P6. Fish Flashcards
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)
after elimination of excess water from surimi, what is added? why?
- cryoprotectants (sugars or sugar alcohols) are added and surimi is then frozen
- adding cryoprotectants = necessary to retard protein denaturation during frozen storage, which would result in loss of functionality
apart from adding cryoprotectants, how can we further process surimi?
- 2 main + role
- can also add (2)
addition of salt (2-3%) + heating
- role of salt = solubilize myofibrillar proteins
- add starch (5%) and egg white
2 stage gelation process of surimi
- setting of gel at 40-50°C: formation of a loose 3-dimensional network by partially denatured actin and myosin
- formation of kamaboko at 80-90°C: conversion of loose network to an ordered, non-transparent gel that is more cohesive and elastic
