Detection of fraudulent addition of whey cheese in milk: Interference of bacterial proteases activity

Abstract

One of the most common economic frauds applied to fluid milk is the addition of the cheese whey. The occurrence this fraud, in Brazil, is suggested by the analysis of the caseinomacropeptide (CMP) index, a portion of the ê-casein molecule soluble in the whey. However, the action of proteases produced by psychrotrophic microorganism can interfere with this test, leading to false-positive results, situation that started to have greater importance after the implementation of Normative Instruction 51 (2002). With the objective to evaluate this interference, Pseudomonas spp. was inoculated to a final concentration of ca 10³ CFU/mL in milk, followed by incubation for 2 and 5 days at 7ºC. The microbial growth was monitored and, after these incubation periods, milk was submitted to thermal treatment (100 ºC/5 min) for elimination of the bacterial cells and the enzymatic extract was obtained for posterior analysis of proteases activity. After that, milk was submitted to incubation 30 ºC/30 d and the CMP index was analyzed at times zero and 30 days. The results of the proteases activity tests at the 2^nd day incubation, using azocasein as substratum, not differed significantly (P > 0,05) from the 5^th day. Differently, the CMP index at the zero time of the treatment 5d/ 7 ºC for the milk inoculated with P. fluorescens, were higher than the same milk frauded with 30% of cheese whey, denoting the necessity to control even more, this important spoilage organism. After 30 days there was a reduction of these indexes, possibly as a result of the CMP degradation by the same proteases produced by P. fluorescens.