urement of firmness , that the differences between cow's Labneh and Labneh made from sheep's and goat's mi lks arc due to the heterogenous nature of the caseins in these mi lks. However , the processing methods (t raditional and ultrafiltration) and the mechanical effect of the lactic curd homogenizer should not be overlooked.

Differences in firmness between the various Lab-

42

nehs cannot be explained merely by their protein contents. There is a probability , however , that the differences in the casein fractions play an im portant role .

Structure The microstructures of Labnehs made by the tradi­

tional method from goat ' s and sheep's milk as visualized by SEM at low magnifications before homogenization (Fig. 1) appeared to be similar to each other and to the Labneh made from cow's milk (Tamime er al., l989a). In contrast, Labnehs obtained by ultrafiltration of yoghurts made from goat's and sheep's milks (Figs. 2 and 3, re­spectively) were not as uniform before homogenization and contained many large (- 20 llm in diameter) void spaces, apparently filled with the aqueous phase in the original samples. Unlike in natural yoghurt (Harwalkar and Kalab, 1986), the void spaces were not associated with colonies of the starter culture and the bacter ia were relatively uniformly distributed in the protein matrices of Labneh. The structure of UF cow's Labneh was uniform (Fig. 4). There is an appa rent relationship between the uniformity of the Labneh structure and firmness (Table 3): the most uniform cow's Labneh was the firmest ( 1470 mN) followed by the Jess uniform sheep's Labneh (628 mN) and the least uniform goat 's Labneh being the softest (476 mN).

Preparation of the samples for SEM involved the extraction of fat. Therefore , on ly fat globule membranes are noticeable in the micrographs, for example, in sam­ples which had been subjec ted to homogenization (Fig. 5). In spite of homogenization using the D-280 head , undis ­turbed compacted protein c luste rs were noticeable in goat's Labneh made by the traditional procedure (Fig. 6) whereas the UF goat's Labneh was broken into smal ler c lusters (Fig. 7).

Interac tions between fat and protein were evident from thin sect ions exam ined by TEM (Fig. 8) , where mi­nute fat particles of the casein particle dimensions may be seen encapsulated in the protein. Adsorption of protein particles on large fat droplets has Jed to the ir integration in the Labneh matrices. There were no major differences in structure between the indi vidual Labneh products which could be noti ced by high magnification TEM.

Conclusions

There were marked diffe rences in the firmness of goat's, sheep's , and cow's Labnehs in spite of their simi­lar total solids contents: the uniformly structured cow's Labneh made either by the traditional procedure or by ultrafiltration was firmest , whereas the goat's Labneh contained large vo id spaces and was the softest, also irre­spective of the method of manufacture. Homogenization of the goat's and sheep ' s Labnehs produced a smoother texture but markedly decreased the firmness.

Acknowledgement

One of us (HAM) is indebted to the Iraqi Govern­ment for financial support. The authors thank Drs . H.E. Medler , V .R. Harwalkar and W . Banks for useful com-

Labneh Made From Cow's, Goat's and Sheep's Milks

mcnts ; and Mrs . lsobel Hamilton, Mrs. Paula Allan Wojtas and Gise le Larocque for sk ilful techni cal assistance. Electron Mic roscope Centre, Research Branch , Agricul · ture Canada in Ottawa provided facil ities. Contribut ion no . 885 from the Food Research Centre in Ottawa.

References

Allan-Wojtas P. (1984). Trans portation of fragile food specim ens suc h as mi lk gels dest ined for electron microscopy. Food Microstruc . .3., 93.

Anifantakis EM. (1986). Comparison of the phys­ico-chemical properties of the ewe's and cow's mil k. IDF Document No. 202, International Dairy Federation , Brussels, Belgium , 42-53.

Bringe NA , Kinse ll a J E. (1990). Acidic coagula­tion of casei n micelles: mechanisms in fe rred from spec­trophotometric studies. J. Dairy Res. ll, 365 -375.

Dalg leish DG , Law AJ R. (1988). pH -Induced dis­sociation of bovine case in micell es. I. Analysis of liberated caseins. J. Dairy Res. ll. 529-538.

Dalgleish DG , Law AJR . ( 1989). pH-Induced dis ­soc iation of bovine case in micelles. II. Minera l solubili­zat ion and its relation to casein release. J. Dairy Res.~. 727-735.

Dannenberg F, Kessler HG. (1988). Effect of de­naturation of iS - lactoglobul in on texture properties of set­style nonfat yoghurt. I . Syneresis. Milchwissenschaft '[1(10) , 632-635.

Haque Z, Kinse ll a JE (1988). In te raction between heated K·Casein and /3- lactog lobulin: predominance of hydrophob ic interactions in the initial stages of complex formation. J. Dairy Res. 22. 67-80.

Harwalkar VR , Kalab M. (1986). Relationshi p be­tween microstructure and susceptib ility to syneresis in yoghurt made from reconst ituted nonfat dry milk . Food Mi crostruc . ~. 287-294 .

Heertje I, Visser J , Smits P. {1985). Structure for­mation in acid milk gels. Food Mic rostruc. ~. 267-277.

