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Chemistry and Structural Composition of Glutinous Rice

Amylose and amylopectin are both components of starch and they are both polysaccharides made from D-glucose units. The big difference amongst the two is that amylose is linear because it only has αlpha-1,4-glycosidic bonds. Amylopectin on the other hand, is a branched polysaccharide because it has αlpha-1,4-glycosidic bonds but with occasional αlpha-1,6-glycosidic bonds approximately every 22 D-glucose units . Glutinous rice is nearly 100% composed of amylopectin (one of the two components that makes up starch) and almost completely lacks its counterpart, amylose, in its starch granules. A non-glutinous rice grain contains amylose at about 10-30% weight by weight and amylopectin at about 70-90% weight by weight . For example, “potato starch, banana, barley, oat and wheat have 20%, 16%, 22%, 27% and 26% of amylose respectively” . Glutinous or waxy type of starches happen to occur in maize, sorghum, wheat, and of course, rice. An interesting characteristic of glutinous rice is that it stains red when iodine is added. Non-glutinous rice stains blue . This phenomena occurs when iodine is mixed with iodide to form tri-iodide and penta-iodide. Penta-iodide intercalates between the starch molecules and stains amylose and amylopectin blue and red respectively. The gelation and viscous texture of glutinous rice is because amylopectin is more hygroscopic than amylose, so it has a higher affinity for water, and thus water enters the starch granule and swells, while the amylose leaves the starch granule and becomes part of a colloidal solution . In other words, the higher the amylopectin content, higher the swelling power of the starch granule Even though the amylopectin content plays a major role in the defined characteristic of viscosity in glutinous rice; factors such as heat also play a very important role in the swelling since it enhances the uptake of water into the starch granule significantly. It is estimate that the swelling increases at about 10% in volume per 10ºC temperature increase .

[[File:Structure of amylose and amylopectin.jpg|thumb|Structure of amylose and amylopectin. Amylose is linear while amylopectin is branched.]]

https://commons.wikimedia.org/wiki/File:Structure_of_amylose_and_amylopectin.jpg

The high amylopectin content of waxy or glutinous starches genetically controlled by the waxy or wx gene. Its quality of greater viscosity and gelation is dependent on the distribution of the amylopectin unit chains . Grains that have this gene are considered mutants and this is why most of them are selectively breed to create a grain that is close to or in the 0% of amylose content . The table below summarises the amylose and amylopectin content of different starches, waxy and non-way:

Table 1. Proportion of Amylose and Amylopectin in various Starch Sources

Starch	Amylose %	Amylopectin %
Potato	20	80
Sweet Potato	18	82
Arrowroot	21	79
Tapioca	17	83
Corn (maize)	28	72
Waxy maize	0	100
wheat	26	74
Rice (long grain)	22	78

During the soaking of the glutinous rice, and elemental step in the preparation of mochi, either traditionally or industrially. Glutinous rice decreases in protein content when it is soaked in water. The chemicals that make up the flavour of plain or "natural" mochi are ethyl ester acetic acid, ethanol, 2-butanol, 2 methyl 1-propanol, 1-butanol, 3-methyl 1 butanol, 1-pentanol and propane acid

Ingredient list of mochi is usually composed only of glutinous rice, however, some variations of mochi may include the addition of salt, spices and flavourings such as cinnamon (cinnamaldehyde) . Food additives such as sucrose, sorbitol or glycerol may be added to increase viscosity and therefore gelatinization increase. Additives that slow down retrogradation are not usually added since amylopectin has a very stable retrogradation shelf life due to its high amylopectin content .