Effect of ultra high pressure homogenization and dimethyl dicarbonate on the inactivation of indigenous microorganisms in the litchi juice and its microbial shelf-life

The inactivation of indigenous microorganisms in the litchi juice, by individual or combined treatment of ultra- high pressure homogenization ( UHPH) and dimethyl dicarbonate ( DMDC) , was investigated in this study.Results indicated that 200MPa of UHPH treatment showed a higher inactivation to indigenous microorganisms in litchi juice, and the inactivation of indigenous microorganisms further increased with the increase of UHPH passes.While the residual microbial counts in the UHPH-treated litchi juice could recover to grow during storage of low temperature and its microbial shelf-life was less than 15d.As 250mg /L of DMDC was added to the litchi juice, indigenous microorganisms of litchi juice, especially yeast, showed a linear decrease.After 12h of DMDC addition, no further decrease in the microbial counts was observed, while the residual Leuconostoc strains recovered to grow during further storage of low temperature, and its microbial shelf- life was less than27d.Moreover, the yeast showed further decreased as 250mg /L of DMDC was added to UHPH-treated litchi juice, whereas the counts of total aerobic bacteria and lactic acid bacteria showed no significant reduction.But the microbial shelf- life of UHPH combined with DMDC was longer as compared with individual DMDC or UHPH treatment, reflecting DMDC and UHPH had additive effect.In addition, the residual microorganisms in the DMDCand UHPH-treated litchi juice were also identified using the methods of 16S rDNA or 18S rDNA, and it was found that the residual microorganisms were Bacillus, Leuconostoc and Mucor circinelloides strain.