Chinese ferment rice in making noodles - pH of fermented rice is low

Soaking

The length of time rice is soaked can range from short period (several hours) for unfermented noodles to long period (several days) for fermented noodles.

Natural fermentation occurs after rice is soaked for a long time.

The purpose of soaking for a short time is to allow water to penetrate into the rice kernel, which results in high moisture content and less rigid structure without initiating fermentation. It takes 2-3 hours in summer and 4-5 hours in winter for the moisture content to reach 26-30% at room temperature.

Soaking rice for a longer time is essential for producing naturally fermented rice noodles. During this procedure, rice is not only soaked to absorb water, but also to allow natural fermentation to occur. Normally, this takes 2-3 days in Summer and 4-5 days in winter. Fermentation is facilitated by various microbes, especially lactic acid bacteria (LAB) and yeasts.

The effect of fermentation by lactic acid bacteria and other bacteria is decrease in pH. The rice and soaking water have an initial pH of 7 (neutral), and this decreases to a pH of 4 when fermentation is complete. In a study by Lu et al, the pH of supernatant decreased to 4.2 after 24 hour and remained constant at 4.1.

The main organic acids in the supernant were lactic acid and acetic acid. Lactic acid was predominant, indicating that some of the LAB involved were heterofermentative. It has been reported that fermentation plays an important role in improving nutritional content, flavor and texture of rice noodles.

http://www.aaccnet.org/publications/plexus/cfw/Documents/CFW-60-3-0515.pdf