Abstract: The purpose of this experiment was to study the relationship between the formation of N-nitrosamine and
proteins in repeatedly frozen-thawed pork. Two model reaction systems in vitro were designed. Model system Ⅰ contained
NaNO2 only, while model system Ⅱ contained DEA·HCl and NaNO2. Muscle proteins (Mep), sarcoplasmic proteins (SP)
and myofibrillar proteins (MP) with different degrees of oxidation from pork subjected to different freeze-thaw cycles
(0, 1, 2, 3, 4, 7 and 10 cycles) were added separately to each of the two systems for nitrosation reaction, and the formed
N-nitrosodiethylamine (NDEA), thiobarbituric acid reactive substance (TBARS), carbonyl and sulfhydryl were determined.
The results revealed that the NDEA formation in model system Ⅱ was 20–80 times higher than in model system Ⅰ,
indicating that the substrate DEA·HCl had a significant impact on the formation of NDEA (P < 0.05). After four freezethaw
cycles, a significant increase in carbonyl content and a significant decrease in sulfhydryl content were observed for
Mep, SP and MP (P < 0.05), suggesting the significant oxidization of all these proteins. Among these proteins, SP were
oxidized fastest and to the largest extent to form the most NDEA, followed by Mep, and MP were oxidized to form the least
NDEA. Our results demonstrated that repeated freeze-thaw cycles could induce protein oxidization and consequent NDEA
formation. Therefore, temperature fluctuation should be controlled during frozen storage of meat.

Key words: porcine longissimus dorsi, freeze-thaw cycles, protein oxidation, N-nitrosodiethylamine