Associate Professor Tang Yong, Xihua University, et al.: The influence of different thawing methods on the thawing quality of raw fish fillets

Thawing is an important part of the processing of raw fish fillets, the essence of which is to make the frozen water in the raw fish fillets back into a liquid state, restore their original state and characteristics, and make their quality as close as possible to fresh, unfrozen raw fish fillets
.
After the raw fish fillets are thawed, the ice crystals inside melt into water, and if they cannot be absorbed by the tissue, part of the water will form a loss fluid, which is not only water, but also water-soluble components, such as proteins, salts, vitamins, etc
.
In addition, improper thawing process can lead to a series of quality deteriorations in raw fish fillets, which will eventually affect its commercial value and eating experience
.

At present, room temperature air thawing (RTAT), cold room thawing (RT) and water bath thawing (WBT) are commonly used thawing methods
in food processing.
In recent years, microwave thawing (MT) has gradually entered people's field of vision and gained a lot of attention
.
Helen Wan and Yong Tang* from the School of Food and Bioengineering of Xihua University took rainbow trout as the research object, and the effects of four different thawing methods of RTAT, RT, WBT and MT on their senses, thawing loss, color, total number of colonies, volatile salt-based nitrogen (TVB-N) content, microstructure structure, texture characteristics, water migration and volatile odor characteristics of fish-eating fillets during processing, aiming to provide basic data
for the selection of thawing methods of raw aquatic products.

1.
Sensory evaluation of raw fish fillets under different thawing methods

It can be seen from Table 3 that the effects of different thawing methods on the sensory quality of raw fish fillets are significantly different (P<0.
05), among which RT raw fish fillets have the highest sensory score of 24.
85 points, followed by WBT and RTAT, MT raw fish fillets have the lowest sensory score, because the uneven microwave heating has caused the fillets to partially ripen, which is seriously affected<b10> in terms of appearance, smell and texture.
This shows that RT raw fish fillets are of better
quality.

2.
Thawing rate and thawing loss rate of raw fish fillets under different thawing methods

As shown in Table 4, the thawing rate of different thawing methods of raw fish fillets differed significantly, the thawing rate of raw fish fillets using RT was the slowest, while the thawing rate of MT was the fastest, 86.
667 °C/min, which was due to the fact that the polar groups in the frozen meat violently oscillated and rubbed under the action of the alternating electric field, thereby converting microwave energy into heat energy, which can have strong penetration ability
at low temperatures.
Compared to RTAT, WBT has a faster thawing rate because water has a larger specific heat capacity than air and transfers heat faster than air, so it can significantly shorten the thawing time
.

Fig.
1 characterizes the thawing loss rate of raw fish fillets under different thawing methods, the thawing loss rate of WBT raw fish fillets is 6.
05%, the thawing loss rate of RTAT and RT raw fish fillets is not significantly different (P>0.
05), while the thawing loss rate of MT raw fish fillets is the largest, 13.
31%.

。 The reason for this result is mainly that microwaves will vibrate polar water molecules, and polar water molecules are unevenly distributed in the fish flesh, which makes the degree of heat absorption of different parts of the fish fillets different, resulting in uneven MT thawing, and even makes some protein structures seriously damaged, and the melted water molecules cannot be combined with it, thus showing a large thawing loss rate
.

Based on the thawing rate and thawing loss rate, the use of WBT helps to reduce the loss
of water during the thawing process of raw fish fillets.

3.
The whiteness value of raw fish fillets under different thawing methods

As can be seen from Figure 2, the whiteness of raw fish fillets after thawing in different ways varies significantly (P<0.
05), and its values are MT group> WBT group> RTAT group > RT group
.
This is because the thawing loss rate of raw fish fillets after MT is the largest, and the water loss is the most during thawing, which enhances the light reflection on the surface of the fillet, resulting in an increase
in whiteness.
Compared with RTAT and RT, WBT has a faster thawing rate, which reduces the contact time between the fillets and the air, so the degree of oxidation of the fat of the fillets is lower, resulting in higher
whiteness.
The above shows that MT and WBT can better alleviate the whiteness loss
after the thawing of fish fillets.

4.
TVB-N content of raw fish fillets under different thawing methods

It can be seen from Figure 3 that different thawing methods have a significant effect on the TVB-N content of raw fish fillets (P<0.
05), and the contents are RTAT group> RT group> WBT group > MT group, but no matter how it is thawed, the TVB-N content of raw fish fillets is less than 15.
00 mg/100 g, and the freshness is high
。 Among them, the content of TVB-N in raw fish fillets after MT was the lowest (10.
74 mg/100 g), probably because its thawing time was the shortest, microorganisms and endogenous enzymes such as (calpain, histamine decarboxylase) decomposed less of their proteins, and produced less
ammonia and alkaline nitrogenous substances such as amines 。 The TVB-N content of WBT sashimi is 11.
76 mg/100 g, which is only higher than that of MT sashimi (1.
02 mg/100 g higher), RTAT has a faster thawing rate than RT, but the former is in a higher thawing environment, which is more conducive to the action of enzymes and the activity of microorganisms, and has a higher degree of protein decomposition of raw fish fillets and produces more alkaline nitrogenous substances such as ammonia and amines, so it is manifested as higher TVB-N content
。 It can be seen that the use of MT and WBT can better delay the production of alkaline nitrogenous substances such as ammonia and amines in raw fish fillets and better maintain their freshness
.

