Fishery products are one of the most important food resources, like agriculture and livestock products. Use of fish meat as protein resources are classified as follows: (i) fresh food for cooking sashimi and grilled/ boiled seafood, (ii) processing raw materials for dried, salted, and canned foods, and (iii) industrial protein material such as frozen surimi. In any application, it is necessary to suppress protein denaturation at various stages from fish catch to the manufacturing process, because fish proteins are thermally and chemically less stable than that of other vertebrates. This special issue focuses on discussing the characteristics of fish meat protein and recent topics on the utilization of seafood protein. Five experts have contributed review articles.
Professor K. Konno (Hokkaido University) introduces the latest findings concerning biochemical properties of fish myofibrillar proteins in the review article, “Recent progress of fish myosin denaturation study”, where he summarized that an understanding of myosin denaturation upon heating and storage of fish meat is essential in evaluating the quality of fish muscle. Subsequently, Professor S. Mizuta (Fukui Prefectural University) explained the biochemical characteristics of collagen derived from a wide range of animals, from fish to marine invertebrates. He also introduced a novel matrix protein in the review article, “Proteins in the extracellular matrix”.
It is known that an appropriate handling of caught fish is important from the viewpoint of control of protein denaturation. Professor I. Kimura (Kagoshima University) explained the utilization of freshness evaluation indices, K value (indicating change in ATP related compounds in fish muscle) and Metmyoglobin ratio (indicating oxidation of myoglobin) in his review article, “Freshness control and improvement in quality of seafood”. He also introduced methods for producing high quality seafood by monitoring these freshness indices in the article. Additionally, Professor T. Ooizumi (Fukui Prefectural University) described the importance of controlling myofibrillar protein denaturation by measuring moisture behavior and protein qualitative change in fish meat during salting and drying in his review article, “Control of protein denaturation and its effect on quality of processed seafood”. This review suggests that monitoring of protein denaturation in food manufacturing processes provides useful information for quality improvement of seafood products.
In the last article of this special issue, “Improvement of food functionality of fish meat protein by introduction of glycosyl units”, I would like to introduce molecular modification technology, which has the potential to improve food characteristics and the health benefits of fish meat protein by improving the protein stability.
I hope this special issue contributes to research and development works of fish meat protein utilization.

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It is generally believed that quality of myosin, the major protein in fish muscle, determines the quality of the meat and meat-based products. Compared with myosin from mammalian muscle, fish myosin is unstable especially from cold-water fish species. Thus, understanding of myosin denaturation upon heating and storage of meat is essential in evaluating the quality of fish muscle. As myosin exhibits ATPase activity, its inactivation is a sensitive index of myosin denaturation. ATPase inactivation provides myosin denaturation at its head region but not tail region. To obtain the information on myosin denaturation at its tail region, chymotryptic digestion technique was employed. Upon heating of myofibrils (Mf), head and tail regions denatured differently and the pattern was fish species-specific. By using the same techniques, myosin denaturation upon freezing of Mf was also studied and it was found that the tail region remained very stable. Chymotryptic digestion revealed actin denaturation proceeded in frozen Mf. A quick myosin denaturation in frozen Mf was explained by the loss of protection by actin. The actin denaturation in Mf was explained by condensation of salt upon freezing of Mf suspension. Such actin denaturation was not found with frozen stored meat. Quality of Bluefin tuna meat was also evaluated by studying myosin denaturation. The denaturation was determined by the body temperature and pH of the meat. By assuming changes of body temperature and pH, the myosin denaturation process was simulated and it was found that cooling of fish as quickly as possible is the method to keep the quality of tuna meat high.

