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Overview of grain drying in China

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Wang Hanbin
Section Chief, Grain Storage Division,
Grain Storage and Transportation
Bureau,
Ministry of Commerce,
People's Republic of China

Abstract

There are many links in the grain postharvest chain, including threshing, cleaning, drying, storage, transportation, and processing. Grain drying is one of the key links in terms of minimising postharvest losses, since it directly affects safe storage, transportation, and processing quality, as well as distribution.

Great losses to stored grain occur every year around the world. For instance, moulding during storage and transportation, deterioration in quality and an increase in brokens and cracked kernels can be attributed to delayed or improper drying.

China is one of the world's largest grainproducing and consuming countries. To minimise grain postharvest losses, particularly in the case of wet grain, has always been a key issue in the management of grain. The following introduction addresses three aspects of the problem.

The geographic distribution of wet grain in China

China is a vast country, spanning thousands of kilometres from north to south. Geographic and climatic conditions vary from place to place, and so do the varieties of grain. This has resulted in a wide range of grain moisture contents as well as postharvest technologies in China.

Wet grains are mainly maize and paddy, two major crops in China, with very small quantities of soybean, rapeseed, and wheat. They are geographically distributed in the following provinces, cities, and autonomous regions: Heilongjiang, Jilin, Liaoning, Inner Mongolia, Beijing, Hebei, Anhui, Xinjiang (Sinkiang), Jiangsu, Hubei, Jaingxi, and Zhejiang.

Maize is produced mainly in the north of China, with high yields but longer maturing periods. The weather is not favourable, with a short no-frost period, low temperatures during the maize maturing period, and low hours of sunshine, which results in huge amounts of wet grain after harvest.

When maize comes into the state grain depots, its moisture is normally as high as 2226%. In the years when frost comes earlier, it can be as high than 30%, and sometimes over 40% in some places. Every year, the grain depots in the north receive wet maize in very large quantifies.. To reduce the moisture content to a safe storage level is a prime activity in the depots during the whole winter period.

Paddy is produced mainly in the south of China. Late-season paddy harvested there is wet because of higher rainfall and relative humidities at harvest time.

Most paddy is harvested manually and sun dried in the field for one or two days after harvest (depending on the weather), so the moisture content of wet paddy is not as high as that of maize in the north. Normally, it is between 16 and 18%. In a bad year it could be around 20% and even 24%, when drying becomes imperative before long-term storage and transportation.

In terms of quantity, the amount of wet grain procured by the government is around 20 million tonnes, about one quarter of the total state procurement. This is made up of about 15 million tonnes of maize, with the rest mainly paddy. The wet grain is more concentrated in the north-east, where 70% of the total procurement is wet maize. In recent years, grain output has increased markedly and so too has the amount of wet grain.

Methods of grain drying

Generally speaking, grain drying can be classified into sun drying and mechanical drying.

China is a large agricultural country at a low level of agricultural mechanisation. particular, mechanical grain drying capacity cannot keep pace with the increase in grain production. At present, only about 30% of wet grain in state depots is mechanically dried; the rest has to be sun dried. All grain retained at farm level for consumption by farmers is sun dried.

Sun Drying

As is well known, sun drying has greater requirements of labour and space, especially in the case of large-scale and centralized treatment Although labour is relatively cheap in China, the cost of commercial-scale sun drying is still very high.

In addition, sun drying depends very much on the weather, and takes more time. If there are long spells of bad weather, there is a high risk of grain losses. Also, the handling losses during sun drying are not insubstantial.

However, sun drying does have some advantages. There appears to be no appreciable reduction in grain quality associated with the process, and grain can be kept fresh, of good colour, and free of contaminants.

There are many ways of sun drying in China. The most popular method is to spread wet grain on the ground, turning it from time to time to remove extra moisture.

In the north-east, farmers usually put maize cobs into hubs. Moisture is removed by natural aeration during storage. Farmers also take measures before maize is harvested. When it is in its waxy ripening stage, farmers tear the husks off the cobs for sun drying while on the stalk. This method not only removes moisture but also promotes maturity, thus increasing yield. Under the climatic conditions of the north-east, it takes about 10 days to reduce maize moisture by 4%.

Clearly, the main disadvantages of sun drying are that it is more labour intensive and takes longer. Some local governments give farmers incentives to encourage sun drying at farm level so as to solve the problem of insufficient capacity in state depots. This has not been successful to date.

