Original Paper
Production of esthetically pleasing care foods using enzyme diffusion during thawing
2021 Volume 27 Issue 2 Pages 229-239
Details
2021 Volume 27 Issue 2 Pages 229-239
Freeze-thaw enzyme impregnation methods can be used to produce esthetically pleasing care food. Such methods are difficult to implement in domestic or small-scale cooking facilities because they involve a vacuuming process. While investigating the diffusion of enzymes into foods during freezing and thawing, we found that external enzymes diffused into the food during thawing and softened the product. We therefore investigated the effects of pre-heating, freezing, thawing, and enzymatic preparation methods on promoting enzyme diffusion. The results clarified the optimal processing conditions for promoting enzyme diffusion. Softening seven commercially available vegetables resulted in all vegetables having firmness values of less than 5 × 104 Pa. Unlike conventional freeze-thaw enzyme impregnation methods, enzyme impregnation by diffusion does not require a vacuum device and can be used for home cooking of care food that has retained its esthetic characteristics.
The number of patients with eating and swallowing disorders is increasing among the elderly. As a result, the preparation of foods intended for the elderly needs to consider safety and functionality, and foods need to be prepared in a form that facilitates chewing and swallowing. In Japan, the standard criteria used to assess food intended for the elderly, who often have difficulty masticating, include firmness and viscosity. These products are often minced, liquefied, or jellied and have consistencies that are markedly different from those of the original foods, which reduces their appeal. In addition to considerations such as nutrition, color, and flavor, the shape of food should also be considered as an important quality criterion (Kohyama, 2011).
A freeze-thaw impregnation (FTI) method (Sakamoto et al., 2004; Sakamoto et al., 2006) was developed to produce shape-retaining care foods, i.e., care food that has retained its esthetic characteristics. Freezing results in the expansion of cells, intracellular spaces, and loose connective tissue. FTI reduces pressure, expands the food, and introduces an exogenous enzyme solution (Shibata et al., 2010a). This method allows the firmness of the food to be modified while conserving the product's appearance. FTI retains the nutritional value and flavor of the food because it softens tissues enzymatically rather than thermally (Nakatsu et al., 2010a; Sakamoto et al., 2004). There have been numerous reports on the efficacy and applications of FTI (Nakatsu et al., 2010b; Nakatsu et al., 2014a; Nakatsu et al., 2014b) and its associated technology (Shibata et al., 2010b; Kajihara et al., 2011; Watanabe et al., 2013). The production of soft, shape-retaining care food by FTI for use in nursing homes involves a vacuum process, but this process can be difficult to implement in domestic or small-scale cooking facilities.
In this study, we evaluated an enzyme impregnation method that does not require vacuum process. A simple way to introduce a substance into food is to use diffusion, which proceeds through moisture in foods. Cooking by heating is commonly used as a method for introducing a substance by diffusion. However, heating denatures enzymes, which lose their activity as a result. In the process of investigating the diffusion of enzymes into food during freezing and thawing, we found that external enzymes diffuse into the food during thawing, thereby causing softening. There are two ways to keep the food in the enzyme solution during thawing. One method involves immersing the food in the enzyme solution before freezing, and the other involves thawing the frozen food while immersing it in an enzyme solution. If food can be impregnated with enzymes, it can be softened while maintaining its shape in a manner that is similar to FTI. To this end, it is necessary to confirm the extent of diffusion of enzymes during freezing and thawing, and to study the optimal pretreatment and enzyme treatment conditions for the food-softening method using the diffusion of enzymes. In this study, we report in detail on the development of a technique for softening food products using the diffusion of enzymes during thawing while retaining their shape.
