Water is a principle factor determining the texture and quality of food. Water distribution in spaghetti during cooking and standing, and in rice grains during water soaking, were investigated using MRI. Formation of a moisture gradient was observed in a cross-section of boiled spaghetti strands, as a result of water diffusion from the surface to the core with starch gelatinization. Homogenization of water distribution was observed during standing of the boiled spaghetti. Water distribution in five types of cooked spaghetti was compared using MR images and water distribution profiles, enabling quantitative analysis of water diffusion. As for milled rice grains, water first penetrated the embryo attachment site and the surface of the ventral side of the endosperm, then migrated along the central line and transverse cracks, and finally diffused to all parts of the endosperm. The central part of the endosperm allowed more water to penetrate. In milled rice grains of the cultivar Yamadanishiki, which has a white core, water quickly infiltrated into the cracks or chalky parts on the dorsiventral line and then diffused to the lateral side of the grain. The route, pattern and speed of water penetration are determined by the morphological structure, crack formation and hardness distribution associated with the filling of starch granules in the grains.
Retrogradation is a general term for the behavior of recrystallization of gelatinized starches on cooling and storage. This retrogradation is often enhanced when subjected to freezing and thawing treatment. However, few reports have dealt with the effect of additives on the freeze-thaw stability of starch. Therefore, the effects of polyols and emulsifiers on the freeze-thaw stability of starch were investigated using. The polyols and emulsifiers were added at concentrations of 5.0% (w/w) and 0.01% (w/w), respectively, to starch paste (potato starch content, 5.5% (w/w)). The temperature in the chamber was maintained at 30°C for 2 hours. Then, the chamber was cooled to -20°C at a cooling rate of -1.3°C · min-1, and the sample was frozen and stored at the same temperature for 24 hours. The sample was thawed in the chamber at a heating rate of 1.3°C · min-1 to 30°C, and thereafter maintained for 3 hours. The use of the two food additives was successful in improving the freeze-thaw stability of starch. It follows that these effects might be attributed to the water-structure formation effect of polyols and the complexation of starch chains with emulsifier molecules. Demulsification occurs by freeze-thaw processing, and fats and oils tend to separate from oil-in-water (O/W) emulsions. It is necessary to improve the freeze-thaw tolerance of emulsifications for the production of frozen emulsified foods. Therefore, the effects of polyols and emulsifiers on the freeze-thaw stability of O/W emulsions were studied. To clearly understand the influence of freeze-thaw processing on the emulsification stability of O/W emulsions (mass ratio: 0.099 oil, 0.896 water, 0.005 emulsifier), emulsions were prepared using fully hydrogenated palm oil, which has proven difficult in preparing emulsions with excellent emulsification stability. The temperature in the chamber was maintained at 25°C for 1.5 hours. Then, the chamber was cooled to -30°C at a cooling rate of -2.0°C · min-1, and the sample was frozen and stored at the same temperature for 24 hours. The sample was thawed in the chamber at a heating rate of 2.0°C · min-1 to 10°C, and thereafter maintained for 4.5 hours. Lipophilic sucrose esters of stearic acid (SES) produced a more stable emulsion. The monoester in SES might migrate to the oil-water interface from the oil phase, and be adsorbed. Di-/tri-esters in SES might penetrate into the oil phase and form tightly packed interfacial layers with monoesters. Polyesters in SES may act to accelerate crystallization and hinder the polymorphic transformation of oil. It appears that the content ratio of monoester/di- and tri-esters/polyesters in SES plays an important role in improving the freeze-thaw tolerance of O/W emulsions. Polyols enhanced the freeze-thaw stability of a soft-serve ice cream mix. This effect might be attributed to the water-structure formation effect of polyols.