Reduction of Tetrazolium Salt XTT with UHT-Treated Milk: Its Relationship with the Extent of Heat-Treatment and Storage Conditions

UHT-treated milk reduced 3'-{1-[(phenylamino)-carbonyl]-3,4-tetrazolium}-bis(4-methoxy-6-nitro)benzenesulfonic acid hydrate (XTT). The reducibility was caused by the Maillard reaction intermediate between milk protein and lactose. First, the assay procedure of the XTT reduction was optimized using the heated model solution containing casein and lactose and UHT-treated milk. When the XTT reduction assay was applied to two kinds of UHT-treated milks under optimum conditions, the ability of each milk sample to reduce XTT significantly reflected the extent of the heat treatment as well as the evaluation method using the hydroxymethylfurfural (HMF) value. In contrast to the HMF value, the ability of UHT-treated milk gradually decreased depending upon the storage period and temperature. These results suggest not only that the present XTT assay is applicable to estimate the extent of heat treatment but also that the reducibility of the milk sample heat-treated under a given condition can serve to estimate the storage period if the storage temperature is known or vice versa. The method is much simpler (only mixing of the XTT solution containing menadione with milk sample) and quicker (about 20 min) with a satisfactory reproducibility than the conventional methods to estimate the extent of the Maillard reaction such as the lactulose or HMF determination.