When vinegar is first added to warm rice, it gets more slippery and
then it sticks up when it cools. Is it the proteins being denatured in
rice with a H+ catalyst from vinegar?
No. Good speculation but, I believe the major cause of slipperiness is
simply the dilute water-starch mix at the cooked rice surface. Think paste.
Did you ever make flour paste? The water and flour are quite slippery
when dilute. Try heating corn starch with 3 volumes of water; it makes a
dandy lubricant - until it dries a bit; then it becomes sticky - just like
sushi rice.
Among our chemists, there was some speculation that the vinegar could cause
acid hydrolysis of starch producing slippier pectins and oligiosaccharides
(but I doubt it'd be significant).
I think the role of the vinegar is primarily for flavor and as a spoilage
retardant. For instance, the "tezu" used to keep the rice from sticking to
unwanted parts is made from 1 cup of water with 2 tablespoons of rice
vinegar and Ѕ teaspoon of salt. That's a 1:8 ratio of vinegar to water --
pretty dilute vinegar.
http://www.madsci.org/posts/archives/ju ... .Ch.r.html