PARAMETERS OF TEXTURE CHANGE IN PROCESSED FISH: MYOSIN DENATURATION

Abstract The white muscle of the Sacramento blackfish ( Orthodon microlepidotus ) was processed by freezing, dehydration, and cooking. Myosin was extracted immediately afterwards or following a period of storage in order to examine evidence for denaturation. The tests used were the solubility of whole muscle protein and the intrinsic viscosity, isoelectric point, ATPase activity, ultra-violet absorption spectrum, and optical rotatory dispersion of purified myosin extract. Almost all measures used showed that denaturation increased in the order: fresh < frozen < frozen-stored < dehydrated < dehydrated-stored < cooked.     

PARAMETERS OF TEXTURE CHANGE IN PROCESSED FISH: CROSS-LINKAGE OF PROTEINS

Soluble myosin and insoluble protein (i. e., soluble only in 5 % sodium dodecyl sulfate) were obtained from fresh and processed Sacramento blackfish and analyzed for the existence of possible cross-links. Free sulfhydryl groups decreased somewhat in freezing and more in frozen storage. None were present after dehydration or cooking nor in any insoluble fractions, so that presumably these were oxidized in the formation of cross-links. Ester bonds were much more numerous in insoluble protein than in soluble myosin, but their relative content was not clearly related to processing. Aldehyde groups decreased in myosin after cooking and dehydration, and were absent from insoluble protein. The presumptive Schiff base content was somewhat greater in soluble protein than in myosin and appeared to increase upon freezing and dehydration.     

TEXTURE OF FISH MUSCLE

Morphological and chemical aspects of fish muscle texture are reviewed and differences from red meat are pointed out. A unique feature of fish muscle is its low connective tissue content which accounts for easy disintegration of fish flesh on heating. Thus, the muscle fibers are the main textural elements in cooked fish meat. Because of this, most of the popular texture testing instruments are not applicable to fish. Best results are obtained with the thin blade shear/compression cell.
Texture of fish muscle is affected by the species, age and size of the fish within the species, and nutritional state. Postmortem factors influencing texture include glycolysis, rigor mortis and the accompanying contraction of the muscle often leading to the separation of muscle segments (gaping), temperature profile during storage, temperature of cooking, pH and presence of NaCl.     

A SURVEY OF HISTAMINE LEVELS IN COMMERCIALLY PROCESSED SCOMBROID FISH PRODUCTS12

Analysis of 100 samples of each of 3 types of commercially processed scombroid fish products obtained at the retail level showed mean histamine concentrations (mg/100 g) of 3.58 for chunk light tuna, 1.50 for albacore tuna, and 2.56 for mackerel. Only 2 samples of chunk light tuna and 2 of mackerel contained greater than 10 mg% histamine. The highest level of histamine found in albacore tuna was 5.35 mg/100 g, while 99% ofthe samples had less than 5 mg% histamine. Chunk light tuna samples contained up to 28.0 mg/100 g, and mackerel samples contained up to 31.5 mg/100 g; 92% of the chunk light tuna and 95% of the mackerel had less than 5% histamine. The histamine levels found in these products were far below any level associated with food poisoning incidents.     

QUANTITATIVE ANALYSIS OF TEXTURE CHANGE IN COD MUSCLE DURING FROZEN STORAGE

The textural deterioration of cod muscle during frozen storage was investigated by objective measurement using the Instron Universal Testing Machine. The rates of texture change as a function of storage time revealed that the increased toughness and decreased cohesiveness could be described by a first-order relationship. Higher storage temperatures resulted in more rapid rates of textural deterioration. Larger activation energy constants for toughness of cod muscle as compared to cohesiveness indicated that toughening was more temperature dependent than cohesiveness. The increased toughness and the decreased cohesiveness of frozen-stored fish appeared to be related to the protein denaturation of the cod muscle and the loss of water-holding capacity, respectively.     

PROTEIN QUALITY OF PROCESSED WHOLE KERNEL GLANDLESS COTTONSEED

Protein quality of raw, cooked and roasted glandless whole kernal cottonseed flour was determined. The adjusted protein efficiency ratio (PER) of cooked (2.10) cottonseed was significantly (P = 0.01) higher than roasted (1.77) cottonseed. Protein retention efficiency (PRE) for roasted cottonseed (58.08) was lower than values for raw (60.54) and cooked (62.95) cottonseed. Relative protein values (RPV) indicated a utlization of 91, 91 and 96% of the protein in raw, roasted and cooked cottonseed, respectively. Cooked glandless cottonseed protein quality was superior to that of raw cottonseed. The roasting process adversely affected protein quality. Supplementation of roasted cottonseed with 0, 0.2, 0.4, 0.6 and 0.8% L-lysine indicated a peak PER response at 0.45%. The adverse effect of roasting on the protein quality of cottonseed was overcome by addition of L-lysine which made the protein quality of roasted cottonseed comparable to that of cooked cottonseed.     

MEASUREMENT OF TEXTURE PARAMETERS OF FREEZE-DRIED BEEF*

A modified texture profile analysis with the Instron Universal Testing Apparatus was used to measure hardness and chewiness of precooked freeze-dried beef cubes in order to determine the influence of processing and reconditioning. Two different plate temperatures were used during freeze-drying, and the resulting product was reconditioned at four different water activities in addition to the dry and fully rehydrated product. Reconditioning temperatures of 52, 70, and 100°F were investigated. The results revealed that the hardness and chewiness values were the highest at water activities of 0.4 to 0.6 and with the product freeze-dried at a plate temperature of 105°F. The influence of reconditioning temperature was inconsistent, apparently due to an interaction between the water activity and the plate temperature.     

PARAMETERS THAT DETERMINE TRIPALMITIN CRYSTALLIZATION IN SESAME OIL

The crystallization of tripalmitin (TP) in sesame oil (0.98, 1.80 and 2.62% w/v) was investigated under isothermal conditions using a cooling rate similar to the one achieved by industrial crystallizers (1 K/min) and utilizing a modification of the Avrami equation. It was concluded that z, the crystallization rate constant, and the associated energy of activation (Ea) values determined through Avrami's original equation yield erroneous results. Additional results indicated that the increase in supercooling at constant oil phase viscosity (e.g., 5.25–5.5 dynes/cm²) during crystallization, produced a higher degree of nucleation without an appreciable effect on TP crystal size. In contrast, as viscosity of the oil phase decreased at equal supercooling conditions (e.g., 22.0 - 22.5C) the main crystallization process evaluated through z and Ea was crystal growth. It was also concluded that the characteristic crystallization stages of induction time for nucleation, crystal nucleation and growth, and end of crystallization were evident in the viscosity profile of the solid fraction developed during TP crystallization. The predominance of any of these processes (i.e., nucleation or crystal growth) as a function of supercooling and viscosity of the oil phase determines, to a great extent, the rheological profile during crystallization and the final viscosity value of the crystal suspension.