Lipid deteriorations of freeze-dried fish meats at different equilibrium relative humidities.

Abstract

The rates of lipid deteriorations of freeze-dried big-eye tuna, a typical red muscle fish, and halibut, a typical white muscle fish, were studied as a function of moisture equilibrium relative humidity at 25°C. At relative humidities of 0 and 11%, corresponding to below the monomolecular layer of water, the lipids of both fish underwent oxidation, as estimated by TBA value measurements. The rates of oxidation were faster in the big-eye tuna than in the halibut. Furthermore, the triglycerides of halibut were hydrolyzed enzymatically even at a relative humidity of 11%, though no hydrolysis of the triglycerides of big-eye tuna was detectable at the same relative humidity. On the other hand, at higher relative humidities, such as 52 and 71%, neither fish underwent oxidative deterioration. However, the lipids were hydrolysed enzymatically during storage, with the exception of the phospholipids of big-eye tuna at 52% r. h. The hydrolyses of lipids were more significant in the halibut than in the big-eye tuna.