Tuna is the general name of several genera with breastplates in Osteichthyes, Pereiformes and Scombridae. It is a highly migratory fish living in the upper and middle water area of the ocean, mainly distributed in the tropical, subtropical and temperate waters of the Pacific, Atlantic, Indian Ocean. There are 5 genera and 17 species of tuna. It mainly includes long fin tuna, Atlantic tuna, yellowfin tuna, bigeye tuna, bluefin tuna, bonito and so on.
Tuna meat is tender, delicious, with high protein content and high bio-price as high as 90; rich in polyunsaturated fatty acids such as docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and other biologically active fatty acids; at the same time, rich in methionine, taurine, minerals and vitamins, it is a green, non-polluting and healthy food recommended by the International Nutrition Association. In recent years, the consumption of tuna in the world is more than 3 million tons per year, and that in China is about 200,000 tons, which is increasing year by year. Japan, Europe and America are the main consumption markets of tuna. Oceanic deep-sea fish, represented by tuna, are becoming more and more popular in the worldwide market. Based on the research status of tuna processing and utilization by world’s researchers, we hereby summarizes the nutritional value of tuna meat, meat quality characteristics, freezing processing, ultra-high pressure processing, canning and processing of fish pine, and the research progress in the separation and extraction of polyunsaturated fatty acids, fish oil, oligopeptides and other natural components from processing by-products, and the processing and utilization of tuna. The purpose of the article is to discuss tuna processing technology in order to provide reference for the study of tuna processing and utilization technology in the future.
1. Nutritional value and meat quality of tuna
Because tuna lives in the upper and middle ocean waters, it often keeps fast swimming and its meat is tender and delicious, so it is a healthy food for modern people. More and more scholars pay attention to its nutritional value, flavor components and meat quality characteristics.Researches found the nutrient contents of yellow fin tuna dorsal muscle composition was 72.35% moisture, 25.53% protein, 1.07% crude fat and 0.94% ash. Amino acid analysis showed that tuna dorsal muscle was rich in lysine (130%) and valine amino acid was the first limiting amino acid. Although the fat content in tuna back muscle is low, the long chain_-3 polyunsaturated fatty acid content is rich, DHA 25.34%, EPA 6.32%. Researchers determined the nutrient composition of tuna bone. The results showed that the contents of protein, ash and fat in tuna bone were 57.0%, 33.1% and 8.0%, respectively. The unsaturated fatty acid and saturated fatty acid in fish bone oil accounted for 39.6% and 56.5%, respectively. DHA and EPA were 15.9% and 1.8%, respectively. 17 mineral elements in fish bone were determined. Calcium and phosphorus were 5.3% and 2.9%, respectively. In order to investigate the flavor components of tuna meat, researchers sampled by headspace solid-phase microextraction and analyzed by GC-MS, identified 36 and 47 volatile components of fresh and heat-treated large-eyed tuna meat, respectively. Aldehydes and alcohols in fresh samples played a major role in the flavor of tuna meat, aldehydes and furans in heat-treated samples played a major role. In order to maintain the high quality of tuna meat for a long time, the storage temperature of tuna meat should be below – 50 C. However, in the process of processing, storage, transportation and marketing, the characteristics of tuna meat often change badly because of the influence of environmental conditions. In order to investigate the effect of repeated freezing-thawing conditions on the quality of tuna meat, researched measured the hardness and other texture characteristics of yellowfin tuna abdomen. It was found that with the increase of freezing-thawing times, the hardness, elasticity and chewiness of tuna meat stored at – 60 C and – 18 C decreased significantly, which indicated that repeated freezing-thawing could induce the hardness, elasticity and chewiness of tuna meat. The quality of tuna meat decreased. For details of how freeezing-thawing affect meat quality, please refer to our article ‘Effects of freezing and thawing on meat quality’
2. Tuna Processing Technology
2.1 Frozen Processing
Fish are vulnerable to spoilage during storage and transportation after they leave the water environment on which they live. In order to prevent the quality of fresh fish from declining rapidly after capture and prolong the shelf life of fish aquatic products, freezing processing technology is one of the main methods used in aquatic products processing at home and abroad. About half of the world’s total tuna production can be used as sashimi. After fresh fish are caught in the ocean, if fresh fish are transported to other places, the freshness of live fish will decrease even if they are transported by air or highway. In order to eat tuna as sashimi, frozen processing is considered the best method. In tuna frozen storage, due to the oxidation of hemoglobin and myoglobin, the red browning of the original fish meat makes people feel stale. Therefore, there are many studies on the color change of tuna frozen products.
