Iron, zinc and phytic acid in rice from China: wet and dry processing towards improved mineral bioavailability

Rice and rice products supply two thirds of Chinese people with their staple food. Mineral deficiencies, especially of iron and zinc, are prevalent in China, and are caused by insufficient intake and poor bioavailability. Rice and rice products contribute more than 50% of the antinutrient phytic acid consumed in the average diet, which has a significant negative impact on mineral bioavailability. This thesis reports studies of dry and wet rice processing methods on levels and in vitro solubility of minerals, and of phytic acid with the aim to contribute to the improvement of the bioavailability of minerals in rice-based diets. Iron levels in 56 rice varieties from China ranged 9-45 mg kg^-1, zinc 13-39 mg kg^-1, and phytic acid ranged between 7.2-11.9 g kg^-1. Molar ratios of phytic acid to minerals indicated that the bioavailability of minerals in brown rice will be < 4%. Dry processing by abrasive milling showed that short-, medium- and long-grain varieties suffer different losses of yield during abrasive removal of the bran. Phytic acid and iron are gradually removed at similar rates, while zinc is retained better. This results from the different spatial distribution: the highest concentration of iron and phytate was observed in the testa and the interface of embryo and perisperm, whereas zinc was concentrated in the endosperm and in the embryo. This offers limited opportunities for optimization of abrasive milling for maximum phytate removal, at minimum losses of yield and minerals. Wet processing (soaking under various conditions, phytase application, fermentation, and germination) revealed that physical structure (intact or damaged kernels) affects migration of components into the water phase, and that phytase and acidic conditions favour the removal of phytic acid. Simultaneously, minerals may be lost by diffusion into the water phase. Although molar ratios of phytic acid to iron and zinc indicated higher in vitro solubilities, these improvements were hardly significant. An exception is the promising dephytinization of bran by phytase treatment while extracting significant levels of minerals. This treatment should receive further investigation because it potentially offers a way to valorize the by-product bran into a concentrate of soluble minerals. Rice products available in supermarkets in China were investigated for their levels of phytic acid and minerals, and mineral solubility. Brown rice had much higher calcium, iron and phytic acid levels than white rice, but mineral solubility in both types of products was equally low. Rice noodles and rice crackers had low phytic acid and varying mineral contents and high solubility of especially calcium. In all product categories - primary and intensively processed - residual phytic acid levels were too high and mineral levels insufficient to be considered as good dietary sources of minerals. There is scope for better rice-derived noodles or crackers that are dephytinized, and preferably fortified with minerals. The implications for the food chain are discussed. In the short term, dietary intervention by mineral supplementation and food dephytinization and fortification should be considered. On the longer term, selection and breeding of new rice varieties attuned to local agricultural practice, and nutrition education of the general public to stimulate a balanced diet and healthy lifestyle, will be sustainable solutions for the future.