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The extracellular matrix plays important roles in cell adhesion and in supporting the morphology of tissues, organs and bodies of multicellular animals. Although it contains a number of proteins such as collagen, elastin, fibronectin, and so on, this review introduces molecular species of collagen in commercially important aquatic animals and novel glycoprotein in the dermis of sea cucumber, named 400kDa glycoprotein, focusing on not only their biochemical characteristics but also on methods for recovering them, from a viewpoint of effective utilization of marine organisms.
For various tissues in fish, the existence of at least 8 types of collagen (type Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ, Ⅵ, Ⅺ, and E) has so far been reported. Interestingly, type Ⅰ collagen includes a unique alpha component, α3(Ⅰ), which is characteristic of fish, and existence of two molecular forms, [α1(Ⅰ)]₂α2(Ⅰ) and α1(Ⅰ)α2(Ⅰ)α3(Ⅰ), has been deduced. Native collagen or collagen peptides from skin or scale of salmon, red sea bream, tilapia etc, are now commercially used as materials of foods and cosmetics. Extraction with dilute acid solution or limited digestion with proteases is mainly adopted for preparing native collagen from fish tissues. In addition, considerable attention has focused on the skin or cartilage of sturgeon, which has recently become a target of culture fishery, as collagen resources.
Biochemical characteristics are described here on major or minor collagen in crustacean, cnidarian, cephalopod, bivalve, and cordate for invertebrate animals. Many of their collagens are highly insoluble in dilute acid solution, so limited digestion with proteases is often essential to solubilize native collagen. In contrast, collagen in jellyfish can be autolytically solubilized by stirring or freezing-thawing using the activity of endogenous proteases. A preparation method for jellyfish collagen has been established by applying the phenomenon of autolytic tissue disintegration.
The 400 kDa glycoprotein is one of the major proteins in the dermis of sea cucumber, occupying about 30 % of its total protein and has a heterodimeric structure consisting of two subunits linked to each other by disulfide bonds. It can be easily extracted with water or salt solution with low-ionic strength and be purified by salt precipitation. So this protein can be expected to be a new marine protein resource.

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Traditional Japanese cuisine was added to UNESCO's intangible cultural heritage list in 2013. Japanese cuisine has spread throughout the world. Seafood is one of the main Japanese foods. The export volume of marine products from Japan has increased year by year. As the special feature of Japanese seafood cuisine is “eating raw,” such as sashimi and sushi, the evaluation and control of the quality of raw fish is very important. The practical evaluation standards and methods of controlling the quality of raw fish such as sashimi are, however, not yet established globally. In this paper, I introduce research on utilization of freshness index K-value and the suppression effect of ATP on freeze denaturation of fish myofibrillar protein and myoglobin. K-value is an excellent scientific index for freshness of fish.

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Effects of drying temperature on dehydration progress and the quality of salted-dried meat were investigated. When drying at 20 ˚C, dehydration of salted fish meat proceeded more rapidly than unsalted fish meat. Although moisture in the inside of unsalted fish meat barely decreased during the dehydration process, a moisture decrease was observed in the inside as well as on the surface of salted fish meat, indicating that moisture migration from the inside to the surface of fish meat was promoted by salting. NaCl content on the surface of salted fish meat was much higher than that in the inside irrespective of drying duration, suggesting sluggish internal migration of NaCl during the dehydration process. A NaCl gradient formed in the inside of fish meat generated the difference in water holding capacity on the surface and the central part of fish meat. Consequently, internal moisture migration was promoted along with the NaCl gradient in salted fish meat. When drying at 40 ˚C, internal moisture migration was promoted due to lowering of the water holding capacity of unsalted fish meat. In addition, the NaCl gradient was not observed in the inside of salted fish meat by the promotion of NaCl migration. As a result, the promoting effect of salting on the progress of dehydration was no longer seen when drying at 40 ˚C.
Inactivation of Ca-ATPase and decrease in salt solubility of myofibrillar protein in salted-dried fish meat proceeded extensively when drying at 40 ˚C, demonstrating much more rapid progress of protein denaturation than at 20 ˚C. The changes in mobility of moisture and NaCl above mentioned were probably because denaturation of myofibrillar protein progressed at 40 ˚C. Extensive protein denaturation proceeded even in semi-dried product with more than 60% moisture, by the dehydration at 40 ˚C. The amounts of centrifugal and cooking drip of semi-dried product dried at 40 ˚C were larger than those at 20 ˚C, suggesting that water holding capacity of fish meat decreased due to protein denaturation.
These results suggested changes in the biochemical properties of myofibrillar protein in fish meat by salting and dehydration were closely related to the progress of dehydration as well as the quality of salted-dried fish meat.

