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Mochi (餅?) is Japanese rice cake made of mochigome, a short-grain japonica glutinous rice. The rice is pounded into paste and molded into the desired shape. In Japan it is traditionally made in a ceremony called mochitsuki. While also eaten year-round, mochi is a traditional food for the Japanese New Year and is commonly sold and eaten during that time. Mochi is called môa-chî (麻糬) in Taiwan.
Mochi is a multicomponent food consisting of polysaccharides, lipids, protein and water. Mochi has a heterogeneous structure of amylopectin gel, starch grains and air bubbles. This rice is characterized by its lack of amylose in starch and is derived from short or medium japonica rices. The protein concentration of the rice is a bit higher than normal short-grain rice and the two also differ in amylose content. In mochi rice, the amylose content is negligible which results in the soft gel consistency of mochi.
Traditionally, mochi was made from whole rice, in a labor-intensive process. The traditional mochi-pounding ceremony in Japan is Mochitsuki:
- Polished glutinous rice is soaked overnight and cooked.
- The cooked rice is pounded with wooden mallets (kine) in a traditional mortar (usu). Two people will alternate the work, one pounding and the other turning and wetting the mochi. They must keep a steady rhythm or they may accidentally injure one another with the heavy kine or by a mochi machine.
- The sticky mass is then formed into various shapes (usually a sphere or cube).
Mochi can also be prepared from a flour of sweet rice (mochiko). The flour is mixed with water to a sticky opaque white mass that is cooked on the stovetop or in the microwave until it becomes elastic and slightly transparent.
Popular uses for mochi
Many types of traditional wagashi and mochigashi (Japanese traditional sweets) are made with mochi. For example, daifuku is a soft round mochi stuffed with sweet filling, such as sweetened red bean paste (an) or white bean paste (shiro an). Ichigo daifuku is a version containing a whole strawberry inside.
- Oshiruko or ozenzai is a sweet azuki bean soup with pieces of mochi. In winter, Japanese people often eat it to warm themselves.
- Chikara udon (meaning "power udon") is a dish consisting of udon noodles in soup topped with toasted mochi.
- Zōni. See New Year specialties below.
New Year specialties
- Kagami mochi is a New Year decoration, which is traditionally broken and eaten in a ritual called Kagami biraki (mirror opening).
- Zōni is a soup containing rice cakes. Zoni is also eaten on New Year's Day. In addition to mochi, zoni contains vegetables like taro, carrot, honeywort and red and white colored kamaboko.
- Kinako mochi is a mochi dish that is traditionally made on New Year's Day for luck. This style of mochi preparation includes roasting the mochi over a fire or stove, then dipping it into water, finally coating with sugar and kinako (soy flour).
- Dango is a Japanese dumpling made from mochiko (rice flour).
- Warabimochi is not true mochi, but a jelly-like confection made from bracken starch and covered or dipped in kinako (soybean flour) with sugar. It is popular in the summertime, and often sold from trucks, not unlike ice cream trucks in Western countries.
- More recently, "Moffles" (a waffle made from a toasted mochi) has been introduced. It is made in a specialized machine as well as a traditional waffle iron.
Viscoelasticity of Mochi
Mochi's characteristic chewiness is due to the polysaccharides in it. The viscosity and elasticity that account for this chewiness are affected by many factors such as the starch concentration, configuration of the swollen starch granules, the conditions of heating (temperature, heating period and rate of heating) as well as the junction zones that interconnect each polymer chain. The more junction zones the substance has, the stronger the cohesiveness of the gel, thereby forming a more solid like material. The perfect mochi will have the perfect balance between viscosity and elasticity so that it is not inextensible and fragile but rather extensible yet firm. 
Many tests have been conducted on the factors that affect the viscoelastic properties of mochi. As puncture tests show, samples with a higher solid (polysaccharide) content show an increased resistance and thereby a stronger and tougher gel. This increased resistance to the puncture test indicate that an increase in solute concentration leads to a more rigid and harder gel with an increased cohesiveness, internal binding, elasticity and springiness which means a decrease in material flow or an increase in viscosity. These results can also be brought about by an increase in heating time. Sensory assessments of the hardness, stickiness and elasticity of mochi and their relationship with solute concentration and heating time were also performed. Similar to the puncture test results, sensory tests also determine that hardness and elasticity increase with increasing time of heating and solid concentration. However, stickiness of the samples increase with increasing time of heating and solid concentration until a certain level, above which the reverse trend is observed. It is important to understand these relationships because too hard or elastic a mochi is undesirable, as is one that is too sticky and will stick to walls of the container.
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Similar foods in other countries
- "Mochitsuki: A New Year's Tradition". Japanese American National Museum.
- Isono, Yoshinobu; Emiko Okamura; Teruo Fujimoto (1990). "Linear Viscoelastic Properties and Tissue Structures of Mochi Cake". Agric. Biol. Chem. 54 (11): 2941–2947. doi:10.1271/bbb1961.54.2941.
- Bean, M.M; Esser, C.A. and Nishita, K.D. (1984). "Some Physiochemical and Food Application Characteristics of California Waxy Rice Varieties". Cereal Chemists 61 (6): 475–479.
- "Not-So-Stressful Microwave Mochi". The Fatty Reader.
- Itoh, Makiko, "Rice takes prized, symbolic yearend form", Japan Times, 30 December 2011, p. 14.
- Kapri, Alka; Suvendu Bhattacharya (2008). "Gelling behavior of rice flour dispersions at different concentrations of solids and time of heating". Journal of Texture Studies 39: 231–251. doi:10.1111/j.1745-4603.2008.00140.x.