3.1. Processing of food proteins
Soy protein suspended in water and packed in plastic tubes is completely coagulated at 400 MPa. Beef pressurized at 400 MPa and room temperature for 10 min is like a raw ham and the taste of the pressurized beef is intact. When the surface of the pressurized beef is slightly baked, it tastes like a rare steak. Boiled shrimps turn red and curve with the coagulation of the meat, but shrimps pressurized at 400 MPa for 10 min show no apparent change in color or shape but have coagulated meat as boiled shrimps. Oysters also show no change in size and shape, but keep the raw taste and flavor, after pressurization at 300 - 400 MPa and room temperature for 10 min. The pressurized oysters may be microbiologically safe.
In general, pressure-coagulated food proteins, i.e., egg, soy protein, beef, pork and fish meat, are more glossy, transparent, dense, smooth and soft compared to boiled ones. These unique textural properties obtained by the pressurization offer ways to create new food materials (5-10).
Pressure treatment for processed foods including meat, fish and plant are the recent developments. McFarlane (18) in Australia used high pressure to treat meat where the pressure was applied as an electric shock to make meat tender. Meat tenderization is now possible by pressure control of intracellular proteases in meat (8).
High pressure effects on proteins and the fine structure of meat and fish have been studied extensively (see Table 2).
R & D on High Pressure (HP*) Food Science and Technology in Japan, 1987-1993 -
A summary of Conferences (6-10)
Chemistry and chemical reaction: Difference in liquid and solid compression, difference and similarity of T and P effects, organic synthesis under HP, HP effects on food-related reactions (hydrolysis of protein and starch, Maillard reaction, formation of lysinoalanine), solubility of ethylbenzene, NMR probe.
Enzyme and enzymatic reaction: Peptide hydrolysis and synthesis by proteases, enzyme digestion (soy protein, whey protein concentrates), activation of crude and purified polyphenoloxidases, inactivation of oxidases, HP effects on enzymes (actomyosin ATPase, lysosomal enzymes, muscle proteases, proteases, chymotrypsin, lipase, lysozyme, amylase).
Water and ice: Behavior of water under HP, thermal increase by rapid compression, near IR of water and ice, HP effect onsalt solution, rapid thawing, pressure-shift freezing, sterilization of ice-nucleation bacteria, properties of ice I & III for food preservation.
Protein: HP effect on hydrophobic interaction, compressibility and structure, denaturation mechanism of Taka-amylase, chemical modification by HP (carboxymethylation, ferrocenation), HP effect on conectin, HP effect on thermal denaturation, deuterium exchange and fluorescence labeling, HP-induced change in myosin and subfragment, and cytoskeletal protein in muscle, properties of soy protein.
Starches: HP effect on starches, relationship between heat and pressure gelatinization, cooking properties of HP rice, rice wine (sake) brewing.
Lipids: Properties of lipids, fish lipid, rice lipid, sardine lipid, and milk fat, heating effect on HP milk fat.
Cellular structure: Cell membrane, subcellular structure of yeast, lysosomal granules, sarcoplasmic reticulum, yeast cytoskeleton, milk fat globules.
Biochemical phenomenon: HP extraction of pectin, solubilization of cellular components.
Microbial physiology: Yeast cell, growth of yeast and E. coli under HP, HP-tolerance of heat-shocked yeast, HP-tolerant yeast, HP effect on oxidative stress of yeast, drug sensitivity of E. coli grown under HP, HP effect on deep sea bacteria.
Viruses: Herpes virus.
Parasite: Muscular larvae of Trichinella spiralis.
Gel and sol: Gel-sol transformation of ovalbumin, emulsion properties of proteins, gel properties of fish surimi, gluconolactone gel of soy protein, gel formation of concentrated milk, gel of alginate, properties of HP-extracted pectin, gel properties of low-methoxy pectin, gel-glass transition, thermal effect on HP-gel of actomyosin and whey protein.
Sterilization: Kinetics, mechanism at low temperature, Bacillus spores at high temperature, estimation of sterilization efficiency, effect of antimicrobial substance.
Food sterilization: Meats, fish meats, egg white, milk, milk products, tomato juice, jams, tea drinks, fresh vegetables, mandarin juice, plums, fruits, pickles, coffee, sausage, and sea urchin eggs, pickles in Japan, paste of water melon, soy sauce, seasoning sauce (tsuyu, tare), oyster, soybean cake (tofu), raw ham, raw sausage, lightly roasted beef.
Processing of meat: Ultrastructure and myofibrillar protein, products of beef muscle, properties of myosin B, gel formation of myosin, properties of HP meat, change in myosin B in high salt concentration, SH content and gel formation of myosin B, rheological properties of HP meat, effect of curing agent on HP porcine minced meats, processing system of meat products.
Processing of fish and fish meats: Texture of Alaska pollack, properties of HP fish meat (surimi), water-soluble fish meat, sea urchin, ultrastructure of carp muscle, new meat products with various texture, HP-induced texture of fish sarcoplasmic protein denatured by heating, pH shift or organic solvents.
Processing of agricultural products: Development of jams, fruit sauce and fruit dessert, processing of plum products, orange juice, rice wine (sake), rice cake (moti), and rice crackers, enzyme inactivation of oranges, food processing by ice nucleation-bacteria, properties of pickles, control of bitter taste in grapefruit juice.
Processing of other foods: Texture of pressurized egg white, cheese processing, whey protein concentrates, HP fermentation of cocoa beans.
Preservation: Pickles, soybean paste (miso, shiromisó), rice wine (sake, namazake), sea weeds, preservation under HP and sub-zero temperature, quality keeping of HP jams.
Cooking: Egg, meat, fish, oyster, shrimp, Japanese radish, kinetics of hardness of HP radish.
Combination of pressure and temperature: Theory and possibility, sterilization by HP and high or low temperature cooking.
Related techniques: Packaging materials, HP vessel for microscopy, HP indicator, HP fixation for electron microscope, electron microscopy of animal tissue cell, rapid thawing of frozen fishes, equipping of ultrasonic-wave generator.
Others: Economy, future and development of HP.
*HP also means high pressure-treated or pressurized.
Starches of potato, corn and wheat are gelatinized by the pressure treatment at several hundred MPa and slightly elevated temperature. Pressurization of starches produces unique properties which are different from those formed by heat-
gelatinization. This opens some interesting preparation possibilities. Although heat treatment destroys starch granules and dissolves starches to give transparent solution, pressure-treated starches keep the granular structure intact. Nevertheless, pressurized starches are digested very well by amylolytic enzymes such as a-amylase (7, 9). Thus, pressure treatment opens a new way to process starches with minimum use of heat. However, further extensive study is clearly needed for understanding the effects of high pressure on starches and other food components.
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