Researchers took the back muscle of yellowfin tuna as the research object, and measured the effects of different frozen storage temperatures on the muscle color (a* value), the content of ferrimyoglobin and fat oxidation. The results showed that a* value of muscle changed significantly at different freezing temperatures (P < 0.05). The lower the freezing temperature, the smaller the changes of a* value and lipid oxidation value, and the less the amount of myoglobin oxidized to methemoglobin. Therefore, in order to keep Tuna meat color from browning, the lower the refrigeration temperature, the longer the storage time. Researchers studied the preservation effect of tea polyphenols on tuna meat under freezing storage at – 18 C. The results showed that the pH and TVBN values of tuna meat treated with tea polyphenol solution were significantly lower than those of the control group, while the a* values were significantly higher than those of the control group, and the sensory values were better than those of the untreated control group. When tuna muscle was treated with 6g/L tea polyphenol preservative solution, the freshness index of the first grade could still be reached on the 30th day, and the shelf life of the second grade freshness was prolonged at least 15 days compared with the compared group.
2.2 Fish Floss Processing
Fish floss is a kind of nutritious food, which is made from fish meat by cooking, seasoning and stir-frying. It looks like fluff, loose and delicious. The chewing is more delicious, and it is a nutritious delicacy. Because of the high fat content in tuna meat, fish products have a fishy smell. Extraction method is used to remove the fat from the fish, and the degreasing rate is as high as 95%. The optimum seasoning formula is salt 1.5%, sugar 3.0%, soybean protein isolate 5.0%, ginger juice 3.0%, onion juice 1%, monosodium glutamate 1% and oil 3%. The optimum processing technology is cooking time 75 minutes, pressing water 55%, and initial frying time 10 minutes. The technological process is determined as follows: raw materials thawing rinsing deodorizing cooking deburring pressing rubbing seasoning frying cooling weighing packaging finished products.
2.3 Canned Processing
Processing tuna into canned food can not only prolong its storage period, but also increase its commodity value. Canned tuna is an important variety of aquatic products, and a series of canned products have been formed. The canned tuna are mainly long fin tuna and yellowfin tuna. Various kinds of canned tuna have been produced, such as raw juice, oil-soaked tuna, vegetable juice, corn, jelly, eggplant juice tuna, etc. Researches showed the optimum ingredients for flavor tuna were soy sauce 6.0%, sugar 2.5%, disodium nucleotide (I+G) 0.14%. The optimum ingredients for five spices tuna were 5.0%, soy sauce 6.0%, I+G 0.10%-0.14%, sugar 4.0%. The optimum ingredients for soybean sauce tuna were soy sauce 8.0%, perfume 3.0%. 2.0%-4.0% sugar and I+G 0.12% sugar. At the same time, the soup was mixed with appropriate amount of wine and agar to make the soup. The salinity of the soup was adjusted to 5.0%-6.0%, and the result was better.
The canned tuna can be divided into the following categories according to its contents: 1) large pieces of white and red-and-white fish meat, all of which contain almost no fragments of meat; 2) large pieces mixed with fragments, with no more than 20% of the fragments of meat, the rest of which are large pieces; 3) whole white meat, with all the fragments of meat; 4) small pieces mixed with the fragments of meat, the edible fragments account for more than 60%, and the fragments of meat contain about 40%; 5) froth fragments, soft fragments; Smaller pieces of meat. Of course, although there are so many products with different flavors, it is still necessary to develop more new series products in order to adapt tuna canned to different tastes in more regions and expand the market acceptance capacity of canned tuna.
