moisture sorption behavior and shelf life prediction of

Moisture Sorption Behavior and Shelf Life Prediction 1

RESEARCH ARTICLE

PREFACE API 2015

Moisture Sorption Behavior and Shelf Life Prediction of Teff Seed and Flour

Baba Abdissa*Osmania University

Kodandaramreddy DeshamOsmania University

Rudrayya MathCentral Food technological Research Institute

Srinivasulu KorraCentral Food technological Research Institute

ABSTRACT

Moisture sorption isotherms of Teff seed and its flour were determined over the range of relative humidity (11–92%) using a gravimetric technique at constant temperature (27°C). The results obtained showed that the moisture sorption isotherm of both is almost similar and sigmoid with increasing equilibrium water content at the constant temperature. The curves start raising high above 64% RH indicating the products deteriorates faster above 64% RH. Hence the moisture content equilibrating to 64% RH was taken as critical for both Teff seed and flour variants. The sorption isotherm fits the GAB equation as per GAB generalized equation for sorption phenomena (Bioprocess Technol, 2008) with R2>0.96. A nonlinear regression analysis method was used to evaluate the constants of sorption equation and the values of GAB constants of which Mo=7.5075, C=14.3886, K=0.7051 for Teff seed and Mo=6.9656, C=10.4012, K=0.7255 for Teff flour respectively. The shelf life of the product was predicted using, Mpack software developed in-house to “prediction of the shelf life of moisture sensitive products” using GAB parameters and packaging Schedule.

KEY WORDS

Sorption isotherm, Teff seed, Teff flour and shelf life

Baba AbdissaCorresponding [email protected]

*

Journal of Applied Packaging Research 2

INTRODUCTION

Moisture sorption characteristics provide the necessary information regarding quality, stability, and shelf life during packaging and storage of food powders. The hygroscopic nature of flour causes caking and follows deterioration by oxidation. The moisture content of the flour as a hygroscopic material exerts a strong influence on its quality and technological properties (Abdullah & Nawawi, 2000; Teunou & Fitzpatrick, 1999).

Knowledge of the sorption characteristics is essential regarding stability in storage and accept-ability of food products, drying process modeling, design and optimization of drying equipment, aeration, calculation of moisture changes which may occur during storage, and for selecting appro-priate packaging materials (Ngoddy & Bakker Arkema, 1975; Erol A et al., 1990; Kumar, 2000; Aviara & Ajibola, 2006; Viswanathan et al., 2003; Chowdhury et al., 2005; Akanbi et al., 2006; Sama-pundo et al., 2007).

(Aviara et al., 2006) have reported the static gravimetric technique as the preferred method for determining the moisture sorption isotherms of food products. This method has several advantages over the manometric and hygrometric methods. This pro-cedure is suitable for developing countries because of the ready availability of slats and lack of CO2

absorption by acid when compared to KOH or other salt solutions. The choice of acids as desiccants can, therefore, reduce the risk of uncertainty in determi-nations of water activity during sorption studies.

The European Project Group COST 90 on Physical Properties of Foods and the American Society of Agricultural Engineers has recom-mended some isotherm equations as the preferred ones for food isotherms and hence are the widely adopted food isotherms models (Chowdhury et al., 2005; Aviara et al., 2006). Because crop processing has a marked influence on the physicochemical and

nutritional attributes of food products (Igbeka & Olumeko, 1996; Oyewole & Afolami, 2001; Falade et al., 2003), it was desirable to investigate the use of these isotherms for cereal flours. (Sopade & Ajisegiri 1994) have constructed moisture sorption isotherms of a variety of maize grains.

Fermenting microorganisms, namely yeasts, molds, and bacteria are the cause of spoilage of car-bohydrate-dense. Starch is the important constitu-ent influencing the thermal properties and enzyme susceptibility of quinoa flour (Li., Zhu., 2017).

Many theoretical, empirical, or semi-empirical models have been proposed by various researchers to describe the sorption isotherms. Existing math-ematical expressions usually describe accurately the sorption data in a certain range of aw, but they usually cannot fit the experimental data in the whole range of water activity (0–1.0). These limitations have led to efforts for interpolation or extrapola-tion of experimental results (Yanniotis et al., 1994; Viollaz & Rovedo, 1999) used the fourth parame-ter in the Guggenheim, Anderson, de Boer (GAB) equation to correlate the sorption data for starch and gluten for water activities higher than 0.9. Nonlinear regression was used for the calculation of the values of the parameters. The generalized GAB form leads to the successful description of the sorption data for water activity values from 0 to 1.

