|MadSci Network: Chemistry|
Let's break this into three questions with answers following: 1) 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 ½ teaspoon of salt. That's a 1:8 ratio of vinegar to water -- pretty dilute vinegar. 2) Why is it necessary to add sugar? It's not according to some of the web pages I checked. Again, I believe the sugar is simply for flavor. The combination of sweet and sour (even dilute) is popular in many cuisines. 3) why does the short rice coated with glucose provide stickier results than with brown or long grain? I dunno the "why" but short grained rice is more glutenous than longer grain rice. I suspect it's a surface to volume ratio phenomenon. For brown rice, the bran is highly linked starch e.g. cellulose. The bran doesn't hydrate and thus, doesn't become slippery when wet nor sticky when dry. Here's a few web pages I ran into researching your question: http://sushiref.com/glossary/sushi:sushi.html http://ftp.neosoft.com/recipes/rice/sushi-rice02.html http://www.stickyrice.com/html/rice.html http://ociialf.org.chemie.tu-muenchen.de/people/ralph/cooking/Sushi.html http://www.netscum.com/~spider/food/sushi.html
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