Volume 62, No. 2, 1997—JOURNAL OF FOOD SCIENCE—299 Textural Changes and Functional Properties of Cod Mince Proteins as Affected by Kidney Tissue and Cryoprotectants C.C. CHANG and J.M. REGENSTEIN ABSTRACT The addition of cryoprotectants, e.g., a mixture of sucrose and sorbitol or polydextrose (PDT) with sodium hexametaphosphate (SHMP), to im- prove the texture and water retention properties of cod mince with kid- ney tissue (modified mince) was studied at -14°C. Kidney tissue provided trimethylamine oxide (TMAO) demethylase to accelerate di- methylamine (DMA) and formaldehyde formation. Expressible moisture (EM), water uptake ability (WUA), protein solubility, cook loss, DMA production, and textural profile analysis were determined to detect changes in modified mince during frozen storage. Modified mince with SHMP and sucrose/sorbitol or PDT at -14°C had improved EM, WUA, and cook loss and was more tender. Key Words: cryoprotectants, frozen storage, cod mince, expressible moisture, water uptake INTRODUCTION PROLONGED FROZEN STORAGE can cause changes in the func- tional properties of muscle protein due to denaturation and/or aggregation of myofibrillar proteins (Matsumoto, 1979, 1980). Losses in muscle protein functional properties, such as protein solubility, gel-forming ability, and water retention properties are major problems in frozen stored fish. Usually, a tough texture develops in gadoid fish mince (e.g., cod, haddock, hakes, pol- lock and whitings) during frozen storage. Such textural changes are caused by trimethylamine oxide (TMAO) demethylase which catalyzes the decomposition of TMAO to dimethylamine (DMA) and formaldehyde (FA). FA is hypothesized to be a cross-linking agent in muscle proteins and may thus cause tex- tural deterioration (toughness) in frozen fish (Babbitt et al., 1972; Castell et al., 1973; Dingle et al., 1977). To increase moisture retention and improve texture of frozen fish, various polymerized phosphates may be added. The effects of phosphate type and concentration, level, and meat type on water retention have been reported (Shults et al., 1972; Trout and Schmidt, 1984, 1986; Lewis et al., 1986). The effectiveness of phosphates in maintaining cook yield was as follows: pyro- phosphate  tripolyphosphate  tetrapolyphosphate  hexa- metaphosphate. Cryoprotective compounds have been used to prevent fish protein denaturation during frozen storage. The protective ef- fects of monosodium glutamate, amino acids and related com- pounds, peptides, acetylamino acids, sulfur compounds, carboxylic acids and carbohydrates on fish muscle proteins dur- ing frozen storage have been reported by Noguchi and Matsu- moto (1975a,b, 1976) and Noguchi et al. (1975). Solubility, viscosity, and actomyosin ATPase activity were maintained at higher levels by the addition of cryoprotectants. Compounds in- cluding sucrose, sorbitol, polydextrose (PDT), protein hydroly- sates, isomalt (an equimolar mixture of D-glucosyl-(1,6)-D-glucitol and D-glycosyl-(1,1)-D-manni- tol), lactitol dihydrate, iota-carrageenan, carboxymethylcellulose and xanthan gum have been used to prevent protein denatura- tion, improve gel-forming ability, increase protein solubility and decrease cook loss (Park et al., 1988, 1993; Da Ponte et al., Authors Chang and Regenstein are with the Dept. of Food Sci- ence, 112 Rice Hall, Cornell Univ., Ithaca, NY 14853-5601. 1987; Yoon and Lee, 1990a,b; Sych et al., 1991). The combi- nation of PDT or sucrose/sorbitol with phosphates (sodium tri- polyphosphate and pyrophosphate) was used to study protein stabilization in fish myofibrils (Park and Lanier, 1987) and func- tional properties of pre- and post-rigor unsalted beef during fro- zen storage (Park et al., 1987). However, SHMP, a food-grade polyphosphate, has not been reported in combination with su- crose/sorbitol or PDT. Fish mince is conventionally used to produce surimi. The ad- dition of cryoprotectants (12% w/w sucrose and 0.2% w/w po- lyphosphates) to fresh Pacific whiting stabilized fish muscle proteins and maintained textural quality during frozen storage (Simpson et al., 1994). However, a problem in producing whole fish mince is inclusion of kidney tissue, which is adjacent to the backbone. If not totally removed, residual kidney tissue may be incorporated into mince during deboning. Hence, the quality of the fish mince may be lowered and protein functional properties may deteriorate faster during frozen storage. The objective of our research was to evaluate the effects of cryoprotective compounds, sucrose/sorbitol and PDT, with SHMP on textural changes and protein functional properties of cod mince with kidney (modified mince) during frozen storage. Kidney tissue contains high levels of TMAO demethylase, which is active during frozen storage (-29°C), and acceler- ates the reaction of TMAO to DMA and FA during storage at -14°C. MATERIALS & METHODS Preparation of fish mince Fresh whole cod (gutted) were purchased from Steve Connolly Sea- food Co. (Boston, MA) and shipped on ice to Ithaca, NY prior to each treatment. Blood and slime on the surface of the cod were washed off. The kidney tissue, adjacent to the backbone, was scraped from the fresh cod frames and homogenized in a Thomas Tissue Grinder (Philadelphia, PA). The whole cod were then headed and filleted. The fillets were ground using a Kitchen-Aid Food Processor (KitchenAid Div., Troy, OH) with 1 mm hole plate in a cold room (4°C). Kidney (0.8% of mince weight) was added to some samples. (The 0.8% was based on a prelim- inary measure of the ratio of kidney to muscle in cod.) Minces with kidney tissues were further divided into 4 groups: no chemical addition, the mixture of sucrose (Sigma Chemical Co., St. Louis, MO) and sorbitol (Pfizer Inc., New York) (4%:4%), SHMP (0.5%) (FMC Corp.), and a combination of sucrose/sorbitol (4%:4%) and SHMP (0.5%). All chemicals were dissolved in 10% deionized dis- tilled H 2 O (dd H 2 O), based on mince weight before addition to assure uniform mixing. Dd H 2 O (10% of mince weight) was added to the con- trol without kidney tissues. Another treatment was conducted to determine whether PDT could substitute for sucrose/sorbitol. Minced fish was prepared as described and 8% PDT was substituted for sucrose/sorbitol in the four groups. After addition of various chemicals, 30g minced fillets were packed into Nalgene jars (35.0 mm internal diameter and 40.0 mm internal height, Sybran Nalge Co., Rochester, NY) and stored at -14°C (Kel- vinator, Manitowoc, WI) for 1 day, 2, 4, 8 and 12 wk. Expressible moisture (EM) EM was determined using the procedure developed by Jauregui et al. (1981) and modified by Hsieh and Regenstein (1989). Samples were placed on one piece of polyester mesh (PE. 105, type 502, 690 mesh per cm 2 , Henry Simon Limited, Stockport, Cheshire, England) attached