Control of microbial and enzymatic changes in intermediate moisture sun-dried figs by mild heating and hydrogen peroxide disinfection

Abstract

During cold storage, the enzyme pectin methylesterase (PME) caused softening and lossof desired gummy texture in rehydrated intermediate moisture (IM) sun-dried figs. Heat inactivation studies indicated that the purified PME can be inactivated rapidly at 80 o and 90 oC. However, at or below 70 oC the enzyme showed activation by heating and inactivated very slowly. The in-situ activation of PME occurred much more extensively when sun-dried figs were rehydrated between 70o and 90 oC to produce IM figs with approximately 30 % moisture and this prevented the effective inactivation of enzyme even by rehydrations conducted at 80 o and 90 oC. The partial reduction of PME enzyme activity (almost 30 %) by rehydration of figs at 80 oC for 16 min may be used to delay undesirable textural changes in cold stored IM figs for 3 months. However, for longer storage periods hot reyhdration alone is not sufficient to prevent softening. No considerable yeast and mold growth was detected in IM figs cold stored 3-3.5 months.However, in some samples rehydrated in water at 80 oC, the total mesophilic aerobic counts and total yeast and mold counts showed a considerable increase when storage time exceeded 3-3.5 months. The rehydration of IM figs in 2.5 % H2O2 for 16 min at 80 oC reduced the total mesophilic aerobic microbial count of figs almost 90 %. Due to bleaching caused by H2O2, the brown fig color turned to a desirable and stable yellowlight brown as well. However, during cold storage the O2 gas released due to the decomposition of H2O2 by in situ fig catalase, accumulated within figs and caused some physical defects. Also, the residual level of H2O2 in the homogenates of disinfected figs was too much (300 ppm) and it seemed unlikely to eliminate this amount of H2O2 by physical or chemical means during processing. Pureeing IM figs eliminated residual H2O2 very rapidly. The application of rehydration first in 2.5 % H2O2 solution at 80 oC for 4 or 8 min and then in hot water at the same temperature for 12 or 8 min, respectively, also reduced the amount of residual H2O2 in IM figs considerably.Besides, these two-stage rehydration procedures eliminated the physical defects occurred in IM figs due to O2 gas release and gave firmer IM figs. To reduce the initial microbial load of IM figs, 4 and 8 min disinfections conducted in H2O2 solutions were less effective than 16 min disinfection in H2O2 solution. However, both 4 and 8 min disinfections effectively suppressed microbial load for at least 3.5 months and they may be used in the production of SO2 free light colored fig products.