Sample preparation Carrot (Nagasaki, Japan), lotus rhizome (Yamaguchi, Japan), burdock rhizome (Aomori, Japan), potato (Hokkaido, Japan), Japanese radish (Hiroshima, Japan), sweet potato (Kagoshima, Japan), and boiled bamboo shoot from China (CGC Japan Co. Ltd., Tokyo, Japan) were used as the samples in this study. Carrots, burdock rhizomes, and Japanese radishes were peeled and cut into circular slices perpendicular to the fibers with a thickness of 1 cm. Lotus rhizomes and bamboo shoots were cut into circular slices perpendicular to the fibers with a thickness of 1 cm. Potatoes and sweet potatoes were peeled and cut transversally into slices with a thickness of 1 cm. The foods were preheated by boiling at 100 °C or by steam convection (ACO-040G; IHO Corp., Toyokawa, Japan) at 90 °C or 100 °C in steam or using a combined method, respectively. In experiments to simulate home cooking, bamboo shoots, burdock rhizomes, carrots, lotus rhizomes, potatoes, Japanese radish, and sweet potato were sampled, and 700 mL of soup stock and 9 g of soy sauce and sugar were added. The mixture was heated in a pan for 30 min.
Enzymes The mixed enzymes used in this study were 50 mM sodium citrate buffer (pH 5.0) at 0.5% or 1.0% containing equal amounts of pectinase (Sumizyme SPG; Shin Nippon Pharmaceutical Co., Ltd., Tokyo, Japan) and hemicellulase (Amano 90; Amano Enzyme Co., Ltd., Tokyo, Japan). According to information provided by the manufacturers, the optimum temperatures for pectinase and hemicellulase are approximately 55 °C and 50 °C, respectively. Vgtoron (Christar Corporation, Hiroshima, Japan), a specific enzymatic agent for FTI, was added to the cooked foods. The pectinolytic activity of the mixed macerating enzymes was assayed as follows: one unit was defined as the amount of enzyme required to reduce the viscosity of a substrate solution by half in 10 min at 40 °C and pH 5.0. One milliliter of the enzyme preparation was added to 10 mL of 2.0% (w/v) citrus pectin (FUJIFILM Wako Pure Chemical Co. Ltd., Tokyo, Japan) dissolved in 50 mM sodium citrate buffer (pH 5.0). After the enzymes had reacted for 10 min at 40 °C, the viscosity was measured using a cone-plate viscometer (DV2T; EKO Instruments Co., Ltd., Tokyo, Japan). The results showed that the mixed macerating enzymes and Vgtoron had a pectinolytic activity of 57 000 U/g and 7 600 U/g, respectively.
Procedure of enzyme impregnation using diffusion The process of preheating, freezing, thawing, enzyme reaction, and enzyme deactivation, which uses enzyme diffusion to soften and process foods while retaining their shape, is shown in Fig. 1. To clarify the phenomenon of enzyme diffusion into food during thawing, three stages of immersion in the enzyme solution were established: before freezing, before thawing, and after thawing. In Treatment 1, the food was immersed in the enzyme solution before freezing, followed by freezing and thawing. In Treatment 2, the food was frozen, and then thawed while it was immersed in the enzyme solution. In Treatment 3, the frozen and thawed foods were immersed in the enzyme solution after freezing and thawing. The foods were removed from the enzyme solution and allowed to react enzymatically before being tested for firmness. Carrots and lotus rhizomes were used as samples, and the enzyme was a mixed agent consisting of pectinase and hemicellulase. Preheated foods were used as controls.
Flow diagram of enzyme immersion to confirm the process of enzyme diffusion.
Freezing, thawing, and enzyme reaction and inactivation For the freezing process, four types of processing, namely, air blast freezing at −7 °C and −30 °C, brine freezing at −30 °C, and home-freezer freezing at −18 °C, were used. The pass times for the maximum ice crystal generation temperature zone were about 4.5 h, 1 h 40 min, 40 min and 7 h, respectively. A blast chiller (HBC-6TA3; Hoshizaki Corp., Tokyo, Japan) was used for air-blast freezing. An aqueous propylene glycol solution was used for brine freezing. Thawing was performed in a water bath at 10–50 °C. The temperature changes when the carrots thawed are shown in Fig. 2. For enzyme processing, a short-duration reaction was performed at 50 °C (optimal temperature) for 30–60 min. A long-duration reaction was also performed at 3 °C for 14 h using home-freezer freezing (RFT-120MTCG; Hoshizaki Corp., Tokyo, Japan). Enzymes were inactivated by heating with steam (ACO-040G; IHO Corp., Tokyo, Japan) at 95 °C for 10 min.
