Ageing of rice: A review

Highlights

• Overview of physicochemical changes in rice during storage.

• Comparison of accelerated ageing and natural ageing of rice with fresh rice.

• Treatments available for accelerated ageing of rice.

• Limitations and scope of work related to ageing of rice.

Abstract

Ageing is an intricate phenomenon that starts at pre-harvest and lasts until consumption; but incompletely understood till date. During ageing, significant changes occur in the physicochemical, sensory, cooking and pasting properties of rice. Most changes are time and temperature dependent and their effects on grain quality are irreversible; mostly being beneficial. Aged rice has better commercial value, owing particularly to improved milling yield, higher consumer preference in terms of cooked rice texture, flavor and associated parameters. Natural ageing through conventional processes requires undesirably long storage duration, in addition to associated higher operational and maintenance costs of the warehouse. Alternatively, artificial ageing of rice has been investigated by researchers to achieve similar results in lesser time and lower cost. This review discusses the changes that occur during ageing of rice, and the methods for artificial ageing. The exposition attempts to highlight a comparative evaluation of various techniques and their scopes. Important attributes to identify quality aged rice have also been included to provide an in-depth idea about the topic.

Introduction

Rice (Oryza sativa L.) is a major food crop and is the staple food of more than half of the world's population. It is a rich source of carbohydrates and a range of nutrients. India is the second largest producer of rice, next only to China, with over 43.39 million hectares of land under paddy cultivation, producing over 104.32 million tonnes annually. Rice is a staple food and feeds over 65% of the global population (Department of Agriculture, Cooperation & Farmers Welfare, 2017). It is mostly consumed after hydrothermal cooking, but raw rice is also an important ingredient in many processed foods across ethnicities of the world.

As is true for any food grain, rice has also a limited shelf life, and is therefore stored in warehouses. Interestingly, stored rice is preferred over raw rice, owing to an improved flavor perceived. Therefore, ageing of rice is one of the important steps in post-harvest processing of paddy. During storage, rice undergoes numerous changes in physicochemical properties that can be reportedly identified after three months of storage (Sodhi et al., 2003). Research indicates that changes in physical properties of rice largely depend on its variety, storage conditions and amylose content (Chrastil, 1990). Stages and type of processing of rice are influenced by factors such as variation in its protein and lipid contents, interaction between breakdown products of lipid oxidation and proteins, and by protein-starch interactions (Park et al., 2012).

Storage also has implications on cooking and eating quality of rice (Sodhi et al., 2003). Texture, pasting and thermal properties of rice are important attributes that are the signature of aged rice (Keawpeng and Venkatachalam, 2015, Prabhakaran and Moses, 2016). Textural properties include attributes of cohesiveness, hardness, tooth pull, roughness, particle size, tooth pack and looseness of rice grains. These properties are influenced majorly by storage duration. Studies have also revealed that storage of rice increases its ‘head rice yield’ (i.e. the ratio of mass of head rice obtained after milling to mass of paddy before milling) during milling. Also, ageing is known to aid in higher volume expansion and water absorption upon cooking; and in obtaining a harder and less sticky cooked rice (Chrastil, 1990).

The present review focuses on the changes that occur during ageing of rice, with special emphasis on methods for artificial ageing. These attributes of ageing of rice are important, and a systematic overview of the research findings on the same would aid improved design of storage structures and provide granular analysis of the areas that require further investigation.