Preface

Although quick-freezing technology only start and promoted from the 20th century, the method has been used for thousands of years to extend the shelf life of meat products by freezing. The meat trade in the world is mostly carried out in the form of frozen meat, but the freezing and thawing of meat have an important impact on the quality of meat.

Freezing and thawing change the quality of meat mainly by affectting the moisture content of meat. Water is abundant in and between muscle fibers. When water freezes, the concentration of solute (protein, carbohydrate, lipid, vitamin and mineral) increases, and then the balance of biochemical system in meat is broken. Changes in the environment of muscle fibers will affect the performance of cell membranes and further affect meat quality.

Meat freeze-thaw research mainly focuses on its water holding capacity. Researchers systematically studied the improvement of water holding capacity of meat in the 1970s and 1990s.

The shelf life of meat mainly depends on its appearance, texture, flavor, color, total number of colonies and nutritional value. Among these indicators, the most difficult to measure is the flavor indicators. Flavor compounds mainly come from lipids and polypeptides. All these measurements change during the freeze-thaw process of meat.

Effects of freezing and thawing on meat quality

1.1 Water-holding capacity

Freezing and thawing can change the content and distribution of water in meat. Water holding capacity, as an important index of meat products, can be measured by many indicators, such as thawing water loss rate, cooking yield rate, etc. Nevertheless, up to now, there is no international standard to determine its water holding capacity.

One of the main reasons for the decrease of water holding capacity of meat products is the rigidity of meat after slaughter, pH value decreases, protein deviate from isoelectric point, and the decrease of water holding capacity of protein. Another important reason is that in the process of meat freezing, due to the formation of ice crystals, some of its muscle fibers are mechanically ruptured, juice seeps out, and the water after thawing can not be absorbed by all its tissues, resulting in the loss of some juice.

It was found that the faster the freezing speed, the better the water holding capacity of frozen meat. Compared with the slow freezing method, the quick freezing method has better water holding capacity. In addition, the influence of different thawing methods on their water holding capacity was also very obvious. It was found that the water holding capacity of pig split meat water defrosting method was higher than that of natural thawing.

1.2 Protein Denaturation

Meat protein contains water-soluble sarcoplasmic protein, salt-soluble myofibrillar protein and insoluble matrix protein. Some researchers believe that protein denaturation is caused by the increase of intracellular ionic strength during meat freezing. In addition, some researchers believe that the separation of bound water in meat during freezing leads to protein denaturation. At present, the nature of freezing denaturation of proteins has not been determined.

During the repeated freezing and thawing process, the spatial structure of myofibrillar protein will be affected, which will change in varying degrees, resulting in the decrease of solubility and functional properties of protein. In addition, it was found that the solubility of proteins decreased continuously during the refrigeration process, and the higher the refrigeration temperature, the faster the decline rate.

1.3 Fat oxidation

Biochemical reactions in meat can occur at temperatures above – 20 ℃, because part of the moisture in meat is not completely frozen. It was found that the freezing temperature of meat was below – 40 C, the water content in meat was basically in a complete freezing state, and the biochemical reaction stopped basically. However, due to economic considerations, the frozen storage temperature of meat is often less than – 40 ℃, generally around – 18℃. In the process of meat freezing, the concentration of solute and the speed of biochemical reaction increased. Unfrozen water in frozen meat is the main factor leading to its oxidation. During the freezing process, the cells were mechanically damaged, which increased the contact area with oxygen and eventually led to lipid oxidation. At the same time, during the freeze-thaw process, some antioxidant enzymes in meat cells changed and their activities decreased, which further accelerated the oxidative rancidity of lipids.

Thiobarbituric acid method is the main index for determining oxidative rancidity of fat in meat. It was found that the peroxide value of rapidly frozen meat was low, and increased with the prolongation of freezing time.

1.4 Color Change

Meat color is an important index to measure the sensory quality of meat, which directly affects consumers’desire to buy. During the process of freezing, thawing and freezing, myoglobin of meat will produce partial oxidative denaturation, which will lead to the change of meat color.