Thawing temperature curve.
After freezing while immersed in enzyme solution, enzyme reaction is performed after thawing. Sample: Carrot. Preheating: boiling for 20 min. Freezing temperature: −18°C.
Measurement of activity of enzyme impregnated into foods Carrots that were boiled for 10 min were cut into 2 × 2 cm cubes. Enzyme impregnation using enzyme diffusion was performed. After immersion in a 0.5% mixed enzyme solution, the cubes were frozen at −18 °C for 12 h. The frozen samples were then thawed in cold water at 10 °C for 30 min. Immediately thereafter, the peripheral tissue was cut from the outside to a depth of 1 cm. The remaining part was denoted as the internal part. As a control, carrots boiled for 10 min were subjected to FTI treatment (Sakamoto et al., 2006). After freezing at −18 °C for 12 h, the sample was thawed by immersion in cold water at 10 °C containing a 0.5% mixed enzyme solution and then vacuum-treated at 5 kPa for 5 min in a vacuum chamber. After returning to ambient pressure, the sample was cut into a peripheral part and an inner part. Pectinolytic activity was then measured in each part.
Physical properties The physical properties of the samples were evaluated using a two-bite texture test with a food rheometer (RE-33005; Yamaden Co. Ltd., Tokyo, Japan). The samples were allowed to stand at 20 °C for ≥ 20 min before being tested. Sample firmness was defined as the maximum stress measured when a cylindrical plunger with a diameter of 3 mm fixed to the load cell penetrated ≤ 70% of the thickness of a sample placed on a stage moving at 10 mm/s. To minimize the variations at the measurement sites, the periphery of each sample was tested for firmness. The firmness of the softened foods tested in this study was based on the Universal Design Foods (UDF) standard of the Japan Care Food Conference (2011). The UDF standard has four care food categories (I–IV). Category II (maximum stress ≤ 5.0 × 104 Pa) includes foods that are easily mashed between toothless alveolar arches.
Statistical analysis Statistical analyses were performed with Microsoft Excel (Microsoft, Redmond, USA) by Social Survey Research Information Co. Ltd. (Tokyo, Japan). The firmness values were analyzed with Tukey's multiple comparison test and Student's t-test.
Physical properties of the foods after immersion in the enzyme solution and during freezing and thawing Figure 3 shows the firmness of the food softened by the enzymatic reaction after treatment using the three types of immersion conditions in the enzyme solution shown in Fig. 1. Carrots and lotus rhizomes showed similar trends for firmness. Treatments 1 and 2 softened the samples to less than 5.0 × 104 Pa, while Treatment 3 resulted in a firmness of only 2.0 × 105 Pa. These results suggest that the diffusion of the enzyme occurs in the thawing process, but not in the freezing process. In other words, if the enzyme liquid is present on the surface of the food at the time of thawing, the enzyme can be introduced into the food. It can be seen that there are two ways by which this can be achieved: immersing the pre-heated food in the enzyme solution before freezing, or immersing the frozen food in the enzyme solution before thawing. In the following experiments, one of these two methods was selected to study the detailed processing conditions for the preparation of shape-preserving softened foods.
Effect of timing of immersion in enzyme solution on softening of food.
Treatment 1: After freezing while immersed in enzyme solution, enzyme reaction is performed after thawing. Treatment 2: After freezing, enzyme reaction is performed after thawing while immersed in enzyme solution. Treatment 3: After freezing and thawing, immersion in the enzyme solution and enzyme reaction are performed. Control: Preheated samples. Preheating: boiling for 20 (carrot) or 30 (lotus rhizome) min. Freezing temperature: −18 °C. Thawing: 20 min in 15°C water. Enzyme reaction: 60 min at 50 °C. Mixed enzyme concentration: 1.0%. The data are presented as the mean ± S.E. (n = 5). Significance tests were performed for each food in the processing interval. Mean values with a different letter are significantly different by Tukey-Kramer's multiple comparison test (p < 0.05).
