Detecção da adição fraudulenta de soro de queijo em leite: interferência da atividade de proteases bacterianas

Abstract

One of the most common economic frauds applied to fluid milk is the addition of cheese whey. The analysis of the caseinomacropeptide (CMP) index, a portion of the κ-casein molecule soluble in the whey, is used as an indicator of the occurrence of this fraud in Brazil. However, the action of proteases produced by psychrotrophic microorganisms can interfere in this test, leading to false-positive results, situation that started to have greater importance after the implementation of Normative Instruction 51 (2002). Actually, great part of the milk produced is kept refrigerated for two days or more before being processed, fact that contributes to the selection and proliferation of these organisms. 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 7 ºC. 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, the milk was incubated at 30 ºC/30 d and the CMP index was analyzed at times zero and 30 days. Additionally, the same milk was frauded with increasing volumes of cheese whey and the CMP index was analyzed. P. fluorescens strains growth average was 3 and 4 cycles log, after 2 and 5 d of incubation at 7 ºC, respectively. The results of the proteases activity tests at the 2nd day incubation, using azocasein as substratum, not differed significantly (P > 0,05) from the 5th 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.