Fitting of EMC data to various sorption isotherm models

The Experimental data’s fitness can be observed by using four moisture sorption isotherm models, such as Guggenheim-Anderson-De Boer (GAB), modified Henderson, modified Oswin and modified Halsey models as seen in Table 1, where aw repre-sents the equilibrium relative humidity in decimal; M is the equilibrium moisture content in % (db); t is the temperature in °C; A, B, C are sorption isotherm constants specific to each equation.


Teff (Eragrostis tef ) is an important grain and is among the gluten-free millet. At the time of harvest, Teff is reported to have a moisture content ranging from 9.30% to 11.22%, dry basis (Bultosa, 2007). Drying and size reduction processes are standard practices that are carried out to produce Teff flour as an alternative way of presenting the grain to commerce in tropical developing countries like Ethiopia and Eritrea. In Ethiopia, it is the most popularly grown and consumed cereal.

Even though Teff is the main staple food crop for Ethiopia, its technological advancement is not yet explored. Its storage is still in traditional silos called ‘Goterra’ (made of wooden materials and mud) or bags. This makes its storage uncontrollable, limiting the product shelf life due to deterioration as a result of moisture sorption and variation of temperature. It is also not well known how long farmers/suppliers/manufacturers can keep their Teff seed or flour in the silo or bags without deterioration. The patterns of granary temperature, relative humidity, grain moisture contents, and association of micro-flora with Teff seed and its flour stored in silos/bags under

different agro-ecological conditions are not known. The changes in grain bulk density, grain weight loss, seed germination, and chemical components of Teff seed or flour stored over time are not estimated.

The aim of this study was to determine the moisture sorption Isotherm characteristics of Teff seed and its flour over ranges of relative humidity (11 – 92%) and at a constant temperature of 27°C. The fitness of the sorption isotherm was done using the recommended isotherm model of GAB equation as per GAB generalized equation for sorption phe-nomena (Bioprocess Technol, 2008).

MATERIALS AND METHODS

Materials

White Teff (Eragrostis tef ) of a local variety (DZ-CR 35) that was used in this study was pur-chased from the Debrezeit agricultural research institute, Debrezeit, Ethiopia. The preparation of the flour was by the traditional procedure. The seed was cleaned and winnowed. The Teff seed was

Table 1: Some Mathematical models applied to the sorption isotherms to analyze EMC-aw data


then milled to obtain the Teff flour without decor-tications of the seed due to its small size. Initial moisture content (MC %) of the seed and flour was determined in replicate according to the (AOAC 1990) procedures. The study was fully conducted at CSIR-Central Food Technological Research Insti-tute Resource Centre Habsiguda, Hyderabad, India.

Microclimate conditioning

The required microclimate was prepared from various concentrations of salts in creating relative humidity ranging from 11% - 92% at room tem-perature (27°C), namely: 11% Relative humidity is 11% distilled water + 89% LiCl, 22% Relative humidity is 22% distilled water + 78% K(CH3COO-

), 32% Relative humidity is 32% distilled water + 68% MgCl3, 44% Relative humidity is 44% distilled water + 64% K2CO3, 56% Relative humidity is 56% distilled water + 44% NaBr, 64% Relative humidity is 64% distilled water + 36% NaNO2, 75% Relative humidity is 75% distilled water + 25% NaCl, 86% Relative humidity is 86% distilled water + 14% K2CrO4 and 92% Relative humidity is 92% distilled water + 8% KNO3 (Alakali & Satimehin, 2009).

Experimental procedure

The equilibrium moisture content was deter-mined by a standard gravimetric method by exposing the Teff seed and its flour to a constant relative humidity environment created by the satu-rated solution of a particular salt at room tempera-ture (27°C) (Bell & Labuza, 2000). Nine differ-ent salts viz., LiCl, K(CH3COO-), MgCl3, K2CO3, NaBr, NaNO2, NaCl, K2CrO4, and KNO3 were used to maintain respective water activity (aw) inside separate vacuum desiccators in the range of 0.11 to 0.92. (Lopez et al., 1995).The replicate samples were placed in Petri-dishes inside the desiccators in which salt solutions were used to control the specific relative humidity within the system. The prepared desicca-tors were then placed at room temperature of 27°C.