Measurement of activity of enzyme impregnated into foods The impregnation of enzymes into foods was found to occur due to the diffusion of enzymes during thawing. Therefore, the activity of the enzymes introduced by diffusion was compared with that of foods impregnated by the FTI method. Figure 4 shows the activity of pectinase introduced into foods by the enzymatic diffusion method and the FTI method, using a carrot cube as a sample. The figure shows that there was little difference between the enzymatic diffusion and FTI methods in the pectinase activity parts of the carrot, and that the effectiveness of the enzymatic diffusion method was higher in the outer part. The pectinase activity in the inner part was 6.5 U/g when the enzymatic diffusion method was used and 8.1 U/g when the FTI method was used, and the extent of enzyme diffusion was sufficient for softening. These results suggest that the enzyme impregnation method for foods using diffusion during thawing is a practical technique.
Measurement of the pectinolytic activity introduced into carrot.
After freezing, enzyme reaction is performed after thawing while immersed in enzyme solution. Sample: Carrot. Mixed enzyme concentration: 1.0%. Enzyme reaction: 60 min at 50 °C. Preheating: boiling for 20 min. Freezing temperature: −18 °C. The data are presented as the mean ± S.E. (n = 5). Mean values with a different letter are significantly different by Tukey-Kramer's multiple comparison test (p < 0.05).
Effect of preheating on enzyme diffusion into foods The effect of preheating on the firmness of foods subjected to enzyme diffusion was investigated. Figure 5 shows the relationship between the firmness of the preheated food and the firmness of the food after enzymatic treatment. The firmness of the food after softening by enzymatic treatment was strongly influenced by preheating. Preheating has the effect of damaging the tissue and softening the food. Therefore, the longer the boiling time, the greater the extent of softening after the enzyme reaction. As shown in Fig. 5, in order to soften the food to the desired firmness, a suitable combination of preheating and enzyme reaction conditions is required. When enzyme diffusion is used, an enzyme reaction temperature of 50 °C, which is the optimum temperature, or 3 °C, which is the typical temperature of a refrigerator, can be applied. The enzyme usually needs to react at the optimum temperature. However, from a hygiene perspective, it is desirable to avoid heating the food and to carry out the enzyme reaction at a low temperature. The enzyme reaction proceeds at a low temperature over time, suggesting that the enzyme may diffuse into the food during that time. The results shown in Fig. 5 indicate that preheating affects the diffusion of enzymes and the resulting softening of the foods, but a prolonged enzymatic reaction at 3 °C is suitable for stable softening of the foods. In this case, sufficient softening is expected even if the preheating time is short. On the other hand, when softening in a short time is required, it is necessary to select appropriate preheating conditions. To reduce the firmness of the food to ≤ 5 × 104 Pa, 10 min of preheating was sufficient when the enzymatic reaction was carried out at 3 °C for 14 h. On the other hand, when it was desired to soften the food in a short time of 30 to 60 min at 50 °C in the optimum temperature range of the enzyme, preheating was required for at least 20 to 30 min.
Relationship between firmness after preheating and after enzyme reaction.
After freezing, enzyme reaction is performed after thawing while immersed in enzyme solution. Sample: Carrot. The reaction temperature and reaction time of ●, ▲, and ■ were 30 min at 50 °C, 60 min at 50 °C, and 14 h at 3 °C, respectively. Mixed enzyme concentration: 0.5%. Thawing: 20 min in 15 °C water. Freezing temperature: −18 °C.
