Effects of natural fermentation on the rice slurry properties related to rice paper production

Abstract

The objective of this work was to study the microbiological, chemical and physicochemical
changes during the natural fermentation process of rice slurry. Slurry for rice paper
processing was prepared from rice grain which contained at least 27% (w/w) amylose. Rice
(Surin1) was soaked in water for 12 h, wet-milled into flour, and then fermented for 1, 2
and 3 days. During this fermentation period, clear upper phase of water was changed every
day to control microbial load. The obtained rice slurry was then mixed with 1% NaCl before
steaming and drying (rice paper) in sunlight. The result of total viable count, yeast mold and
lactic acid bacteria count (from 5.39-9.39, 3.85-7.21, and 4.08-9.14 log CFU/g, respectively)
in the slurry increased with the increasing fermentation duration. This caused an increase
in total acidity (from 0.81-1.44% w/v as lactic acid) of the slurry but reduction of pH (from
4.56-3.53) and amylose content (from 36.49–35.72% w/w) through microorganism activity.
The enthalpy level (ΔHgel) (from 1.87-2.17 J/g) of the slurry consistently increased with
the fermentation time. Scanning electron microscopy revealed that starch granule surface
porosity increased with the fermentation time due to the starch hydrolysis and the corrosion
caused by enzymes and organic acids during fermentation. These resulted in the slurryviscosity
reduction (from 52.16-32.42 cP), which produced the rice paper of better texture.
With these results, the fermentation time during rice paper production could possibly be
shortened to modify rice paper properties in conventional settings.