The samples were periodically weighed until they attained practically constant weight or showed signs of mold growth whichever was earlier. After equili-bration, the moisture content (MC %) of the product at different RHs was calculated by adding /sub-tracting % pick up/loss to/from the initial moisture content. The sensory remarks on their quality were taken, and critical moisture content was fixed.

Initial Moisture Content (IMC %)

This was determined by drying the known weight of the sample taken in Petri-dish at 100 ± 5°C till constant weight as per AACC, 1983.

Critical moisture content (CMC %)

According to (Azanha. & Faria. 2005), the critical moisture content is the moisture content at which the product loses its characteristics to a level that would be rejected by the consumer.

Determination of Equilibrium Moisture Content (EMC)

Equilibrium moisture content was determined by calculating the original moisture content and the known change in weight on a dry basis (Oyelade et al., 2008). The relation below is used in calculating the equilibrium moisture content (EMC) of the samples.

Determination of water activity (aw)

The water activity aw of the sample was deter-mined from the relative humidity of the air sur-rounding the sample when the air and the sample are at equilibrium, which is equilibrium relative humidity. The equilibrium relative humidity value was divided by 100 to get the water (aw) value. It is mathematically expressed as follows:


Fitting of EMC data to sorption isotherm models

The Experimental data’s fitness was observed by using the most widely recommended isotherm model of GAB equation as per GAB generalized equation for sorption phenomena (Bioprocess Technol, 2008). The SAS procedure for non-linear regression (Proc NLIN) was used.

Where M is the moisture content (M = f (aw, T)),

k: constant, aw: water activity, C: GAB model parame-

ter, K: GAB model parameter, m0: monolayer moisture

content (kg/100kg dry matter).

Goodness of fit to the sorption isotherm models

The statistical validity of the fit to the models was evaluated using statistical parameters such as the root mean square error (RMSE) and from the determina-tion of coefficient (R2) and mean relative error (MRE).

Where Me is the experimental EMC value, Mp is the predicted EMC value, and N is the number of experimental data. The value of the coefficient of determination (R2) close to 1 and that of root mean square error (RMSE) value close to 0, indicate a better fit, mean relative error (MRE) below 10% indicates a good fit for practical purposes (Lomauro et al., 1985).

Microbiology

Aerobic Mesophilic Plate count

Aerobic (Mesophilic) Plate count of 10g of the sample was conducted after dilution, pour plating, and 48hrs of incubation period at 36°C as expressed in Compendium of Methods for Microbiological examination of foods (Downes & Keith Ito, 2001). Following incubation, all the colonies on the dishes containing 25- 250 colonies were counted and

recorded per dilution. For plates with 25-250 colonies forming units (CFU) of Aerobic Plate Count (APC) or Total Plate Count (TPC) was calculated as follows:

Where N=Number of colonies per ml or g of productΣ c=Sum of all colonies on all plates countedn1=number of plates in the first dilution countedn2=number of plates in the second dilution countedd=dilution from which the first counts were obtained

Yeasts and Molds

The type of media for the growth of Yeasts and molds was potato dextrose agar (PDA). 10gm of the sample was mixed with 90ml of saline and homoge-nized well. 1ml of the homogenized sample was then added into test tubes containing 9ml of saline, and serial dilution was done. After dilution, pour plating and 5days of incubation period at 28°C as expressed in the Compendium of Methods for Microbiological examination of foods (Downes & Keith Ito, 2001). The colonies were counted and multiplied by the inverse of the corresponding dilution and reported as yeast or mold count per g or ml.

Shelf Life Prediction

The shelf life of the Teff seed and its flour was predicted using Mpack software developed in-house- “prediction of the shelf life of moisture-sensitive products” (G.C.P Rangarao, 2001). The packaging schedule considered for predicting shelf life were Water Vapor Transmission Rate (WVTR) of Packaging material, 250g product packed in a pouch of dimensions of 14X19cm and Storage con-ditions of Accelerated condition (380C/90%RH), Indian Standard condition (270C/ 65%RH) and also moisture sorption isotherm of the product.


RESULTS AND DISCUSSION

Moisture Sorption isotherms

To design functional economic package for Teff seed and flour- which are non-hygroscopic, their moisture sorption characteristics were studied. The moisture sorption isotherms are presented in Fig 1. As can be seen in fig, the moisture sorption isotherm of both is almost similar and sigmoid, indicating the typical of a low-fat and fiber-rich products. The curves start raising high above 64% RH, indicating the products deteriorate faster above 64% RH. The products Teff seed and flour with an Initial moisture content of 9.50 to 8.91% on as-is basis equilibrates to 51 and 53% RH, respec-tively. The products equilibrating up to 64% RH in the moisture range of 11.35 to 10.30% was excellent and free-flowing in seed, whereas in flour slightly off flavor observed. Both products; seed and flour equilibrated to 75% with moisture contents of 12.97 to 12.88% respectively, had off-flavor whereas flour had a tendency for caking but could be broken by applying little pressure, and at higher RH they developed mould growth at 92 and 86% RH in 2 and 3 weeks’ time Table 2.

