Natural rennet sterilisation by non-thermic methods for fresh cheese manufacture

Natural rennet is widely used in artisanal cheese shops to produce fresh cheese with particular characteristics such flavour, in comparison to artificial rennet. This rennet is produced by the cheesemakers under non-hygienic conditions. The use of non-thermal technologies such as ultrasound and UV light can be an alternative of rennet sanitisation. The objective of the present work was to evaluate the effect of applying of different times of ultrasound or ultraviolet on the microbial growth in natural rennet for fresh cheese manufacture, its curd yield and curd hardness texture. Natural rennet was prepared and divided into three samples, one of which was exposed to ultrasound for 5, 10 and 15 min, the second one received UV radiation for 30, 60 and 90 min, and the last one was considered as the control treatment. A microbial test was conducted to analyse total coliforms, Escherichia coli, Staphylococcus aereus and Salmonella spp. Fresh cheeses were manufactured using the different rennets and the coagulant effect was tested by way of with a texture test (hardness). The results showed the highest microbial growth in the control treatment; none of the US treatments allowed microbial growth and the UV radiation was effective for all microorganisms at a 90 min exposure. All rennet treatments maintained their coagulant effect. In conclusion, the use of non-thermal methods was effective for rennet sterilisation, although it affected curd yield (2% less). Highlights Ultrasound inhibits pathogenic microorganisms growing in natural rennet. Sterilisation by non-thermal methods does not affect the coagulation process. A 42 kHz ultrasound at, 70 W power caused the best reduction of pathogenic bacteria.