Next, the relationship between the preheating method and the softening of food was investigated for carrots. Preheating included steam convection at 90 °C and 100 °C, boiling, and a combination of steam heating and convection at 100 °C. The enzymes were diffused into the preheated carrots by immersing them in the enzyme solution before freezing. Figure 6 shows that boiling alone was the most effective preheating method. Steam heating and the combination of steam heating and convection were equally effective at softening the food. Boiling led to a higher thermal diffusivity than steam heating and a greater effect on foods (Nagao, 2007). Moreover, it substantially increases the food moisture content. Boiling is advantageous for enzyme diffusion through the water in the food by increasing the water content. However, it may be more appropriate to use steam heating to soften seasoned foods to minimize flavor loss. When choosing a preheating method, the characteristics of the food product and the desired firmness must be considered.
Effects of preheating method on the firmness of carrots.
The diffusion of the enzyme was performed by immersing the food in the enzyme solution, followed by freezing and thawing. Freezing temperature: −18 °C. Thawing: 20 min in 15 °C water. Enzyme reaction: 60 min at 50 °C. Mixed enzyme concentration: 1.0%. The data are presented as the mean ± S.E. (n = 5). Mean values with a different letter are significantly different by Tukey-Kramer's multiple comparison test (p < 0.05).
Effect of freezing and thawing rates on firmness of foods The effect of the freezing rate on enzyme diffusion into foods was evaluated using the firmness after the enzymatic reaction as an index. Rapid freezing produces small ice crystals in foods and has a minimal impact on their structure. As the overall objective of the processing is to soften the food, damage to its tissues is not a concern, since the shape of the material is maintained after enzymatic processing. Enzyme diffusion was performed by thawing the frozen foods for 20 min in an enzyme solution at 15 °C. Figure 7 shows the firmness of the food at different freezing rates and enzymatic reaction conditions. Air-blast freezing at 7 °C was also able to soften the food, as was freezing in a household freezer. Brine freezing and air-blast freezing at −30 °C inhibited enzyme diffusion and also the degree of softening of the foods. The same results were obtained when the enzymatic reaction was carried out at 3 °C for 14 h, which is sufficient for the enzymatic reaction and has a high softening efficiency. From these results, it was observed that in the enzyme diffusion method, the freezing rate of the food affected the diffusion of enzymes, and that a slower freezing rate promoted the diffusion of enzymes, which was advantageous for efficient softening of the food. The effect of loosening the food tissue induced by slow freezing greatly affected food softening by enzymatic diffusion. Compared with rapid freezing, ice crystals in both intercellular and intracellular spaces increase in size during slow freezing (Mousavi et al., 2006; Sun and Li, 2003). It is considered that the increased penetration of the enzyme solution and the water content due to loosening of the food tissue affected the softening of the food. On the other hand, freezing that is too slow can lead to a deterioration in food quality. Next, the effect of thawing speed on the diffusion of enzymes into foods was investigated. Enzyme diffusion into the food was performed by thawing the frozen food while it was immersed in the enzyme solution. The firmness of the food after the enzyme reaction is shown in Fig. 8. At thawing temperatures of 10 °C and 20 °C, the firmness of the food softened to 5 × 104 Pa. On the other hand, when thawing at 50 °C, the firmness exceeded 1 × 105 Pa and the food was not sufficiently softened. In other words, the diffusion of enzymes was enhanced by thawing slowly at a lower temperature. As the thawing temperature increased, the thawing time decreased, and the diffusion of enzymes was inhibited. During the thawing process, the ice in the food first melted near the surface and gradually thawed toward the center. Initially, the enzymes diffused into the water present on the surface of the food. Thus, it can be inferred that as thawing progresses towards the center, the enzymes gradually diffuse towards the interior of the food as the ice melts. This suggests that a certain amount of time is required for the diffusion of enzymes during thawing, and that slower thawing is more effective for the diffusion of enzymes.
Effects of freezing rate on firmness of enzyme-softened carrots.
After freezing, enzyme reaction is performed after thawing while immersed in enzyme solution. Thawing: 20 min in 15 °C water. Mixed enzyme concentration: 0.5%. The data are presented as the mean ± S.E. (n = 5). Mean values with a different letter are significantly different by Tukey-Kramer's multiple comparison test (p < 0.05).
Effects of thawing temperature on the firmness of softened carrot.
