Enzymatic Polishing of Brown Rice

Rice is the unique food grain sustaining two-thirds of the world’s population. It is nutritious and one of the cheapest sources of food energy and protein to the Asians. The way of rice consumption varies from place to place, country to country. Ironically in countries where rice is the principal food, the preference is generally for well-milled rice, which is lower in nutrient because the proteins, fats, vitamins and minerals, concentrated in the germ and outer layers of the starchy endosperm, are removed as bran during whitening or milling operation. On the other hand, brown rice contains more nutritional components, such as dietary fibres, essential amino acids, minerals, proteins, vitamins and other phytochemicals, than the ordinary milled rice. Largest proportion of the gross nutrients concentrated in the bran layers, outside the starchy endosperm of rice are removed during milling operation. Brown rice has lost its appeal due to its hard texture, nutty taste, dark color, higher cooking time and lower shelf life. Therefore, producing palatable rice, with minimum breakage, retaining maximum possible nutrients as well as simultaneously increasing the shelf life is the primary goal of rice processing industry.

Mechanical milling results in lower head rice yield with loss of major nutrients. To do away with the loss incurred during mechanical milling and to resolve the problems associated with brown rice, a novel attempt for an alternative polishing method has been developed to bio-polish the rice with some selected multienzymes having the catalytic properties to degrade the bran layer of rice. The enzymes are the selective biocatalysts, which are very specific to their targeted reactants. Thus through such processes the amount of by-product yield is minimum. These enzymes can be produced in large quantities through biotechnological route where different types of bioreactors can be used. Depending upon the nature of fermentation process, the reactor systems can be selected.

Such bio-polishing has been carried out on germinated brown rice to get softer, nutritious brown rice with the added benefits of germinated rice. Such type of novel approaches for polishing of germinated brown rice has not been reported earlier. The bio-polished rice has been found to be superior in every aspect when compared to milled rice.

Calorific and Nutrient Content of Brown and Milled Rice and Losses during Milling

Parameters	Enzyme treated rice	Milled rice
Protein (%)	7.68 ± 0.312	7.28 ± 0.294
Ash content (%)	0.97 ± 0.09	0.26 ± 0.18
Crude oil (% of bran)	16 ± 0.98	-
Free amino acid (mg equivalent of leucine /g of rice)	1.5 ± 0.093	1.1 ± 0.082
Crude fibre (%)	5.6
Total phenolic content µg equivalents of gallic acid	154	34
Calcium mg/100g of rice	2.92	0.91
Magnesium mg/100g of rice	14.75	13.59
Iron mg/100g of rice	1.92	1.36
Zinc mg/100g of rice	0.65	0.44
Thiamine mg/100g of rice	0.073 ± 0.012	0.032 ± 0.009
Pyridoxine mg/100g of rice	0.726 ± 0.025	0.072 ± 0.011
Niacin mg/100g of rice	0.834 ± 0.058	0.364 ± 0.028
Riboflavin mg/100g of rice	0.042 ± 0.01	-
Optimum cooking time (min)	28 ± 2	18 ± 2
Volume expansion ratio	3.22	4.00
Water uptake ratio	2.8	3.6
Grain elongation ratio	1.49	1.69
Hardness of cooked rice	59.7 ±1.68	56.0 ±1.52
Cost	Rs 100-110/kg*	Rs 55-60/kg

*Calculation done based on the laboratory data and reagents used

Salient features of development

• No loss in head rice yield
• Nutrient rich than milled rice
• Fibre content more than milled rice
• With benefits of germinated brown rice
• Softer with less cooking time than brown rice
• Enzyme treated rice has been found to have better self-life as compared to the non-treated rice
• Enzymatically treated rice possessed higher antioxidant potential, free amino acids, crude fibre and phenolics as compared to the non-treated rice

Brown rice has lost its appeal due to its

• Nutty taste
• Undesirable color
• Longer cooking time
• Difficult for digestion
• Poor storability

Major Disadvantages of Milling

• A lot of weight loss is resulted (~7-9%) with the major loss of vitamin – Thiamine (~80%)
• Low head rice recovery
• Significant loss of Calcium, Magnesium, Iron and Zinc

Advantages of enzymatically polished rice

• Mechanical damage to rice kernel can be avoided
• Parboiling process can be skipped
• Nutrient loss or weight loss can be avoided
• Selective bio-polishing has helped to retain the mineral with minor losses in Magnesium, Iron and Zinc, while Calcium content increased by 10%.
• Enzymatic treatment exhibited more positive effect on water uptake ratio and volume expansion ratio.
• Less cooking time
• Easier for digestion

Suggested minimum plant capacity: 1500 MT per annum
Likely investment: ~ Rs.5 crores

This technology has been filed for Indian patent. For further information, please contact:

Prof. R. Banerjee
Microbial Biotechnology & Downstream Processing Laboratory
Agricultural & Food Engineering Department
Indian Institute of Technology Kharagpur – 721 302
Phone Number: + 91-3222-283104/283105(R)
Email: rb@iitkgp.ac.in / rin_tuin@yahoo.com