2.4 Ultrahigh Pressure Machining
Ultra-high pressure technology is to apply certain pressure to food or other materials in sealed containers with water or other liquids as media to achieve the purpose of sterilization, modification, processing and preservation. The sale of tuna in the whole international market is mainly canned tuna, but with the growth of demand for fresh tuna fillets, such as in Japan, the popularity of tuna sushi, to a certain extent, has affected consumer habits. Therefore, in the process of processing and preservation, to ensure the fresh quality and food safety of tuna is an urgent problem to be solved in tuna processing. Researchers used ultra-high hydrostatic pressure technology to process and preserve fresh tuna. The quality changes of fresh tuna fillets during storage were studied under optimum pressure of 220 MPa and 275 MPa for 3 min and 6 min. The results showed that the total number of colony of fresh tuna slices treated by ultra-high pressure was not exceeded the standard during the 12-day storage period, and the TVBN of the samples treated by ultra-high pressure did not exceed the raw food safety standard of 25mg/100g, which met the food safety standard.
3. Comprehensive Utilization Technology of Tuna Byproducts
3.1 Extraction of Fish Oil and Polyunsaturated Fatty Acids
According to researches, tuna head has high fat content and contains a lot of polyunsaturated fatty acids, especially DHA content is more than 20%, EPA content is 5%-10%. Therefore, tuna head is a good source of DHA and EPA. EPA and DHA have the functions of inhibiting platelet aggregation, anti-thrombosis, vasodilating, regulating blood lipids and improving biofilm liquidity. Researchers studied the separation and purification of_-3 polyunsaturated fatty acids from by-products of tuna canning processing by urea embedding crystallization method. The purity of_-3 polyunsaturated fatty acids was 89.64%. Researchers took yellowfin tuna eye socket meat as the research object. Fish oil was extracted by waterproof cooking, enzymatic hydrolysis, alkali hydrolysis and supercritical carbon dioxide (SCCO 2) extraction. The results showed that the fish oil obtained by SC-CO2 extraction met the first-class standard of refined fish oil in aquatic industry in China, and the content of unsaturated fatty acids in the extracted fish oil was the highest (74.29%), including EPA 5.99% and DHA 27.12%. It is concluded that SC-CO2 has significant advantages over other three methods in extracting fish oil from tuna eye socket meat. Scientists studied the extraction of crude fish oil by double enzymatic hydrolysis using tuna processing waste as raw material. The optimum technological conditions were as follows: the ratio of feed to liquid was 1:3, the amount of protease A and B was 1.5% and the reaction time was 3.2 h, respectively. The obtained enzymatic hydrolysate was frozen at – 20 for 12 hours and thawed at 40 for 1.5 hours to demulsify. Under these conditions, the extraction rate of crude fish oil was (6.58 +0.56)%. The physicochemical indexes of crude fish oil reached the second grade standard of SC/T 3502-2000 Fish Oil.
3.2 Extraction of Oligopeptides
In the process of canned tuna processing, a large number of crushed meat is produced, accounting for about 11% of the raw materials. Except for the limited net weight supplement for canned food, the remaining meat is not well utilized. But it is rich in protein as fish, and is an excellent raw material for preparing various active peptides and amino acids. Studies have shown that oligopeptides bound by several amino acids have better digestive and absorptive properties than amino acids, and have physiological functions such as anti-allergy, cholesterol lowering, blood pressure lowering and immune enhancement. Therefore, the research on hydrolysis technology and hydrolysates of proteins has important significance and application value [25-27].