Mechanical Drying

For mechanical drying, design and manufacture of grain dryers started in the late 1950s. However, early development was not rapid enough. In the late 1970s and early 1980s, successive bumper harvests created a large grain drying problem. Mechanical means of grain drying have been developed more rapidly since then.

Most grain dryers are designed and manufactured by local Research and Design Institutes and end users. There are three main types of grain dryers: tower dryers; rotary drum dryers; and fluidised-bed dryers.

Tower dryers are very popular for maize drying in the north. They can be classified as direct or indirect dryers. In the direct dryers, the heated gas is in direct contact with the grain so giving better drying efficiency.. Anthracite coal is required as fuel to minimise contamination.

Indirect dryers normally use heat exchangers. Since there is no contamination possible, there are no special fuel requirements. However, the energy losses are high during heat exchange.

The capacities of tower dryers using coal as fuel are around 10-20 tonnes per hour with moisture removal of about 10% for each pass.

Rotary drum dryers and fluidised-bed dryers are popular in the south for paddy drying, with capacities ranging from 5 to 15 tonnes per hour and moisture removal of about 3%. They use mainly paddy husks and coal as fuels. Producergas is also used as a heat resource for grain dryers in the south. It can provide stable heat and maintain grain quality, but its use increases the cost of drying.

The grain dryers using solar energy, infrared radiation, and microwaves were tested in the early 1980s, at an experimental level. The technical and economic data collected were not promising for potential commercial utilization.

Apart from dryers, when we say mechanical drying, we should include mechanical ventilation, which is simple in construction, and economic and reliable in operation. Fans, ducts, controllers and suitable ambient air are the total requirements for a grain ventilation system, with the aeration time controller as a key component. In some cases, supplementary heating equipment is probably needed for reducing the relative humidity of the ambient air and raising its temperature. Ventilation technology for wet grain drying is now practiced in the both the north and the south of the country, with good results.

In summary, mechanical drying has many advantages it is independent of the weather, has low operating costs; and is suitable for fast, large-scale and centralised facilities. However, good design and operations are very important, because grain is easily damaged by over-drying.

Current Problems and Developments

At the moment, the major problem faced after harvest in China is a shortage of mechanical drying capacity. The government each year has to make substantial inputs of labour, materials, and money for sun drying in order to protect grain-particularly maize in the north-from moulding or other kinds of damage during storage and transportation,

Under the present management system, the state grain depots have to procure a huge amount of wet maize within a short period after harvest. Although the temperature is low and wet grain can be stored for a certain time without drying, the huge amount of wet maize in depots waiting to be handled by a very small drying capacity is still a great pressure on safe storage. It keeps all store keepers busy with grain drying (mainly sun drying) throughout the winter.

In years when frosts are early, the problem is further exacerbated, with even greater amounts of maze at moisture contents more than 10% over the standard. Depots have to ask local government for help as the drying season is short. Roads, squares, other public spaces, and even airport landing strips have been used to dry all wet maize to a level safe for storage before the temperature rises in the spring.

High moisture paddy in the south represents the second biggest wet grain problem in China. Its moisture content is lower than that of maize, but the ambient temperature in the south is much higher than that in the north. Wet paddy therefore needs to be dried as soon as possible. In addition, exposure to high temperatures awing uneven drying can reduce paddy quality, leading to broken and cracked kernels during milling. It is essential to increase mechanical drying capacity and develop drying technology to solve these problems.

Nevertheless, the development is uneven in the north and south. Large capacity dryers able to remove substantial amounts of moisture in each pass are urgently needed in the north, while in the south, advanced drying technology that preserves paddy quality is the top priority. Here, drying systems based on aeration would probably be more suitable and effective. This would seem to be the best way to develop grain drying policy at present.

China is eager to learn from other countries about postharvest technology in general and grain drying in particular, including:

Efforts should also be made to develop new, high-yielding and early maturing varieties of maize in order, for example, to reduce grain moisture content in the field before harvest.

China is a developing country and, because of financial constraints, the government is unable to tackle all problems immediately. However, there are a few important first steps that could usefully be taken to overcome the grain drying problem:

There are two factors that support this idea. Firstly, grain production in China is decentralized and small scale at present. For this reason, on-farm drying may be easier than centralised drying in state depots. Secondly, farmers have many traditional and effective methods suited to small-scale grain production. No doubt they could do the job well, as long as government policy is practical.


