The main factors that contribute to the loss of quality in frozen food, including water crystallization, deterioration of food components such as water migration, moisture loss, recrystallization of ice, and drip loss during thawing, protein denaturation, and starch deterioration.
Main Characters in the Quality Loss of Frozen Food
Freezing can preserve the taste, texture, and nutritional value of foods better than most other preservation methods. However, freeze damage results in loss of quality in the thawed product. Loss of quality may be embodied in aspects like freezer burn, discoloration, and mechanical damage in the frozen product. In many cases, the loss of quality is not noticeable until after thawing and cooking, e.g., frozen dough-based product.
The poor loaf volume and strong alteration in the textural properties of the baked products from frozen dough mainly attributed to the decreased yeast viability and disruption of gluten network caused by freezing and frozen storage (Wang et al., 2015a). On the other hand, some frozen foods like ice cream and related frozen desserts which are consumed while still frozen, the deteriorated quality like formation of big ice crystals can be distinguished from the appearance and mouth feel. Most of the mechanisms of quality loss are determined by storage temperature and accelerated with time spent above the recommended value. The main factors resulting in the quality loss are due to the water crystallization and food components deterioration upon freezing and further frozen storage.
Water crystallization is the main character during the freezing process. Crystallization is a general term used to describe several different phenomena related to the formation of a crystalline lattice structure (Hartel, 2001). This process involves two main successive stages: nucleation and crystal growth. The ice content, size, shape, and distribution are determined during these two stages.
The main driving force for the nucleation is supercooling achieved in the system. Usually a large supercooling driving force is required for the formation of the nuclei. After the nucleation, the next step of crystallization process is crystal growth. The initial few crystals that appear at the beginning of crystallization provide a structural template upon which all the material is deposited in the form of the crystals (Kiani and Sun, 2011; Mersmann, 2001). However, nucleation and crystal growth can occur simultaneously as well. In addition, the initially formed nuclei may also be numerous, restricting the growth of each crystal. Therefore, the interaction between the growth and nucleation steps defines the crystal size distribution (Mersmann, 2001).
The process of formation and growth of the crystals are complicated and can employ different thermodynamic, mass transfer, and heat transfer principles to explain the crystallization process. For the frozen food quality, formation of large ice crystals results in significant damages to the food structure. Formation of evenly distributed fine crystals leads to the better preserved food quality due to less damage (Kiani and Sun, 2011). Therefore, controlling the ice morphology is always a field of active research for the research community to enhance the frozen food quality. The molecular variation process during crystallization could explain the empirical observation that faster freezing rates give rise to smaller and more numerous crystals. It is also the explanation for the industrial practice of trying to maximize freezing rate, and hence product quality, by maximizing the degree of supercooling and employing low temperatures, exposed surface areas of food, and convective heat transfer coefficients through high air velocity (Sahagian and Goff, 1996).
Deterioration of Food Components in Frozen Food
Food is a multi-component system, which is mainly consisted of water, protein, carbohydrate, fats, vitamins, and minerals. These components are affected at different degree during frozen storage, contributing to the frozen food quality loss according to their specific importance in determining the food quality. The deterioration of the main food components is discussed below:
Water migration is the principal physical change occurring in frozen foods as a direct consequence of ice formation during freezing and frozen storage. This is usually regarded as the consequence of the degradation in other food components. The water migration can be generally divided into several forms: moisture loss by sublimation, recrystallization of ice, and drip loss during thawing (Erickson and Hung, 1997).
Moisture loss by sublimation
Moisture loss is one of the most important aspects of the quality loss, which results in a huge economic loss of the frozen food industry. The moisture loss can bring the freezer burn effects, which appears as grayish-brown leathery spots on frozen food, and occurs when air reaches the food surface and dries the product. Meanwhile, the moisture loss results in the reduction of the weight loss of food and can greatly affect the profitability of the company. Also for the frozen wrapped food like frozen dough, it exerts a partial water vapor pressure in the air boundary layer associated with the surface upon the frozen storage. The transfer of water depends on the water activity of the dough and the saturated vapor pressure of water at the dough surface temperature. The water transfer reaches a balance when the air boundary layer associated with the package surface exerted a partial pressure of water vapor equal to the saturated vapor pressure at the package temperature. The changes in saturated vapor pressure further results in a difference in partial pressures so that the water vapor will transfer from the dough to the package and vice versa (Wang et al., 2015b). Therefore, the transfer of water from frozen dough to package occurs, forming frost inside the package bags. This could further lead to the loss of moisture in bread crumb. Therefore, the firmness of frozen dough bread becomes harder and degrades the bread quality. Water content in the dough can be one of the determinants to sensory quality by affecting the bread crumb grain structure (Mastromatteo et al., 2013).
Recrystallization of ice
Besides the moisture loss, the tendency of the ice crystals to minimize their surface to volume ratio promotes the formation of large ice crystals during frozen storage, also known as recrystallization, will cause further damage to the food quality. The impact of recrystallization on the food quality can be divided into direct and indirect form. For example, recrystallization can have the direct effect on appearance and texture of ice cream, frozen dessert, and delicate plant tissues such as broccoli and strawberry (BahramParvar and Goff, 2013; Damodaran, 2007; Xin et al., 2014). Meanwhile, recrystallization can disrupt the network-based food such as dough and starch paste, which also leads to the unfavored food quality (Charoenrein and Preechathammawong, 2010; Wang et al., 2016). Recrystallization can also cause detrimental effects on the food components such as the depolymerization of protein macro polymers and enhanced starch damage levels in the dough, which would result in the deterioration of bread quality (Tao et al., 2016a; Wang et al., 2014).
