Freezing is one of the most important long-term preservation techniques for foods, and offers a means to suppress microbial growth and to preserve taste and nutritional value superior to any other method. However, during freezing, cold storage and thawing, changes in physical and chemical structures due to the formation of ice crystals, recrystallization or various chemical reactions can affect the quality of foods. The causes, types and degree of quality loss vary depending on the food items, and scientific reports on the mechanisms responsible for the quality loss as well as the technical solutions of pretreatment, freezing, storage and thawing technology have accumulated. Also, various analytical and observational technologies of ice crystal and tissue structure in foods have contributed to the development of effective freezing, storage and thawing technologies. The purpose of the present paper was to review the current state of studies on freezing and thawing technologies, and on evaluation technologies of physical and chemical properties of frozen foods, and to present future issues of the technologies concerned with frozen foods to enable their further development.

We examined the hyaluronidase-inhibitory activity of rhamnan sulfate derived from Monostroma nitidum, cultivated in Kukamoto Prefecture. Rhamnan sulfate containing 20% (w/w) sulfate groups was used in this study, and Lineweaver-Burk analysis indicated a competitive inhibition mechanism. Rhamnan sulfate was desulfated or hyper-sulfated to obtain rhamnan sulfate preparations with different sulfate group contents (0.5, 7, and 41% (w/w)). Subsequently, we examined the hyaluronidase-inhibitory activity of rhamnan sulfate with different sulfate content. Consequently, the rate of hyaluronidase inhibition by rhamnan sulfate increased in a sulfate group content dependent manner. The mixture of rhamnan sulfate and hyaluronidase produced a whitish, opaque material. IR and ¹H-NMR analyses identified the material as a rhamnan sulfate-hyaluronidase complex; the material is thought to have resulted from ionic interactions between the negative electric charge of the sulfate group and the positive electric charge of the lysine residues of hyaluronidase. In rhamnan sulfates with higher sulfate content, the formation yields of the complex increased and the degradation rates of hyaluronic acid in the complex-removed filtrate decreased. Thus, we presume that rhamnan sulfate and hyaluronidase form a water-insoluble complex through ionic interactions and that rhamnan sulfate inactivates hyaluronidase.

Imidazole dipeptides are generally present in animal foods as a mixture of anserine and carnosine. When ingested orally, both dipeptides are absorbed from the intestinal tract and immediately hydrolyzed to their constituent amino acids in blood. A reversed phase-HPLC method using acetonitrile solvent was developed for the quantification of imidazole dipeptides and their metabolites in blood. However, due to the overlapping of serum components and deproteinizing reagents, it was difficult to simultaneously determine the plasma levels of these compounds, especially that of 1- or 3-methyl histidine, using this system. To solve this, we modified the solvent and used an isocratic condition without acetonitrile to prolong the retention time of imidazole dipeptides and their metabolites. Consequently, this system was able to quantify imidazole dipeptides and their related metabolites without removal of the deproteinizing reagents. Further, we evaluated whether there are differences in plasma concentrations and bioavailability of different commercial products. Alterations in plasma levels of imidazole dipeptides and related compounds in 10 normal volunteers who orally ingested two different commercial products in a double-blinded and crossover study were measured using the modified method. Although the plasma levels of anserine were only 1/10 of 1-methyl histidine, the C_max of anserine was observed at 40 min after ingestion and was similar for both commercial products. There were no differences in the bioavailability and plasma levels of the related compounds of the two commercial imidazole dipeptides. These results indicate that the modified HPLC method was applicable for the simultaneous and precise determination of human plasma levels of imidazole dipeptides and their metabolites.