Je nness R. (1982). Inter-species compari son of milk proteins. Deve lopments in Dairy Chemis try- l. Fox PF (Ed.). Elsevier Applied Science Publishers Ltd. , London , U.K. , 87-114.

Juarez M , Ramos M. (1984). Dairy products from ewe's and goat's milk. Dairy Industries Internationa l ±2.(7), 20-24.

Juarez M, Ramos M. (1986). Phys ico-c hem ical character isti cs of goat's milk as distinct from those of cow's milk. IDF Document No. 202 , op. cit., 54-67.

Mahdi HA. ( 1990). Some Aspects of Producti on and Quality of Strained Yoghurt (Lab neh). Ph.D. Thesis, University of Strathclyde, Glasgow, U.K .

Mottar J , Sassier A, Jon iau M, Baert J. (1989). Effect of heat·induced assoc iat ion of whey proteins and casei n micelles on yogurt textu re. J . Dairy Sci.ll, 2247-2256.

Ono T , Creamer LK. (1986). St ructure of goat ca­sei n micelles. N.Z . J . Dairy Sci . Techno!. ll. 57-64 .

Ramos M, Jua rez M. ( 1981). The Composit ion of Ewe 's and Goat's Mi lk. ID F Docu ment No. 140, op. c it., 5-19.

43

Remeuf F , Lenoir J . {1986). Re la tionshi p between the physico-chemical characteristics of goat's mi lk and its rennetabil ity. IDF Document No. 202, op. cit., 68-72.

Reynolds ES. (1963). The use of lead citrate at high pH as an e lectron-opaque stain in e lectron micros­copy. J. Cell Bioi. 17 , 208 2 12.

Richa rdson BC, Creamer LK, Pearce KN. (1974). Comparat ive micelle structure- II: Structure and compo­si tion of casei n micell es in ovine and capr ine milk as compared with those in bovine milk. J. Dairy Res. !1. 239-247.

Richardson BC , Merc ier JC, Ri badeau Dumas B. (1978). The primary st ructure of the ovine iS -case in . XX Int. Dairy Congress 1..£. Congrila it, Paris , France , 215-2 16.

Robinson RK, Vlahopoulou I. (1988). Goat's milk utilisation for fe rmented mil ks. Dai ry Industries In ter ­national ll(12), 33-35.

Roefs SPFM. (1986). Structure of Ac id Case in Gels. Ph. D. Thesis, Agricultura l University , Wageningen, The Netherlands .

Roefs SPFM , Walstra P, Da lgleish DG , Horne DS. ( 1985). Pre lim inary note on the change in casein micelles caused by acidification. Neth. Milk Da iry J . .3..2., 119-122.

Schmidt DG, Poll JK . (1986). Electro-kinetic measurements on unheated and heated casein micelle systems. Neth. Milk Dairy J. 1!), 269-280.

Tamime AY, Kal ab M, Davies G. ( 1984). Micro­structure of set-sty le yoghu rt manufac tu red from cow's milk fortified by different me thods. Food Microstruc . .3., 83 92.

Tamime AY, Davies G, Hamilton MP. (1987). The quality of yoghurt on reta il sale in Ayrshire - Part I . Chemical and microbiologica l eva luation. Da iry Industries In ternat ional. ll(6) , 19-21.

Ta mime AY, Kalab M, Davies G. ( 1989a). Rheology and microstructure of strained yoghurt (Labneh) made from cow's milk by three different met hods. Food Microstruc . .8.. 125-135.

Tamime AY , Davies G, Chehade AS , Mahdi HA. (1989b). The production of Labneh by ultrafiltration - a new technology. J. Soc. Dairy Techno!. 11. 35-39.

van Hooydonk ACM , Hagedoorn HG , Boerrigter 11 . (1986). pH-Induced physico-chemical changes of casein micelles in milk and their effect on renneting. I . Effect of ac idifica tion on physico-chemica l properties. Ne th . Milk Dairy J. 1!), 28 1-296.

van Vliet T, Roefs SPFM, Zoon P, Walstra P. (1989). Rheological proper ties of casein gels. J. Dairy Res. ,22, 529-534.

Visser J. Minihan A, Smits P, Tjan SB , Heertje I. ( 1986). Effects of pH and temperature on milk salt system . Neth. Milk Dairy J. '!l) , 35 1-368 .

Walstra P, Jenness R. (1 984). Da iry Chemist ry and Phys ics. John Wiley and Sons Inc., New York , 146- 155.

Walstra P, van Dijk HJM, Geurts TJ. ( 1985). The syneres is of cu rd . I . General considera ti ons and li te rature review. Ne th . Milk Dairy J. :ll!. 209-246.

See over for Discussion with Reviewer s.