The diffusion of the enzyme was performed by freezing the food and thawing it while immersed in the enzyme solution. Freezing temperature: −18 °C. Enzyme reaction: 60 min at 50 °C. Mixed enzyme concentration: 0.5%. The data are presented as the mean ± S.E. (n = 5). Mean values with a different letter are significantly different by Tukey-Kramer's multiple comparison test (p < 0.05).
Effect of removing enzyme solution in enzymatic reaction process In the previous experiment, the enzymatic reaction was performed by removing the food from the enzyme solution after thawing, but the enzymatic reaction can also be performed while the thawed food is immersed in the enzyme solution. If the food is packaged in film, freezing, thawing, and enzymatic reactions may be performed while the food is immersed in the enzyme solution. Therefore, we compared the effect of allowing the enzymatic reaction to take place while the food was immersed in the enzyme solution and the effect of removing the food from the enzyme solution before allowing the enzymatic reaction to take place. As shown in Fig. 9, the softening effect was comparable in both cases. However, when the food was pre-heated for 10 to 20 min, there was a slight tendency for it to soften slightly when it was soaked and subjected to the enzymatic reaction. After 30 min of preheating, there was no difference between the two treatments. When the enzymatic reaction was performed while the food was immersed in the enzymatic solution, the enzymatic reaction was excessive and the surface of the food partially disintegrated, even though the firmness was comparable. Therefore, it is better to remove the food from the enzymatic solution before the enzymatic reaction is carried out to ensure superior quality and shape retention in the softened food.
Effect of enzyme solution removal on the firmness.
The diffusion of the enzyme was performed by freezing the food and then thawing it while immersed in the enzyme solution. Sample: Carrot. Freezing temperature: −18 °C. Thawing: 20 min in 10 °C water. Enzyme concentration: 0.5%. Enzyme reaction: 14 h at 3 °C. The data are presented as the mean ± S.E. (n = 5). Mean values with a different letter are significantly different by Tukey-Kramer's multiple comparison test (p < 0.05).
Relationship between thickness and firmness of food Next, the relationship between the thickness and firmness of the food after the enzymatic reaction was investigated. Carrots were cut into 0.5 to 2 cm-thick slices, preheated for 10 min, frozen at −18 °C, and thawed while soaking in the enzyme solution at 10 °C for 20 min. As shown in Fig. 10, the thickness of the foods affected the firmness after the enzymatic reaction; the thicker the foods, the harder they were. During enzyme impregnation, the enzyme concentration in the food decreased towards the interior. As a result, the larger the food, the lower the enzyme concentration in the center. Although the thickness problem could be solved by increasing the concentration of the enzyme solution, it was considered more practical to use processing conditions that allowed for softening at the standard enzyme concentration.
Relationship between thickness and firmness of carrots.
The diffusion of the enzyme was performed by immersing the food in the enzyme solution, followed by freezing and thawing. Freezing temperature: −18 °C. Thawing: 20 min in 10 °C water. Enzyme reaction: 14 h at 3 °C. Mixed enzyme concentration: 0.5%. The data are presented as the mean ± S.E. (n = 5). Mean values with a different letter are significantly different by Tukey-Kramer's multiple comparison test (p < 0.05).