The CMC of the products is 11.35 to 10.30%, respectively beyond this MC. The product is not acceptable, and lump formation in flour is observed.

Hence the moisture content equilibrating to 64% RH was taken as critical for both Teff seed and flour variants. The moisture tolerance of Teff seed and flour are 11.35-9.50=1.85% and 10.30-8.91=1.39% (The dif-ference between initial and critical moisture contents), respectively. The Teff seed appears to be slightly better in storage stability due to its higher water holding capacity. The critical RH being low, it requires a high moisture barrier for a required shelf life of a year. The sorption isotherm fits the GAB equation as per GAB generalized equation for sorption phenomena (Bio-process Technol, 2008), with R2>0.96. The GAB con-stants are Mo=7.5075, C=14.3886, K=0.7051 for Teff seed and Mo=6.9656, C=10.4012, K=0.7255 for Teff flour respectively. The GAB model has been used due to its theoretical bases, it describes the sorption behavior in a wide range of aw (0 - 0.9). Thus, it was found to be suitable for analyzing more than 50% of fruits, meat and vegetables. The major advantages of the GAB model are the following: it has a viable theo-retical background since it is a further refinement of Langmuir and BET theories of physical adsorption; it provides a good description of the sorption behaviour of almost every food product (aw <0.9);

Fig.1. Moisture sorption isotherm of Teff Seed & Flour at 27°C


Table 3: Number of colonies or cfu/gm for Teff seed after four weeks of moisture sorption study

Table 2: Humidity moisture content data of Teff seed and its flour at 27°C.


Microbiology

The microbial count of the Teff seed and Teff flour is shown in Table 3. The APC -Aerobic (meso-philic) plate count of the Teff seed and its flour were in the range of (i.e., below the limit) Turkish legal limits (1.0 × 105 CFU/g) (Turkish Food Codex., 2001) over the relative humidity of 11 -92%. But the APC of teff seed at the relative humidity of 86 and 92% were found to exceed the limits while Teff flour totally complies with it. A number of studies on wheat flour report also showed mean total aerobic counts in the order of 104 CFU/g or below (Potus and Suchet., 1989; Spicher., 1986; Cicognani et al., 1975, and Ottogalli and Galli., 1979; Richter et al., 1993; Berghofer et al., 2003). The result obtained for both Teff seed and Teff flour also conforms to this. The Yeast and Mould counts (CFU/g) for both Teff seed and flour were high at high water activity, particularly at the relative humidity of 86 and 92%. There were no observable mold and yeast growth in Teff flour at the relative humidity of 56, 64, and 75%. Mean mold counts are usually around 103 CFU/g (Potus and Suchet., 1989; Spicher., 1986; Cicognani et al., 1975; Ottogalli and Galli., 1979). Our results do not resemble these findings. Considerably higher levels of mold loads in cereal samples in Turkey, however, i.e., in the order of 105 -106 CFU/g, have been reported by (Aran and Eke, 1987).

Contamination of stored grain by bacterial pathogens is generally not a major issue, as patho-gens such as Salmonella survive poorly on grain. Furthermore, most grain Stored grain in Australia 2003 58 destined for human food use will undergo processing steps that will destroy bacterial patho-gens before the commodity reaches the consumer. Mycotoxin contamination of grain is not always a result of poor storage, as some mycotoxins, particu-larly Fusarium toxins, may already be in the grains when they are brought into storage. However, if molds do start to grow in stored grains, mycotoxins may be formed if the fungal succession develops to a

point where mycotoxigenic species can grow, gener-ally above about relative humidity of 83% (0.83aw). Whether or not mycotoxins are an issue, any mold development in stored grains is undesirable.

Mold growth in grains may cause deleterious changes in addition to the formation of mycotox-ins. Many spoilage fungi cause loss of germination in seed grains, discoloration, and darkening of the grains, reduction in protein content, musty odors, and changes in fatty acid profiles and other con-stituents of the grains. Mould development may also encourage mite and insect infestation. Mould development during storage can be controlled or prevented by ensuring that grain is adequately dry at intake. Further protection can be provided by preventing the development of temperature and moisture gradients by cooling and aeration of the grain. Protection from insect infestation will also help prevent mold development in stored grains, both in bulk or bag storages.