It was found that the redness, whiteness and yellowness of meat decreased with the increase of freeze-thaw times. In the process of freezing and thawing, the pH value of meat decreases, the acidic substances increase, and the amines and free radicals formed by oxidation in protein harden, resulting in the production of yellow pigments. In addition, during freezing and thawing, myoglobin is easier to oxidize and its color is darker.

1.5 pH value reduction

After freezing and thawing, the pH value of meat was significantly lower than that before freezing. During the process of freezing and thawing, some protein solutions denatured, the concentration of hydrogen ion increased, and the pH value decreased. In the process of freezing and thawing, the loss of juice also increases the concentration of hydrogen ion, and also leads to the increase of acidity and the decrease of pH value. In addition, some studies have found that protein deaminates under the action of microorganisms and enzymes, releasing hydrogen ions, which further increases the acidity of meat.

1.6 Texture Change

The texture of meat affects the edible quality of meat, which is an important index of the edible quality of meat. It was found that the palatability of frozen meat was worse than that of fresh meat, and its tenderness was lower. Different freezing methods have different effects on it. The quality of quick-frozen meat is better than slow-frozen meat. In the process of frozen storage, the palatability of meat products will also decrease, and the acceptability of consumers will be reduced.

Technology for Improving the Quality of Frozen Meat

During the process of freeze-thaw and freeze-storage, the quality and commercial value of meat will decrease in varying degrees, which will affect the consumption in terminal market. With the development of society and technology, many new technologies and means have emerged in the field of food freezing to improve the quality of frozen meat.

1.1 Accelerated freezing and thawing rates

The faster the freezing speed is, the smaller the ice crystal particles will be and the smaller the mechanical damage to meat products will be. Ultra-low temperature quick freezing and high pressure freezing are two commonly used methods at present. High-pressure freezing is to freeze meat under high pressure. Under high pressure, water has low freezing point and ice crystals have not yet formed. Then rapidly reducing pressure will form uniform and fine ice crystals. The volume of ice crystals will not increase rapidly, and the mechanical damage to meat fibers will be small. Ultra-low temperature and quick freezing technology can also shorten the residence time of frozen meat in the maximum ice crystal formation zone, and keep the quality of meat unchanged to a greater extent.

The quality of frozen meat by flow hydrolysis is obviously higher than that by traditional natural thawing. The water holding capacity and color of frozen meat by flow hydrolysis are better, and the total number of colonies is relatively low.

1.2 Adding food additives such as antifreeze protein

Antifreeze protein is a kind of food additive which can inhibit ice crystal formation. It can be extracted from bacteria, fungal plant protein, fish and insects [10,11]. Adding antifreeze proteins to meat can inhibit the formation of ice crystals, reduce the mechanical damage to muscle fibers and reduce juice loss. On the other hand, adding antifreeze proteins can reduce the formation and recrystallization of ice crystals in frozen meat during storage, thereby improving meat quality. But at present, the price of antifreeze protein is high, and its application is greatly limited.

1.3 Saline Injection

In poultry processing, before freezing, poultry meat is often injected with salt and phosphate to improve its juiciness, water holding capacity, tenderness and flavor. It was found that the water holding capacity and palatability of beef could be improved by injecting salt and phosphate before freezing. This method of saline injection has low cost and can improve the quality of frozen meat. It is more and more popular among meat processing enterprises worldwide.

Summary

In summary, during freezing, storage and thawing, frozen meat will undergo many changes, such as fat oxidation, protein denaturation and the increase of the total number of bacteria. The processing performance and commercial value of frozen meat will decline. With the deepening of the research on the mechanism of meat freezing, some new technologies and equipment to improve the quality of frozen meat are emerging. But at present, most of these technologies are not mature, and most of them are not promoted commercially. In the future, we should do further research to provide technical support for the popularization and application of new technologies in industrialization.

Frozen meat packaging

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