The combined effects of pressure levels and hypochlorite concentration
on the inactivation of microbial populations on sprouts
obtained from treated mung bean seeds compared to those from
untreated seeds, were investigated by examining the response surface
plots generated by holding constant temperature at 32.5 C, as
shown in Fig. 2a–d. The reduction of all microbial groups studied
increased with increasing pressure and hypochlorite concentration.
For moulds and yeasts reduction, the effect of hypochlorite
concentration was more pronounced at low pressures. In the same
way, the effect of the level of pressure was more important at low
hypochlorite concentrations. Reductions of total aerobic mesophillic
microorganisms, total and faecal coliforms counts ranged between
3 and 6 log units were shown by mung bean sprouts
developed from seeds soaked in 18000 ppm of calcium hypochlorite
and treated at pressures between 100 and 300 MPa (Fig. 2a–
c), compared to reductions of about 2 log units on sprouts from
seeds treated only by the chemical (results not shown). A previous
work carried out in our laboratory demonstrated reductions of
these microbial groups ranged between 1.2 and 3.4 log units in
sprouts from seeds treated at 32.5 C and 100–300 MPa (Peñas
et al., 2008). No significant differences were found on the reductions
on mould/yeasts populations (Fig. 2d) between the treatments
with 3000–18000 ppm of the chemical when seeds were
pressurised at 250–300 MPa. However, differences with the chemical
concentration were observed at lower pressures. The results
suggest that the combination of treatments, soaking the seeds with
18000 ppm calcium hypochlorite and further pressurisation at
250 MPa before sprouting seems to be the optimal for obtaining
sprouts with more than 5 log cfu/g reductions in all microbial
groups achieving studied, with the additional advantage of maintaining
a relatively high (about 80%) germination capability of
seeds subjected to these combined treatments. The results also
indicated the additive effect of pressure and chemical treatments
applied to seeds for improving microbial quality of developed
sprouts.