5 Chemical Deterioration of Muscle Proteins During Frozen Storage JUICHIRO J. MATSUMOTO
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Department of Chemistry, Sophia University, Kioi-cho 7, Chiyoda-ku, Tokyo, Japan 102
Frozen storage of meat, poultry meat and fish i s one of the most important preservation methods for these foods. During frozen storage, deteriorations due to putrefaction and autolysis are decreased and the foods are s a t i s f a c t o r i l y preserved from the hygienic point of view. However, several undesirable changes still occur i n the frozen stored meats. Changes in quality of the frozen meats are demonstrated by several features. In frozen and thawed raw meat, there i s in creased loss of water, some changes i n flavor and taste and an undesirable softening. When the frozen and thawed meat i s cooked, the succulence and water-holding capacity i s decreased and there are undesirable changes i n texture such as toughness, coarseness and dryness. As compared with unfrozen fresh meat, the functional properties such as emulsifying capacity, lipid -binding properties, water-holding or hydrating capacity and gel capacity are decreased i n the frozen stored meat. In order to ovecome these defects, much research has been done in an attempt to c l a r i f y the mechanisms and causes of these changes during frozen storage of meats.
Such s t u d i e s have i n
cluded a wide v a r i e t y o f animals i n c l u d i n g beef a n i m a l s , hogs, p o u l t r y , f i s h , s h e l l f i s h and other i n v e r t e b r a t e aquatic a n i m a l s . The number o f papers published i n t h i s area amounts to several hundred. Most o f the s t u d i e s i n d i c a t e t h a t denaturation o f muscle p r o t e i n s plays the dominant r o l e i n the q u a l i t y changes o f the frozen s t o r e d meats. The muscle p r o t e i n s o f f i s h and other a q u a t i c animals have been found to be much l e s s s t a b l e than those o f beef a n i m a l s , pigs and p o u l t r y Q ) . The present paper w i l l be l i m i t e d p r i m a r i l y to f i s h muscle as one r e p r e s e n t a t i v e o f v e r t e b r a t e muscle and i t w i l l a l s o deal p r i m a r i l y w i t h the behavior o f f i s h p r o t e i n s a t sub-zero temperatures. In order to do a thorough a n a l y s i s w i t h i n the space l i m i t p e r m i t t e d , focus w i l l be on the changes o f the p r o t e i n s per_ se l e a v i n g p e r i p h e r a l problems to other reviews ( 2 - 1 8 ) .
0-8412-0543-4/80/47-123-095$07.25/0 © 1980 A m e r i c a n C h e m i c a l Society In Chemical Deterioration of Proteins; Whitaker, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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C H E M I C A L DETERIORATION OF PROTEINS
S t r u c t u r e and Composition o f Muscle
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Anatomical s t r u c t u r e . The s t r i a t e d muscles o f v e r t e b r a t e s con s i s t o f bundles o f muscle f i b e r c e l l s covered w i t h connective t i s s u e . A muscle f i b e r i s composed o f bundles o f s t r i a t e d myo f i b r i l s surrounded by sarcoplasmic r e t i c u l u m , mitochondria and other o r g a n e l l e s . The unique s t r u c t u r e o f the s t r i a t e d myo f i b r i l s , composed o f t h i n ( a c t i n ) - f i l a m e n t s and t h i c k ( m y o s i n ) f i l a m e n t s , i s common to a l l s t r i a t e d muscles o f vertebrates i n cluding f i s h (19,20). Chemical c o n s t i t u t i o n . The approximate composition o f mammalian muscle i s : 16-22% p r o t e i n ; 1.5-13% l i p i d ; 0.5-13% carbohydrate; ^1% i n o r g a n i c matter; and 65-80% water (21). P o u l t r y muscle contains l e s s l i p i d . The composition o f f i s h muscles i s : 15-24% p r o t e i n ; 0.1-22% l i p i d ; 1-3% carbohydrate; 0.8-2% i n o r g a n i c matter; and 66-84% water (21-23). V e r t e b r a t e muscles c o n t a i n s i m i l a r types o f p r o t e i n s , a l though some d i f f e r e n c e s do e x i s t i n t h e i r r e l a t i v e amounts and i n the p r o p e r t i e s o f each p r o t e i n . The p r o t e i n s are c l a s s i f i e d i n t o three groups based on s o l u b i l i t y ( 2 2 , 2 4 ) . These a r e : 1) the sarcoplasmic p r o t e i n s , e x t r a c t a b l e at low i o n i c strength (0.5), which i n c l u d e a c t i n , myosin, tropomyosin and t r o p o n i n . In mus c l e , F - a c t i n f i l a m e n t s , a polymerized form o f monomer G - a c t i n , and tropomyosin and t r o p o n i n , the r e g u l a t o r y p r o t e i n s , compose the t h i n f i l a m e n t s , w h i l e o r d e r l y aggregated myosin molecules form the t h i c k f i l a m e n t s ; 3) the stroma p r o t e i n s - the r e s i d u a l group which i s not e x t r a c t e d by s a l t s o l u t i o n s or d i l u t e a l k a l i n e or a c i d s o l u t i o n s . This group i n c l u d e s c o l l a g e n and e l a s t i n , the connective t i s s u e p r o t e i n s , and a new p r o t e i n , c o n n e c t i n , found i n the f i n e s t r u c t u r e o f the t h i n f i l a m e n t s (25). The a p p r o x i mate p r o t e i n compositions o f various animal muscles are shown i n Table I (26-32). The amount o f stroma p r o t e i n s i s l e s s i n f i s h muscles (3-5%) than i t i s i n beef o r r a b b i t muscles (15-18%). This may e x p l a i n why raw f i s h f i l l e t s are acceptable i n Japanese d i s h e s , whereas beef, r a b b i t and pork are r a r e l y served raw. According to Fennema e t a l . ( £ ) , tenderness i s p r i m a r i l y r e l a t e d to c o l l a g e n content, w h i l e toughness and w a t e r - h o l d i n g c a p a c i t y are a s s o c i ated w i t h the m y o f i b r i l l a r p r o t e i n s . Many papers on cooked meat mention both tenderness and toughness, w h i l e those on cooked f i s h note the problems o f toughness r a t h e r than tenderness. This a l s o might be r e l a t e d t o the d i f f e r e n c e i n content o f the stroma proteins.
In Chemical Deterioration of Proteins; Whitaker, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
5.
MATSUMOTO
Table I .
Deterioration
During
Frozen
P r o t e i n Composition o f Muscle
Sarcoplasmic
Myofibrillar
Stroma
proteins
proteins
proteins
{% o f t o t a l
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97
Storage
Reference
proteins)
Beef
VI7
^68
15
(26)
Rabbit
