|
|
|
||||||||
In Pursuit of Prime Quality Seafood
Superchilling As A Viable Alternative to Traditional Post Harvest Handling Techniques
SUPERCHILLING - History & Rational
Superchilling temperatures are commonly achieved by using a mixture of ice, water and salt to depress the freezing point. Increased salt concentrations in the tissue have been a question of concern regarding superchilling. Toledo-Flores (1988) found salt uptake by the flesh to be approximately 1% higher in fish superchilled in salt/ice mixture, compared to regular ice storage, however, taste panelist found the flavor of the superchilled fish to be superior to ice stored fish. Janke and Gooch (1997) found a similar salt uptake 0.62%, in superchilled rough scad.
Syneresis (drip) can be found in appreciable amounts in thawed, previously frozen fish muscle. In a comparison of drip in frozen versus superchilled Japanese anchovies, Kakuda and uchiyama (1983) found no detectable drip in superchiled samples while copious amounts were found in samples previously frozen at -30C. Examining structural changes that had occurred to mullet, Lee and Toledo (1984) found that structural changes due to ice crystals at -2C appeared to be minor compared to those that occurred during freezing at -20C as revealed by microscopic examination.
Examining yellowtail fish, Choi and Park (1991) did find breakdown of glycogen granules and some damage to the mitochondrial inner membrane in tissue stored at -3C, indicating that some deleterious changes do occur at these temperatures. However, in samples stored at -20C, broken fibrils were frequently found with most damage occurring at the Z-disk. This type of damage would manifest itself in a noticeable textural change, most likely softening.
The lesser extent of damage that occurs to muscle stored at -3C compared to -30C, as well as the difference in intercellular freezing suggested that -3C storage should not be classified as a freezing technique, hence the term "partial freezing". Examining enzymograms from samples at freezing and partial freezing temperatures, Gould (1970) found that latent enzyme activity occurred in both samples, a phenomena that does not occur in fresh fish. Her conclusion from this study was that superchilled fish should be labeled as "frozen fish". Ravazir-Shiraya (1990) were also able to show difference between 3 day yellowtail fillets stored at -3C and those on ice using a Torrymeter. Results in subsequent studies were not as conclusive (Ravazir-Shiraya, 1990).
The effect of superchilling on autolytic changes to the fish muscle were examined by Uchiyama and Kato (1974) using sea bass. Fish stored at -3C reached a K-value of 20% in 14 days, whereas samples stored on ice reached 58% after 5 days storage. In studies with mackerel, Cho et al. (1984) found K-values of 63% after 12 days storage at 0C compared to a value of 35% after the same number of days at -3C. The difference in K-value between 0C and -3C was remarkable due to the small variation in temperature. Date et al, (1984) examined sardines for changes in ATP and lipid oxidation using peroxide value (POV) and thiobarbituric acid value (TBA). After 8 days storage, sardines stored on ice reached a K-value of 34.3% while those at -3C were at 14.9%. Both measures of lipid oxidation were also found to be significantly lower in superchilled samples versus those stored on ice. Total volatile base nitrogen (TVB-N) values for sardines stored at -3C were approximately one-half the values obtained for product stored at 50C, 19.1 mg/100g versus 40 mg/100mg (Ehira et al., 1984). A threshold value of primary putrefaction is approximately 30 mg/100g.
Gutted samples were shown to store better than whole fish under superchilling conditions. Using rainbow trout, Fik et al., (1986) found that gutted fish had lower protease activity, less`rancidity, and higher sensory scores than whole fish stored at -2C. These researchers also found that trout stored under superchilling conditions (both whole and gutted) outperformed controls stored on ice.
In a recent interview with Dr. Norman Haard, who has written extensively on postmortem biochemical changes and prime quality, has stated that superchilling is superior to other methods for the short term storage of fresh seafood, if the temperature variations could be kept at less than 0.5C with a resting temperature of between -3C to -1C. Paul Dion working with Sensitech, Inc. temperature monitoring instruments has worked extensively at scaling up the superchilling process off the coasts of Chile and Oregon. He has documented only small fluctuations in temperature (+/- 0.5C) throughout the storage up to three weeks using his system. Dion (1996) has also documented rapid chilling rates as an integral part of superchilling. Fish that have been allowed to enter rigor prior icing at high ambient temperatures and chilled relatively slowly to low holding temperatures appear not to maintain "prime quality" texture of those fish handled shortly after harvest.
Return to HOME PAGE