Softening of seasoned foods The aforementioned results indicated that preheating, slow freezing, and slow thawing were most effective for promoting enzyme diffusion into food. However, the extent of food softening varies with enzymatic reaction conditions and enzyme concentration. In the preparation and production of food for care homes, it is necessary to optimize the conditions for each processing environment. Factors to be considered include fabrication costs, hygiene constraints, time expenditure, and home cooking. As the demand for home care increases, so does the need for home-care food preparation. Home cooking can markedly improve the quality of life of patients. In this study, home cooking was simulated by adding soy sauce, sugar, and salt to the foods, which were softened using the enzyme diffusion method. In this experiment, “Vgtoron”, used in FTI, was applied as the enzyme. As shown in Fig. 11, all of the samples used in the experiment had a firmness of < 5 × 104 Pa. According to the UDF standards, these samples could be crushed by the gums. Foods can be softened by freezing and thawing in much the same way as in home cooking. “Vgtoron” is commercially available as a softening enzyme agent for FTI and is mainly used for industrial purposes. It was found that the food softening method using enzymatic diffusion can be used for shape retention softening of cooked food with the same enzyme agent and enzyme concentration as FTI. The enzyme diffusion method evaluated in this study impregnates enzymes into foods during the thawing process. As enzymes are high-molecular-weight proteins, they do not readily diffuse. Nevertheless, shape-retaining softened foods could be prepared by immersing them in an enzyme solution either before freezing or during thawing. As enzymes diffuse through the water in the food, the rates of diffusion increase with increasing water content. To optimize diffusion and food softening, it is necessary to select the most appropriate method for each process. To this end, manufacturing, time costs, and adaptability to particular foods must be considered. The enzyme concentration and reaction time may be optimized for each individual's preheated food. When this method is applied to a mixture of cooked foods, it is impossible to adjust all foods to the same firmness as they are impregnated with enzymes under the same conditions. Thus, it may not be possible to adjust the physical properties of the cooked food to the same level. However, the proposed method for softening cooked foods is convenient and practical for preparing care food at home.
Firmness when various seasoned foods are softened with shape retention using the diffusion of enzymes. The diffusion of the enzyme was performed by immersing the food in the enzyme solution, followed by freezing and thawing. The seasoned food were prepared by adding 700 mL of broth, 9 g of sugar and salt, and putting them in a pan for 30 min. Freezing temperature: −18 °C. Thawing: 20 min in 10 °C water. Enzyme reaction: 14 h at 3 °C. Enzyme (Vgtoron) concentration: 5%. The data are presented as the mean ± S.E. (n = 5).
The mechanisms by which an enzyme diffuse during thawing are currently unknown. In studies of thawing processes (Sun, 2006), it has been reported that the recrystallization of water occurs in tissues at elevated temperatures. This phenomenon may also affect enzyme diffusion, due to ice formation. Although diffusion coefficients of salt and sugar in food have been reported as studies on the diffusion of substances into food (Akiba et al., 1967; Odake, 2000; Hashiba, et al., 2013), no studies have examined the enzyme diffusion coefficient during thawing. As Fick's first law states, the diffusion coefficient increases with enzyme concentration. Consequently, applying enzyme powder or spraying a concentrated enzyme solution on the surface of foods is more effective than immersion. As described above, immersion in an enzyme solution cannot be used for seasoned foods due to loss of the seasoning liquid. In the next study, we plan to report on the usefulness of enzyme powder application method and enzyme solution spraying on food surfaces. Furthermore, the effects of this technique on proteinaceous animal foods are unknown. It is necessary to select more effective enzymes and to test a variety of activators that promote diffusion. Previous clinical trials have tested the relative efficacy of freeze-thaw impregnation for effectively softening and preserving the nutrients in enzyme-treated foods that have been prepared for people that have difficulty with chewing and/or swallowing (Nakatsu et al., 2017).
In the present study, the methods for enzyme diffusion during the pre-heating, freezing, thawing, enzyme reaction, and enzyme deactivation processes were investigated. The longer the boiling time, the greater the quantity of enzyme-treated food that could be softened. The food was softened more effectively by performing the enzymatic reaction at a low temperature (3 °C) for a longer time than by performing it at the optimum temperature (50 °C) for a short time. Boiling was more effective than steaming. Slower freezing and thawing speeds were more effective for softening foods by enzyme diffusion. It was considered that the removal of the enzyme solution during the enzyme reaction was effective for suppressing excessive enzyme reaction and for maintaining the esthetic characteristics of the foods. Home cooking was also simulated by adding soy sauce, sugar, and salt, and the foods were softened using the enzyme diffusion method. The findings showed that enzyme impregnation by diffusion during thawing does not require a vacuum device and can be used for home cooking of care food while retaining the esthetics of the food.
Acknowledgements This study was supported by a Grant-in-Aid from JSPS KAKENHI (Grant number JP16K07759).
