Rice shinpaku structure or not?

Rice structure or not rice structure? Extending the review from last post with three more papers.

Key to sake production is the shinpaku or white opaque center. The shinpaku is generally important because of its increased ability to gelatinize during steaming, has a softer center for koji to invade once it breaks through the firmer outer layer, and is more highly convertible by the koji enzymes. All of this is true no matter the milling rate. For these reasons much of the breeding of new rice varieties has involved attempts to improve the significance of the shinpaku. However, as ginjo and daiginjo become more important types of sake, high millability is also becoming more important.

In particular, the papers analyze why some are better suited for high polishing levels while others are not. This characteristic is hugely important for ginjo and daiginjo styles of sake which have been on the rise in importance for some time now. This week I will look at three more articles on the same basic topic. However, before covering the additional papers, I would like to clear up a loose end from the last article.

The papers discussed in the last article, discussed the characteristic type of shinpaku each varietal has. These characteristic types include: non-lined (I think no-shinpaku would be a better name), lined, bellied, dotted, and ellipsoidal. As I mentioned in the article I was not sure what was meant by lined. Actually, I was thinking that it was a dense shell around the shinpaku. However, this was wrong. The lead author on the three papers, Masahiko Tamaki, sent me some nice pictures that make it clear. I don’t have permission to post the pictures so I will attempt to provide a diagram that gets the information across.

In the following diagram there are five characteristics represented with each having a side angle view of a rice kernel on the left and a cross section view looking down the long axis on the right. The first characteristic pattern, non-lined, represents rice without a significant shinpaku (white area in the center of the rice). Most eating rice has this characteristic. Next is the lined shinpaku which is the best structure for high milling levels. The bottom two bellied and ellipsoidal both have larger shinpaku but they are also more brittle. This gets in the way of milling and creates more broken rice. Dotted, is the final characteristic and is somewhere between non-lined and lined in terms of its suitability for high milling levels.

Rice kernel characteristics of shinpaku

OK, now that we have cleared up what was meant in each characteristic it is time to move on to the three additional papers. Masahiko Tamaki is the lead author for this set of papers as well. They are:

The first of these three papers looks at differences between rice kernels with shinpaku and without where the kernels are of the same variety. Two varieties (Senbon nishiki, Yamada nishiki) were analyzed each of which were grown in three different areas of Hiroshima (Shobara, Takamiya, Miwa).

The hardness of the center of each type kernel was measured with no significant difference between the three plots but quite significant difference between the shinpaku and non-shinpaku kernels within the same type. Other locations, four more, were also tested with similar results. In addition, starch characteristics were analyzed finding again, no significant difference between the shinpaku and non-shinpaku kernels. Given all this the conclusion continues to be that the differences in the structure of endosperm cells are responsible for all the properties of the shinpaku.

The second paper looks closer at the differences in hardness of the rice kernel and its shinpaku and their likelihood of breakage. They considered four types of rice: Kairyo omachi, Hattan nishiki No. 1, Senbon nishiki and Yamada nishiki. The first two of these have a significant higher breakage rate as compared to the second two. Very cool pictures taken with a Scanning Electron Microscope (SEM) show that Yamada and Senbon nishiki have much more tightly packed starch than does Kairyo Omachi and Hattan nishiki. This tight packing makes the rice harder and less susceptible to breakage.

Paper three analyzes the same rice varieties as paper two but compares starch properties rather than hardness. This study suggests that the shinpaku tissue may also play a part as well as the temperature of the early ripening period. This is, of course, in addition to the endosperm structure’s demonstrated impact on millability.