Flour is regarded as a microbiologically safe product as it is a low water activity commodity (ICMSF., 1998). Although the growth of pathogenic bacteria may not be supported by such properties, pathogens that contaminate flour may survive for extended periods (Berghofer et al., 2003). There are reported cases of food poisoning resulting from contaminated flour. Australian, European, and US studies indicate that Salmonella spp., Escherichia coli, Bacillus cereus, and other spoilage micro-organisms are present in wheat and flour at low levels (Cicognani et al., 1975; Ottogalli and Galli., 1979; Spicher., 1986; Richter et al., 1993). In addition, mold growth in flour is known to decrease its quality significantly. Mould contamination on cereals, which can exist at the farm or at the site of storage, affects the yield, quality, and nutritional value of the products (Aran and Eke., 1987). When the water content exceeds the alarm level for wheat flour (13%-15%) molds start growing (Jay., 1996). This situa-tion illustrates how some chemical parameters could


affect microbiological parameters. Fungal contami-nation of flour has been the subject of various inves-tigations (Aran & Eke., 1987; Beuchat., 1992; Wei-denbörner et al., 2000).

Shelf life Prediction

The shelf life of the product was predicted using Mpack software developed in-house to “prediction of the shelf life of moisture-sensitive products” using GAB parameters and packaging Schedule. The developed method has been validated with a biscuit, which is also a Moisture sensitive product as per Prediction of Moisture changes in packed

glucose biscuits stored under variable Temperature and RH conditions (ICFoST, 2012). As seen from Table 4 the predicted shelf life of two Teff seed and flour are two years and < 1 year when packed in packaging material of WVTR 0.1, 1.0, 2.0, and 5.0, respectively. When many available packaging materials were screened, 7µ foil laminates (10µ PET/7µ Al/40µ PE) have WVTR of about 0.1, 10µm Met PET/40µm PE of Optical Density (OD)>2.5 has WVTR close to 1.0 and 75µm CPP and 37.5µm CPP with WVTR 2.0 and 5.0 respectively offer about >365 days shelf life under 65%RH/27oC, and it fails to offer <1year shelf life in polypropylene and in

Table 4: Prediction of shelf life of Teff seed and its flour in various packaging configurations and storage environments


MET/PET/PE under 90%RH/38oC. However 7µm Al Foil laminate offers > 2-year shelf life under 65%RH/27oC and >1 year under 90%RH/38oC.

Adsorption isotherms of amaranth flour can be predicted by many models, but a better understand-ing of the sorption behavior was obtained by GAB equation. Water adsorption estimates ranged from 6.4 g to 7.2 g of water per 100 g of dry material. It was suggested that maximum moisture of amaranth flour should be13 g of water per 100 g of dry material and stored at <60% relative humidity. High moisture barrier packaging materials are required for the high humid condition storage (Balderrama, Carballo Cadima, 2014). The shelf life of RTE cut salads stored under the cold chain can be predicted if the storage temperature data is available. Such models will help the development of new products in the fresh vegetable sector (Tsironi et al., 2017).

CONCLUSIONS

The moisture sorption isotherm characteris-tics of Teff seed and its flour over ranges of relative humidity (11–92%) and constant temperature (27°C) were determined by the standard gravi-metric method using various saturated salt solu-tions. For the study illustrated that there was a significant effect of relative humidity on the equi-librium moisture sorption in the range of relative humidity studied at a constant temperature for both Teff seed and Teff flour. The percent equilib-rium moisture content increased with constant tem-perature at increasing water activity. The state of water in foods has a direct effect on their quality and stability through its effects on chemical and enzymatic reactions. For this reason, control and determination of EMC and aw have been an area of concentrated research. The more water activity to which the study materials subjected, the faster rate of deterioration and the higher the growth of yeast and mold limiting the shelf life of the product

depending upon the properties of the packag-ing material. The sorption isotherm fits the GAB equation as per GAB generalized equation which exhibits sigmoidal shape, is a characterized as type II isotherm with determination of coefficient R2 for sorption phenomena (Bioprocess Technol, 2008), The GAB appeared to have an extended range of application relative to BET and this sorption model was proved to be an adequate representation of the sorption behavior of Teff seed and Teff flour.


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