Improving frozen food quality through modified starches

Starch is an important ingredient in many foods and is used as a thickener, stabilizer, gelling agent, and more. But it has some limitations, like low resistance and tendency to change over time. To fix this, starch can be modified to improve its qualities for use in food. The most common way to improve its stability in frozen food is through genetic, physical, or chemical modification.

Genetical Modification

Genetically modified starch refers to starch from genetically engineered plants, such as those that have been genetically modified to produce novel fatty acids or carbohydrates which might not occur in the plant species being harvested. For native, unmodified starches, freeze-thaw stability requires high-amylopectin concentrations as shown for rice starch (Varavinit et al., 2002), wheat starch (Yi et al., 2009), barley (Bhatty, 1999), and a very short-chained starch isolated from red algae and functionally analyzed (Bojko et al., 2002).

Hence, strategies for obtaining freeze-thaw stable starches rely on reducing the amylose content and/or decreasing the chain length of the amylopectin. Based on this, genetic engineering technologies has been developed for cold-resistant starches by antisense downregulation of three starch synthase genes (GBSS, SSII, and SSIII) into starch, creating a waxy starch with short-chain amylopectin (Jobling, 2004). Jobling et al. (2002) reported the freeze-thaw stability of this starch was excellent. A slightly less freeze-thaw stable starch was also produced when expression of the GBSS and SSIII genes were inhibited. In both of the starches, the granule abnormalities seen in the parent lines (in which the activities of the soluble starch synthases are inhibited) were abolished, suggesting that it was the amylose or its interaction with the altered amylopectin that caused disruptions of the granule matrix (Fulton et al., 2002). Large-scale field trials of waxy mutants has been conducted on maize, barley, and potato for many years (Nakamura et al., 1995; Shure et al., 1983; Visser et al., 1991), but these crops was still going through the regulatory approval process till now. The flour with waxy mutants showed a greater refrigeration and freeze-thaw stability than the nonwaxy starch and noted that stickiness and extensibility of dough with waxy mutants did not change as much during frozen storage (Abdel-Aal et al., 2002; Yi et al., 2009). Nuclear magnetic resonance studies showed that frozen dough with higher waxy flour content had lower transverse relaxation, which could be attributed to the ability of waxy flour starch to better hold water and limit the recrystallization of ice (Yi et al., 2009).

Physical Modification

The physical modification of starch is accomplished by moisture, heat, shear, or radiation and this modification has been gaining wider acceptance because of the absence of chemical reagents in the modified starch. Physical modification of starch can be safely used in food products and can be achieved through alcoholic-alkaline, micronization, and drum-drying. New methods are being evolved and one of the most common methods of physical modification of starch is pregelatinized starch which has found wide applications in food industry, including for extending the shelf-life of frozen food (Din et al., 2015).

Pregelatinized starches, also called instant starches, are precooked and dried with the help of a drum that enable the products to from a relative stable suspension in cold water compared with the native starches (Hodge and Osman, 1976). These types of products are extensively used in the textile industry, as a thickening agent in the food industry, and as an adhesive in foundry core binders (Colonna et al., 1987). Recently, pregelatinized starch was found to effectively enhance the French-type bread quality elaborated from frozen dough (Ortolan et al., 2015). This was due to its unique physiochemical properties. Pregelatinized starch has the characteristic of dispersing more easily and absorbing more water compared to its respective unmodified starch, forming a gel at room temperature, which can create a protective layer around other molecules (Demiate and Kotovicz, 2011) and microorganisms, such as yeast. Substances that retain water in their structure can contribute to decrease the content of free water. This behavior could minimize water available for ice crystallization in frozen dough, reduce the amount of water drawn from yeast during the freezing cycle, enhancing cell viability (Casey and Foy, 1995). Besides its possible yeast protective effect, the water retained in the starch would help to maintain the crumb moist, reducing its firmness. Pregelatinized starch is also ready for attack by amylases from wheat and, thus, could be a good source of fermentable sugars for the yeast, increasing proofing speed. Similarly, Guo (2013) found incorporation of pregelatinized starch also enhanced the frozen noodle quality in the aspect of hardness and elasticity.