A. Y. Tamime, M. Kalab, G . Davies and H.A. Mahdi

Discussion with Reviewers

D.N. Holcomb: Were the organoleptic properties of the various Labneh samples determined? Authors: The Labneh samples were stored at 7 oc for two d:1ys and only the smoothened products were evalu­ated by to panelists according to the scheme of Pearce and Heap (Pearce LE , Heap HA. Evaluation of yoghurt and cottage cheese. Town Milk. J. N.Z. Milk Board ll(2), 18-20 , 1974). The scores were awarded for ap­pearance and color (5), body and texture (5) and flavor (10). The results were reported in detail by Mahdi (1990) and may be summarized as follows:

a) The panelists awarded the highest rating to Labneh made from cow's milk, followed by sheep's and goat's Labnehs.

b) Slightly lower scores of body and texture of sheep's Labneh were awarded by the panelists because the smoothened product was not as firm when compared with cow's Labneh.

c) Labneh made from goat's milk received the lowest rating because it was soft and the "goaty" flavor was unacceptable to the panelists.

P. Walstra: Electron microscopy does not really yield observations that would help in interpreting the differ­ences in properties. Application of present-day knowl ­edge about microstructure, rheological (and fracture) properties and syneresis of acid casein gels could lead to far better interpretation but the scarcity of experimental results would not allow for such a treatment. Neverthe­less, it is strange that no reference is made to the Danish product Ymer, which shows similarity with Labneh. Authors: The comments regarding the above parameters should have been investigated in more detail. However, in this preliminary study the main objective was to ob­serve if the warm yoghurt made from sheep's and goat's milk could be ultra filtered for the manufacture of Labneh. The results suggest that further stud ies are required to identify the processing conditions which are required to produce a firmer sheep's and goat's Labneh by ultrafiltration.

While it is true that there are some similarities between Labneh and Ymer, there are also significant dif­ferences, namely: (a) the manufacturing stages, and (b) the chemical composition of the end product. In brief, the production of Ymer under factory conditions in Den­mark could be described as follows: Skim-milk is ultrafil­tered at 50°C to concentrate the protein content ~ 6% (which is the Danish standard) . The UF retentate is standardized to - 3.5% fat (a lso Danish standard), pre­warmed to 74°C, homogenized at 13.7 MPa, deaerated, heated to 95~ l00°C for 1 minute , cooled to 22 oc , inocu­lated with 3% mesophilic starter culture (e.g., mixed strains which contains flavor and aroma producer) and incubated for 20-22 hours. On the following day the cultured milk is thoroughly mixed for 1 hour , homoge­nized at 4.9 MPa to smoothen it , cooled to l2°C and packaged (J. Kirkegaard, personal communication). It is for these reasons that Ymer was not included in the discussions in this paper.

44

P. Walstra: A major variable may be pH, and its effect should have been studied. For acidified cows' milk, e.g., yoghurt, it makes a marked difference in rheological properties and syneresis, whether the pH is 3.95 or 4.3. How this would be for sheep's and goat 's milk is presumably not known , although some French literature may exist. Authors: The pH values of yoghurts made from fresh cow's, sheep's and goat's milks were 4.60, 4.60 and 4.40, respectively. In preliminary experiments, these va lues were found to be optimal concerning the firmness of the yoghurts required for the production of Labneh by ultrafiltration. The report by Mahdi (1990) and in un­related experiments (M. Kalab, unpublished data) it was found that goat's milk must be acidified to a considerably lower pH than cow's milk to coagulate, and yet the firmness of goat's milk gel was markedly lower. For the manufacture of UF Labneh , the yoghurts were concen­trated directly after the incubation period and the pH of all the Labneh were very similar (Table 1). However , for the production of traditional Labneh, the cow's and sheep's yoghurts were removed from the incubator at higher pH (- 4.7-4.8) and goat's yoghurt at pH 4.4 because they were cooled slowly in bulk overnight. It was found that if goat's milk yoghurt was removed at higher pH, structural damage of the coagulum was inevi ­table, and for this reason the acidity of goat's Labneh (traditional) was higher as compared with other milk products. Regrettably, yoghurts having other pH values were not made. In retrospect, it is agreed that yoghurts with different pHs could have been made and the effect of this on the microstructure could have been included in the study.

P. Walstra: Centrifuged skim-milk contains about 0 .05% , could the authors explain the low fat content (i.e., 0.01% -Table I) in the filtrate, which is very unlikely? Authors: It is true that very low fat content was observed in the cloth bag filtrate which was due to unavoidable processing conditions. Extreme care was taken when the refrigerated fermented milks were transferred to the cloth bag. Despite the fact that the bags were placed in a bucket and the ferment milks were poured gently into the cloth bags, the first flow of filtrate was "milky" or "turbid" due to the permeability of the material of the bag. After few minutes of drainage, the appearance of the filtrate became comparable with UF permeate, and it is for this reason that minute fat losses were observed in the filtrate. The fat loss is an inherent feature during the manufacture of traditional Labneh.

L. K. Creamer: On the basis of the authors impressions, what factor(s) is the most likely to be responsible for the lower firmness of goat's Labneh? Authors: It is very difficult to identify the factor(s) which is responsible for reduced firmness of goat's Lab­neh from the limited trials conducted: however, one possi ­ble factor could be the low level of as-casein in goat's milk (Table 2) which plays a major role in the formation of acid milk gel.