Drip loss during thawing
Drip loss during thawing is induced by irreversible tissue damage during the freezing, storage, and thawing processes. It is usually relatively important for the frozen food such as meat, vegetables, and fruits (Fuster et al., 1994; Kong et al., 2016; Ngapo et al., 1999). The drip loss reduces visual attraction and causes water-soluble nutrient loss.
Freeze-induced denaturation of protein sometimes is the limiting factor of the frozen food shelf-life. For example, gluten protein deterioration is a well known phenomenon and major problem in the quality loss of frozen dough. The detailed deterioration mechanism can be concluded as: Depolymerization of glutenin macropolymers is conducted through breakage of interchain disulfide bonds, leading to reduced viscoelasticity. Gliadin could further promote this depolymerization behavior. Frozen storage also induces the conformational changes in glutenin and gliadin, resulting in the more disordered spatial aggregation and exposure of hydrophobic moiety. This further weakens water-holding capability and promotes ice recrystallization, which is contributive to the disruption of glutenin network and diminished viscoelasticity. Meanwhile, frozen storage induces the degradation in the foaming properties of gliadin, which are mainly attributed to the reduction in the flexibility of molecular chain, surface hydrophobicity, and absorption ability at the air-water interfaces of γ-gliadin (Wang et al., 2015a). Meanwhile, for some marine fish, formaldehyde derived from trimethylamine oxide in frozen tissue has been shown to modify myofibrillar proteins and collagen by crosslinking adjacent polypeptides, thereby forming insoluble aggregates. Recent studies have also demonstrated that oxidative reactions, such as lipid peroxidation, are involved in denaturation and deterioration in functional attributes of muscle proteins during frozen storage (Erickson and Hung, 1997; Suvanich et al., 2000).
Starch is quantitatively the major component of most staple food, whereas freezing can also modify its property. The most significant notorious consequence for the starch-based frozen food is the textural changes related to amylose and amylopectin retrogradation and may make such products unacceptable to consumers (Varavinit et al., 2000).
For the frozen precooked food like rich starch paste and bread, the native starch is gelatinized before subjecting to the freezing treatment. Freezing results in the disrupted microstructure of starch network, a high degree of syneresis in gelatinized starches, and enhances the starch retrogradation, thereby inducing the harder texture of frozen food products (Charoenrein et al., 2011; Meng et al., 2014).
In the frozen uncooked food such as frozen dough, the ungelatinized starch is also deteriorated from several aspects. Starch constitutes the largest volume fraction of solids in dough, comprising about 75% (Colonna et al., 1990). The freezing treatments have been shown to alter the structural and functional properties of wheat starch. Pressure is developed due to phase transformation from water to ice crystals upon freezing. This freezing pressure induces irreversible changes in amylose and amylopectin (Ribotta et al., 2003). Wolt and D’appolonia (1984) suggested the amylose-amylopectin ratios were also negatively correlated with the frozen storage time, whereas highly significant positive correlations were found between amylose-amylopectin ratio, proof time, and loaf volume. Autio and Sinda (1992) observed that the onset temperature of starch gelatinization was increased in frozen dough. They attributed this increment to either a delay in the diffusion of water into the starch granules or to increased growth of ice crystals in the frozen doughs. Lu and Grant (1999) compared the thermal properties of wheat starches isolated from original wheat flour and frozen dough after subjecting to 16 weeks of frozen storage. The onset and peak temperature of starch gelatinization and melting enthalpy of the frozen dough starch increased with frozen storage time. The increase in melting enthalpy of the starches after frozen storage suggested that the retrogradation might take place within the starch granules during frozen storage.
Meziani et al. (2011) have developed a new technique for studying the changes occurring during frozen dough storage, which is based on the absorption bands of the crystalline and amorphous zones of starch (Van Soest et al., 1995). To this end, sweet dough were frozen at different freezing rates, samples were analyzed by fourier transform infrared spectrography. By applying this methodology, a rapid retrogradation was found in frozen dough by decreasing the amorphous material with an increase in crystallinity.
To elucidate the comprehensive deterioration mechanism of starch in frozen dough, a series of studies were conducted. Tao et al. (2016a) applied the fractionation and reconstitution methodology to study the role of starch in the frozen dough. The starch fraction in the reconstituted dough was substituted with freeze/thaw-treated starch. A lower specific volume and firmer crumb were observed in the bread made from freezing-treated starch (Tao et al., 2016c). The breads containing freeze/thaw-treated starch exhibited higher retrogradation enthalpy and recrystallinity during the storage period. From the findings presented here, it was postulated that wheat starch accelerated the bread staling by altering the integrity of starch granules and leaching starch-associated materials after multiple freeze/thaw treatments. During freezing, water inside the starch granule expands channels in the granule envelope and causes leaching of the materials (Szymonska et al., 2000; Tao et al., 2016b). In this manner, a coarse surface and broad granular channel allowed for the intragranular release of amylose, which promoted chain interactions to form a network of amylose, leading to an increase in the firmness of bread crumb (Ba´rcenas et al., 2003; Tao et al., 2016a). In addition, wheat starch was fractionated into large (A-type granules) and small (B-type granules) groups. Their structural and functional properties subjected to freezing treatment were investigated and concluded as: (1) Freezing resulted in dissociation of amylose-lipids and proteins leaching for B-granules; (2) Bread made with frozen B-granules had a smaller specific volume and firmer crumb texture; (3) B-type granules were more sensitive to the freeze/thaw treatment than A-granules and noted that the greater specific surface area and more rapid water uptake of B-type granules might offer some explanation for the relative granule physicochemical properties. It seemed that the deterioration in frozen bread quality derived from starch could be minimized by increasing the A-granules content (Tao et al., 2016b).