Reserchers used two-step enzymatic hydrolysis with tuna crushed meat protein as substrate. The results showed that the best hydrolysis effect was obtained when pepsin was used as the first step, pepsin was added 800 U/g, enzymatic hydrolysis time was 3 h, temperature was 35-40 C, pH 2.0, substrate concentration (i.e. the ratio of fish to water quality) was 1:7. Flavor enzymes were used as the second step of enzymatic hydrolysis, adding 52000 U/g, enzymatic hydrolysis time was 2 h, and temperature was 50-55. The hydrolysis effect was the best at temperature and pH 7-7.5.
Scientists used tuna minced meat as raw material and trypsin and papain as two enzymes to form a double enzyme reaction system. The optimum hydrolysis conditions of tuna minced meat protein were obtained as follows: reaction temperature 45 C, reaction time 6 h, trypsin 15 000 U/g, papain 45 000 U/g and system pH 7.0. Under these conditions, the degree of hydrolysis can reach 15.83%. Ultrafiltration membrane was used to separate the enzymatic hydrolysate, and three components were obtained: more than 10 000, 5 000-10 000 and less than 5 000 D. The yields were 7.69%, 30.71% and 19.23% respectively.
In addition to extracting oligopeptides from crushed tuna meat, scientists studied the hydrolysis of yellow fin tuna pancreas with papain and alkaline protease. The results showed that the optimum hydrolysis conditions of papain and alkaline protease were as follows: enzyme activity ratio 1:1, feed-liquid ratio 1:10, enzyme dosage 30 mg/g, pH 7.55, enzymatic hydrolysis time 3.39 h, and enzymatic hydrolysis time 3.39 H. The temperature of enzymatic hydrolysis was 55.73 C. The degree of hydrolysis was up to 60.22%.
3.3 Other Byproducts
At present, the processing of tuna is mainly based on the production of sashimi and canned tuna. The by-products produced during canning process account for 50%-70% of the total quality. These by-products include fish head, bone, skin, viscera and minced meat. In addition to extracting oligopeptides and proteins, fish oil and polyunsaturated fatty acids, fish bones can be processed into fish bone meal, enzymes can be extracted from fish viscera, gelatin can be extracted from fish skin, collagen can be extracted from fish skin, fish bones and fins. Collagen is a kind of high quality raw material, which can be widely used in food, medicine, cosmetics and other industries. Researchers studied the technological conditions of extracting collagen from tuna skin. The extraction rate was 27.1%. Among them, the basic amino acid containing imino accounted for 20.5%.
4. Safety of tuna processing
Because tuna is rich in free histidine, when tuna dies, its own epidemic prevention mechanism collapses, microorganisms multiply rapidly, and can produce a large number of histidine decarboxylase. Under weak acid conditions, histamine decarboxylase produces histamine, which can cause headache, nausea and vomiting, diarrhea, itchy rash, hypotension and a series of allergic reactions. Therefore, histamine has become an important factor restricting the development of tuna processing industry. Researches studied the change rule of histamine in tuna processing. It was found that the increase of histamine mainly occurred in the thawing stage. When the thawing temperature was higher than 4 C, the histamine content of fish began to increase sharply. The higher the histamine content of raw material, the higher the rate of histamine increase. During cooking and standing, the histamine content continued to increase.
At present, the main products of tuna meat processing are canned and sashimi. The research of processing technology focuses on the formulation of canned condiment, the quality change and control of sashimi during freezing and refrigeration. The application of modern food engineering high-tech in tuna meat processing is obviously insufficient. Secondly, tuna processing produces a large number of by-products, mainly in the separation and extraction of protein and fish oil, and has not yet been put into the market. Therefore, with the increase of tuna processing capacity, a large number of by-products will be produced. Paying attention to the high-value utilization and comprehensive development of by-products, studying the application of high-tech in tuna meat processing, developing more and better products, controlling the changes of histamine content and color in fish meat processing and storage, and increasing the added value of tuna processing may be the development direction of tuna processing and utilization research.