5.
Texture characteristics of raw fish fillets under different thawing methods

It can be seen from Figure 4 that different thawing methods have little effect on the elasticity, cohesion and resilience of raw fish fillets, but have a greater
effect on hardness, viscosity, adhesion and chewiness.
Among them, due to uneven microwave heating, the protein structure of MT raw fish fillets is seriously damaged or even partially matured, and the large thawing loss rate leads to more juice loss, so the raw fish fillets passing through MT show greater hardness and lower viscosity
.

Fig.
5 shows that the shear forces of raw fish fillets after thawing by different thawing methods are significantly different (P<0.
05), but the effect of RTAT and WBT on the shear forces of raw fish fillets is not significantly different (P>0.
05).

Among them, RT has less damage to the muscle fiber structure of raw fish fillets than other thawing methods, so the shear force of raw fish fillets after RT is the smallest, which is 14.
81 N, thus showing high tenderness
.
In the process of MT, the water loss of raw fish fillets is the most, and the contraction of muscle fibers increases the density of fibers, thereby increasing the force required to cut the fibers, and its shear force is 17.
47 N, which is the largest among the 4 thawing methods and has the lowest
tenderness.
The above shows that raw fish fillets after RT have better texture characteristics
than other thawing methods.

6.
Water distribution of raw fish fillets under different thawing methods

It can be seen from Figure 6 that there are three water groups in the four thawing methods of raw fish fillet, namely bound water, non-flowing water and free water, of which non-flowing water accounts for most of the total water and is the main water group
in raw fish fillet.
Consistent with the results of Li Dongmei et al.
on fresh fish, the content of non-flowing water in fillets is higher than that of free water and bound water, located within myofibrils, and is its main water group
.
In addition, the peak relaxation time of the sample shifted after thawing by different methods, indicating that the thawing method affected the degree of freedom of water in the muscles of
raw fish fillets.

Fig.
7 shows the change of the peak area ratio of each water group in the raw fish fillets under different thawing methods, in which the relative peak area of the bound water ranges from 4.
57%~5.
20%, and the fluctuation is minimal, which indicates that the relative content of the bound water is very little affected by the thawing method, which may be attributed to the fact that the bound water is a water group closely bound to muscle protein, and is not affected by
any mechanical stress, freezing and heating or microstructure changes 。 In the comparison of three water groups, it can be seen that the relative content of non-flowing water reached more than 90% in all thawing methods, and the relative content of free water in WBT raw fish fillets was the lowest, which indicated that WBT was more conducive to the water retention of raw fish fillets, which was consistent
with the results of the thawing loss rate Xin Tan Mingtang et al.

7.
The total number of colonies of raw fish fillets under different thawing methods

As can be seen from Figure 8, different thawing methods significantly affected the total number of colonies of raw fish fillet (P<0.
05), from largest to smallest: RTAT group (3.
45(lg(CFU/g))> RT group (3.
40(lg(CFU/g))> WBT group (3.
34(lg(CFU/g))> MT group (2.
88(lg(CFU/g))), all of which did not exceed the limit of 6.
00 (lg(CFU/g)
).
。 It can be concluded that MT can better inhibit the growth and reproduction of microorganisms in raw fish fillet, and the total number of colonies of raw fish fillets under RTA and RT is comparable, which may be due to the high thawing environment temperature of MT, RTAT and RT raw fish fillets in contact with air for a long time and high humidity, both of which provide favorable conditions
for the growth and reproduction of microorganisms.

8.
Electronic nose analysis of raw fish fillets under different thawing methods

It can be seen from Figure 9 that the electronic nose responds to the raw fish fillets under four different thawing methods and the change trend is consistent, among which the W5S and W1W sensors have the strongest response to the sample, followed by W1S and W2S, and the other sensors have low response to the sample and the difference between different thawing methods is not obvious.
However, since W1C, W3C and W5C are reverse sensors, the response value of these three sensors is inversely proportional to the concentration of volatile substances in the top air of the sample, and its response value is close to 1, which indicates that the response of these three sensors to the sample is not strong, and nitrogen oxides, sulfides, methane, ethanol and other aromatic compounds play a major role
in the characteristic odor of raw fish fillets after thawing 。 The production of nitrogen oxides and sulfides is mainly the result of freshness changes caused by protein decomposition and fat oxidation, indicating that different thawing methods will have different degrees of impact on the freshness of raw fish fillets, among which the WBT method has the lowest response in W5S and W1W sensors, which indicates that WBT can effectively alleviate the decline
of freshness of raw fish fillets during the thawing process.
The stronger response of W5S than W1W indicates that the change of nitrogen oxides in raw fish fillets after thawing is more obvious, which is consistent
with the results of TVB-N content.
Moreover, with the change of thawing method, the radar chart outline of raw fish fillets showed differences, indicating that different thawing methods had an impact
on the characteristic odor of raw fish fillets to a certain extent.

9.
Microstructure analysis of raw fish fillets under different thawing methods