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Introduction of glycosyl units into food proteins is a useful procedure for improving processing characteristics of proteins. Thermal and chemical stability, solubility, and emulsifying properties of food proteins have been enhanced by conjugating with glycosyl units (monosaccharides, oligosaccharides, and polysaccharides). The use of cross-linking reagents and reductive alkylation are popular methods to synthesize protein-sugar conjugates, whereas “protein glycosylation using the Maillard reaction” is an effective method to improve food protein characteristics. The Maillard-type glycosylation is superior to other types of chemical modifications targeting food proteins as the reaction proceeds under mild and safe conditions without chemicals. So far, more than 200 reports have been published to discuss improvement by the Maillard-type glycosylation, on processing characteristics of food proteins. In the case of fish meat protein, which is thermally less stable than other vertebrate protein, the food functionality was effectively improved by introducing reducing sugars via the Maillard reaction under controlled humidity states and in the presence of a thermal denaturant such as sorbitol. The improvement of the protein functionality depended on the type of sugars attached to the protein. Protein conjugation with alginate oligosaccharide produced a thermally stable and water soluble fish myofibrillar protein, which has strong emulsifying properties. In addition, the glycosylated protein-hydrolysate had strong anti-inflammatory activity. These results indicate that Maillard-type glycosylation could contribute to development of multifunctional food materials from fish meat.

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Yellow extracts from outer skins of yellow onions have been used as a natural food color resource. Such outer skins result from desiccation of outer scales of onion bulb. The chemical nature of the yellow pigments has not been characterized although cyanidin-glycosides have been identified as the red-purple pigments in outer skins of red onions. Quercetin, a major flavonoid in onion scales, has for long been presumed to be a yellow pigment of outer skins. However, it is now clear that this presumption was erroneous, from results for UV-visible absorption spectra. In fact, a novel xanthylium compound, cepaic acid, was identified as one of the yellow pigments. This highly hydrophilic compound comprises two phloroglucinol moieties. Their structures strongly suggest that yellow pigments are formed by coupling of oxidative degradation products of quercetin during drying of outer scales.

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Genetic manipulation techniques for Bifidobacterial strains, such as transformation and gene-knockout, have been considered ‘tough nuts to crack’. We have developed new tools to make breakthroughs in this regard. The generally low transformation efficiency has been overcome by a new concept of ‘plasmid artificial modification’, which protects external plasmids against host restriction systems. We also constructed a temperature sensitive plasmid, pKO403, which can be cured at 42 °C. Positive/negative bi-functional selection has been developed with a focus on the pyrE gene, encoding orotate phosphoribosyl transferase, which can be positively selected on uracil(-) medium and negatively selected with 5-fluoroorotic acid. Combining these newly developed tools, marker-less gene-knockout can be performed at the practical routine level. Multi-gene knockout is also feasible by repetition of these techniques.

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The porridge-type barley preparations were born about eight thousand years ago in Western Asia. The method used for cultivating cereals such as barley and wheat was spread to various places in the Eurasia and Africa Continents between 5000~2000BC. The barley porridge made with the cultivation technique of barley and rice was introduced from China to Korea in 2000BC. Barley porridge was the staple food in Korea in those days. The various porridges with rice and many other kinds of foodstuffs were born in the area from the barley porridge, about three thousand years ago.
In particular, many kinds of the porridges were born in the Ri-dynasty Era. There are porridges for the traditional events and Yakuzen porridges (medical porridges) among the Korean porridges. Presently, people eat porridges as one of their staple foods.

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