P6. Fish
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are fish proteins digested faster than muscle proteins of land animals?
yes! substantially faster
which 3 aa are more abundant in fish vs beef muscle?
• aspartic acid • cystine • lysine
distribution of 3 types of protein in fish? (give percentage range)
• 65-75% myofibrillar protein • 20-30% sarcoplasmic protein • 3-10% connective tissue proteins
myofibrillar proteins - 1 similarity + 1 difference compared to mammalian protein
• similarity: relative proportions of individual proteins (myosin, actin G, tropomyosin, troponin…) are similar • difference: proportion of myofibrillar proteins in fish total protein is higher than in mammalian total protein
• heat stability of fish myofibrillar proteins is higher/lower than mammalian counterparts • protein hydrolysis by trypsin is faster/slower –> especially which protein? - these properties underlie what?
• lower • faster –> myosin! actin is similar • good digestibility of fish protein
fish sarcoplasmic proteins - mostly what? –> correspond to those of mammalian muscle tissue? - when these proteins are separated electrophoretically, what happens? helpful for what? - aa composition of what protein in fish is different from that of mammalian counterpart? how?
• enzymes –> yes! • specific patterns are obtained for each fish species –> helpful chemical means of fish taxonomy • fish myoglobin = different than mammalian myoglobin –> fish contains cysteine residue
myoglobin and other pigments for fish are concentrated in _____ muscle
• dark muscle! ish mackerel (vs light tissue has less myoglobin)
how much hemoglobin/myoglobin do light fish muscles contain? vs dark muscle like mackerel?
• light: 0.1 g/kg of both • dark: 3.9 g/kg of myoglobin, 5.8 g/kg of hemoglobin (and 0.13 g/kg of cytochrome c)
what is absent in some mollusks and in antarctic fish with colorless blood?
hemoglobin
in strongly pigmented fish (ex.?), pigment degradation reactions can induce what?
• tuna • induce meat discoloration –> observable greening in canned tuna meat
connective tissue protein same in fish than in mammalian muscle? - main components? (2)
• lower in fish (1.3%) than in mammalian muscle • collagen (up to 90%) and the rest is elastin
shrinkage temp of fish collagen vs mammalian collagen
fish: 45°C mammalian: 60-65°C
what makes fish meat more tender than mammalian meat? (2)
• lower content of connective tissue protein • lower shrinkage temperature
why do some fish not freeze in -2°C water?
because they have antifreeze serum proteins –> congealing temp of blood serum for polar fish of Arctic or Antarctic regions is about -2°C –> significantly lower than that of other fish (-0.6 to -0.8°C)
what makes congealing temp of blood serum of polar fish so low? - MW? - structure?
antifreeze glycoproteins –> aa sequence of this class of proteins is characterized by high degree of periodicity - 10.5-27 kDa - several a-helical regions
antifreeze glycoproteins: - highly _______ in solution - antifreezing effects attributed to what? (2)
• hydrated • disaccharide residues as well as methyl side chains of peptide moiety
what is a predominant free aa in fish with dark-colored flesh (tuna, mackerel)? - content %? - vs light colored flesh?
histidine! - content in flesh is around 0.6-1.3% of fresh weight –> can even exceed 2% - light color: 0.005-0.05% free histidine
what happens to histidine during bacterial decay?
• converted to histamine! can cause allergic reactions
what free peptides other than histidine are present in fish muscle tissue? (3)
• free l-methylhistidine • anserine (b-alanyl-L-methylhistidine) • carnosine (b-alanyl-L-histidine)
fish muscle tissue has high content ( 500 units?) of _________ –> derived from _______ - is it an aa?
• 500 mg/kg • taurine (2-aminoethanesulfonic acid) • derived from cysteine • sometimes referred to as an aa, but lacks a carboxyl group
what is surimi?
concentrate of insoluble fish muscle proteins (20%) that forms a solid cohesive gel when warmed
how to make raw surimi? (4 steps ish)
highly mechanized process 1. mechanical filleting and deboning of fresh, lean (white-muscle) fish (mostly Alaskan followed) 2. comminution at 5-10°C 3. minced fish is then extracted with water until only myosin, actin, actomyosin and small amounts of collagen remain –> purpose = remove strong flavor compounds and pigments + water-soluble proteins that can interfere in gelation of myofibrillar proteins 4. results in odorless and flavorless high-protein fish flesh (no lipid and almost no carbs) = raw surimi
3 properties of raw surimi compared with original minced fish
enhanced: - gel-forming - water-holding - fat-binding properties
addition of what enhances gel-forming properties of raw surimi?
paramyosin (protein similar to myosin that is present in thick filaments of invertebrates)