EFFECTS OF IRRADIATION AND STORAGE ON THE GAMMA-GLUTAMYL TRANSPEPTIDASE ACTIVITY OF GARLIC BULBS CV RED

The effects of 50 Gy gamma-irradiation 30 days after harvest on gamma-glutamyl transpeptidase (GTP) activity (the first enzyme in the catabolism of gamma-glutamyl peptides) of garlic bulbs cv 'Red' during storage for 300 days were evaluated. GTP activity was determined by spectrophotometry using gamma-glutamyl-p-nitroanilide as exogenous substrate, and was correlated with parameters related to the metabolic-respiratory activity, such as sprouting index in control bulbs, and cumulative weight losses (CWL) and non-enzymic or control pyruvate (CP; metabolite of the respiratory chain) in irradiated and control bulbs. GTP activity was also correlated with flavour parameters, such as enzymic pyruvate (EP; metabolite of the reaction of alliinase and sulphur amino acids in crushed garlic) and primary sulphur compounds. From these results, three storage stages are suggested: (i) the internal dormancy period, (ii) the first post-dormant stage, and (iii) the second post-dormant stage. During the first 90 days of storage (first stage) all the parameters remained nearly constant in the controls, while GTP activity and CP content increased in irradiated garlic because of radioinduced metabolic-respiratory activation. From 90 to 180 days of storage (second stage) the correlation between the increases of GTP and EP in irradiated garlic and controls was due to the action of GTP on gamma-glutamyl peptides that finally released substrates of alliinase. Both enzymes increased EP contact, which was higher in irradiated garlic (major flavour enhancement) than in controls. After 180 days of storage (third stage) EP and primary sulphur compounds decreased in irradiated garlic and in the controls, while GTP, CWL and CP kept increasing in both samples with lower rates of increase in irradiated garlic. These increases were related to metabolic activation, reserve exhaustion and finally rotting. Therefore, irradiated garlic was of better quality at the end of storage.