Drying and Storage of Cereal Grains

B. K. Bala

• Publisher: Wiley-Blackwell
• Year: 2016
• ISBN:
  9781119124597
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

Finite Element Analysis and Computational Fluid Dynamics have been introduced in modelling and simulation of drying and storage systems, these techniques are expected to dominate the future research and development of drying and storages, and should reduce losses and improve the quality of agricultural products, enhancing food security globally.

Drying and Storage of Cereal Grains, Second Edition, covers the wide spectrum of drying and storage methods applied to economically important cereal produce, providingnumerical examples for better understanding the complexity in drying and storage systems through modelling and simulation, aiding design and management of drying and storage systems. Chapters 1 to 8 look at air and grain moisture equilibria, psychrometry, physical and thermal properties of cereal grains, principles of air flow, and provide detailed analyses of grain drying.Chapters 9 to 13 focus on temperature and moisture in grain storages, and provide comprehensive treatment of modern grain storage systems. The book also includes a number of unsolved problems at the end of each chapter for further practice.

This revised second edition includes new sections on -

• heat of sorption
• finite element modeling of single kernel
• CFD modeling of fluidized bed drying
• exergy analysis and neural network modeling
• numerical solution of two dimensional temperature and moisture changes in stored grain

This book will provide students in agricultural engineering and food engineering with a wide spectrum of drying and storage studies previously unavailable in a single monograph. It will also serve as an excellent reference for practicing agricultural engineers, food engineers and food technologists.

1
Principles of Drying

1.1 Introduction

Drying is a common activity which has its origin at the dawn of the civilization. It is interesting to note that the knowledge of how to dry and store crops developed enough before how to cultivate crops was discovered. But scientific studies on crop production started before such studies on drying and storage. However, considerable research has been done on drying but surprisingly limited research work on storage has been carried out.

Annual loss of grain from harvesting to consumption is estimated to be 10–25%. The magnitude of these losses varies from country to country. These losses are significantly high in the developing countries because of favourable climates which cause deterioration of stored grains and also because of lack of knowledge and proper facilities for drying and storage. Great efforts are being made to increase crop production, but until now little or no effort is being made to improve drying and storage facilities, especially in developing countries. Most developing countries are facing acute shortage of food and they need food, not production statistics. The post-harvest loss is proportional to production and increases with increased production. A programme to reduce drying and storage loss could probably result in 10–20% increase in the food available in some of the developing countries, and the increased food supply could be used for the nourishment of hungry people in the developing countries.

Drying and storage are a part of food production system consisting of two subsystems – crop production and post-harvest operation. Efficiency of the system can only be increased by a coordinated effort of a multidisciplinary team consisting of agriculturists, agricultural engineers, economists and social scientists for increased crop production and reduction of post-harvest losses. The reduction in post-harvest losses depends on the proper threshing, cleaning, drying and storage of the crops. A reduction in crop loss at one stage may have a far-reaching effect on the overall reduction of the loss. For example, overdrying of paddy will increase the storage life but it will also increase the breakage percentage of the rice during milling. This suggests that a systems approach is essential for increasing the efficiency of food production system. Food security can be increased through increasing production and reducing post-harvest losses of the crops (Majumder et al., 2016). This implies that considerable emphasis should be given not only on crop production but also on drying and storage process.

1.2 Losses of Crops

As mentioned earlier proper harvesting, drying and storage are essential to reduce losses of farm crops. Loss of harvested crops may be quantitative or qualitative and may occur separately or together. One of the basic problems in loss estimates is the definition of the term ‘loss’. The following brief descriptions are intended to demonstrate the different types of loss.

Weight Loss: Weight or quantity loss is the loss of weight over a period under investigation. There are two types of weight loss – apparent weight loss and real weight loss. Apparent weight loss is the loss of weight during any post-harvest operation under study. This loss does not consider the effect of the moisture content or the contamination with insects, fungi and foreign materials. The real weight loss is the apparent weight loss with the correction for any change in moisture content, plus dust, frass, insects and so on.

Nutritional Loss: Any loss in weight of the edible matter involves a loss of nutrients. Thus, weight loss can be used to estimate nutritional loss.

Quality Loss: Damaged grains and contaminants, such as insect fragments, rodent hairs and pesticide residues, within the grain cause the loss of quality, resulting in monetary loss. Similarly, changes in the biochemical composition, such as increase in free fatty acid content, may also rank as losses in quality.

Loss of Viability: Loss in viability of seed is one of the losses easiest to estimate and is apparent through reduced germination, abnormal growth of rootlets and shoots and reduced vigour of the plant.

Indirect Loss: Indirect losses involve commercial relationship which may not be quantified easily. This includes goodwill loss and social loss.

The crop losses discussed in the preceding text are mainly quantitative and qualitative losses. The major factors in quality loss appear to be from insect damage, damage by fungi, broken grain, dust and other foreign materials.

1.3 Importance of Drying

Drying has the following important advantages:

1. Drying permits the long-time storage of grains without deterioration of quality.
2. Drying permits farmers to have better-quality product for their consumption and sale.
3. Drying permits the continuous supply of the product throughout the year and takes advantage of higher price after harvesting season.
4. Drying permits the maintenance of viability and enables the farmers to use and sell better-quality seeds.
5. Drying permits early harvest which reduces field damage and shatter loss.
6. Drying permits to make better use of land and labour by proper planning.

1.4 Principles of Drying

Drying is the removal of moisture to safe moisture content and dehydration refers to the removal of moisture until it is nearly bone dry. Generally, drying is defined as the removal of moisture by the application of heat, and it is practised to maintain the quality of grains during storage to prevent the growth of bacteria and fungi and the development of insects and mites. The safe moisture content for cereal grain is usually 12–14% moisture on a wet basis.

Heat is normally supplied to the grains by heated air naturally or artificially, and the vapour pressure or concentration gradient thus created causes the movement of moisture from inside of the kernel to the surface. The moisture is evaporated and carried away by the air.

Drying capacity of the air depends on air temperature, moisture content of the grain, the relationship between the moisture content of the grain and the relative humidity of the drying air and grain type and maturity. The temperature of the drying air must be kept below some recommended values depending on the intended use of the grain. Safe maximum temperature of drying seed grains and paddy grains is 43°C, and for milling wheat the maximum recommended temperature is 60°C. Excessive high-temperature drying causes both physical and chemical changes and, especially in the case of rice, increases the percentage of breakage of whole rice and reduces the quantity and quality of rice. However, in cases of malt and tea, high-temperature drying is essential for desired physical and chemical changes for their ultimate use as drinks.

Reference

1. Majumder, S., Bala, B.K., Fatimah, M.A., Hauque, M.A. and Hossain, M.A. 2016. Food security through increasing technical efficiency and reducing post harvest losses of rice production systems in Bangladesh. Food Security, 8(2): 361–374.

Further Reading

1. Adams, J.M. 1977. A review of the literature concerning losses in stored cereals and pulses published since 1964. Tropical Science, 19(1): 1–28.
2. Bala, B.K. 1997. Drying and storage of cereal grains. Oxford & IBH Publishing Co, New Delhi.
3. Hall, C.W. 1980. Drying and storage of agricultural crops. AVI Publishing Company Inc, Westport, CT.

2
Moisture Contents and Equilibrium Moisture Content Models

2.1 Introduction

Moisture contained in a grain is an indicator o...