Chemical Modification

Chemical modification is carried out by introducing a functional group into the native starch molecule that leads to dramatic changes in its physicochemical properties. Consequently, the gelatinization, proximate composition, pasting, and retrogradation properties of starch granules will also be altered. Chemical modification results in the stabilization of intra- and intermolecular bonds at different positions and locations. Factors such as starch source, reaction conditions, degree of substitution, type, and distribution of substituting agent along the molecule of starch affect the functional and chemical properties of the modified starches. Chemical modification of starches is generally accomplished through derivatization such as acetylation, cationization, acid hydrolysis, oxidation, and cross-linking. In addition, the introduction of acetyl, phosphate, and hydroxypropyl groups and cross-linking into the starch molecule had been shown to interfere with the alignment of amylose chains and the outer linear chains of amylopectin during retrogradation process (Hoover, 1995). This could contribute to enhance the freeze-thaw stability of the colloidal starch dispersion.

Acetylation is one of the common chemical method of starch modification; it is achieved by esterification of native starch with either acetic anhydride or vinyl acetate in the presence of alkaline catalyst (Ashogbon and Akintayo, 2014). Numerous studies have been reported that acetylation increased solubility, swelling power, and viscosity but decreased the gelatinization temperature of starch (Jeong et al., 1993). The results of freeze-thaw stability showed that retrogradation of normal rice starch was alleviated in the acetylation and the cross-linking process (Liu et al., 1999). Cross-linked starch is treated with a bi- or polyfunctional reagent (e.g., phosphorus oxychloride, sodium trimetaphosphate, and mixtures of adipic anhydride and acetic anhydride) so that a small number of the starch polymer chains are chemically linked at hydroxyl groups by the cross-linking reagent (Thomas and Atwell, 1999). Deetae et al. (2008) proposed a combined method of dual modification using cross-linking and phosphorylation on rice starch, provided modified rice starch with good freeze-thaw stability than the single chemical-modified starch.

Hydroxypropylated starch derivative formed by reaction of starch with propylene oxide was shown to be the most effective way to improve the shelflife, freeze-thaw stability, cold-storage stability, and clarity of starch paste (Miyazaki et al., 2006). When introduced into starch granules, the hydroxypropyl groups weakened the internal bond structure and held the granule together. This could prevent water in the starch paste from separating through syneresis when subjected to freeze-thaw cycling (Yook et al., 1993).

The quantification of liquid in the starch gel after centrifugation is an indicator of the freeze stability. Using this indicator, the hydroxypropylated potato starch (Senanayake et al., 2014), waxy wheat/barley starch gels (Reddy and Seib, 2000; Yangsheng and Seib, 1990), pea starch gels (Hoover et al., 1988), and maize and tapioca starch gels (Takahashi et al., 1989) stored under the frozen storage all showed alleviated syneresis effect during freeze-thaw cycle, suggesting the improved freeze stability. White et al. (1989) has estimated the energy required to break down recrystallized starch molecules after 10 cycles of freezingethawing using differential scanning calorimetry (DSC). Using this approach, the hydroxypropyl distarch phosphate completely inhibited the recrystallization of starches after 10 freeze-thaw cycles.

In a serial study, a fundamental rheological method was conducted to study the freeze-thaw stability of hydroxypropyl potato starch pastes. The changes in complex modulus (G*) and phase angle (δ) upon freeze-thaw cycles were found correlated with well syneresis by centrifugation and DSC measurement. The results of this rheological method gave good information on the destabilization process and could provide a simple method for detection and prediction of the extent of freeze-thaw stability of starch paste (Eliasson and Kim, 1992). Subsequently, the effects of molar substitution and cross-linking on the freeze-thaw stability of hydroxypropyl potato starch pastes were studied by following the changes in the rheological behavior and by determination of syneresis. With increasing molar substitution, the destabilization and syneresis of starch paste were delayed with regard to the number of freeze-thaw cycles. The rheological results suggested the hydroxypropyl starch pastes were gradually transformed into coarsely aggregated structures while the hydroxypropyl cross-linked one became diluted starch dispersions due to shrinkage and disruption of swollen starch granules (Kim and Eliasson, 1993). Different cooking conditions and concentrations could also affect the freeze-thaw stability of hydroxypropyl potato starch paste. With the increased pasting extent and starch concentration, the freeze stability of starch pastes were enhanced. The results of this study further showed that the amount of intermingled amylose and amylopectin in the dispersion of hydroxypropyl starch paste was the main controlling factor in the rheological response as well as in the syneresis (Kim et al., 1993).

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