Some imperatives in crop drying research

Dante de Padua

Program Officer,
Agriculture, Food and Nutrition Division,
Post Production Systems,
International Development Research Centre,
Asia Regional Office, Singapore

Abstract

Crop drying research has produced components of a drying system. There is now a need to consider the integration of the knowledge gained about the drying characteristics of products, the drying load, the product quality preferences of consumers, and the dryer with its supportive infrastructure. Procedures for generating the requirements of the marketing system, for evaluating available component hardware, and for determining optimum combinations for different commodities, users, environments, and marketing channels have to be developed. If they are to be adopted on a sustained basis, system dryers must increase the profitability of the operations of the end user.

Introduction

There has been no sustained adoption or increase in the utilisation of more 'efficient' drying technology for crop preservation in the region, despite continuing investments in research in the past 10 years towards providing drying capability. This does not mean that advances have not been made. Progress has been made in the understanding of the technical, economic, and people or system issues involved. There is a continuous proliferation of learned papers on the subject. But if continued support for drying research is to be considered, we need to make an analytical review of the situation and come up with a more strategic plan. The development of drying technology seems to be towards a blanket prescription for the industry, or an assumption that, given a good drying technology, the agricultural industry in the region will adopt it. The question is, what is a good drying technology, and who are the targets in the industry? The engineers complain about the non- engineered designs, and the economists claim the problem is a failure of the market to respond. They are probably right, but what are we supposed to engineer, and how does one go about developing a responsive market?

I would like to try the hypothesis on this exalted group of donors that system dryers are needed, but that no one drying technology is appropriate for all users and for all conditions. They key words here are system dryers. This is akin to the terminology: drying strategy. The term 'system dryers' is used in this paper to connote dryers that are designed to be compatible with the system where they are used. They could be simple batch units with forced or natural convection modes of moving the drying air, continuous flow dryers that complement sun drying, or the Australian instore drying technology, or a combination of these technologies.

Sun drying of paddy on the highways is not a technology, it is a practice which is one of the indications of a problem. The terms mechanical dryers and artificial dryers are used in the literature. This may be semantics, but this is a clue to the isolated or component thinking, and besides, there is nothing mechanical or artificial about dryers. To discuss drying strategies and to be able to design a system dryer, we need to identify commodities, users, environmental conditions, the production and marketing systems of which the system dryer is a subsystem, the costs and benefits, and the distribution of costs to beneficiaries. In this context, the problem issues have to be identified and defined, the probable causes of the problems, and then decide the nature of, and direction for research. This paper contains only perceptions derived from working with the grain industry to trigger the relevant discussions and to enable those concerned to make their own decisions.

What is the need for crop dryers in the Southeast Asian region?

Drying is one of the most practical methods for primary preservation. A corollary to the hypothesis is that system dryers offer an advantage over the traditional drying practices under certain conditions. The agricultural commodities that are traditionally dried are paddy, maize, groundnuts, soybeans, coffee berries, some fruits (e.g. mangoes and longan), spices (e.g. chill), garlic, onions), fish, meat, chipped root crops (e.g. cassava and sweet potato), and coconut meat (cope). Direct sun drying or free/natural convection drying over a fire is the traditional practice. The need for system dryers varies with each commodity, the area where they are grown, and how they are marketed. System dryers for paddy are acutely needed in, say, Indonesia and the Philippines, but not for the majority of production in Thailand. System dryers for maize are needed very badly in the Philippines and Thailand, but maybe not in Indonesia.

To focus discussion, we shall now restrict reference to commodities to the two staple food crops in the region: rice and maize. A major portion of these two commodities is harvested during the rainy season where sun drying is not dependable. The consequence of this lack of drying capacity is physical and qualitative loss of commodity. While loss figures are available which are often used for justifying intervention, the quantified benefits of intervention attributable to investment in system dryers are also necessary for decision making. For example, an analysis of paddy drying in the Philippines by Habito (1986) states that in a mechanised rice postharvest system and a purely traditional one, the former can have the advantage of providing a more stable net income (to farmers) due to its independence of the weather, particularly the task of grain drying. However, he further indicates that the mean incomes obtained under each system were not significantly different and therefore concludes that there is no strong incentive for farmers to mechanise these operations. Habito, in the same paper, ventures the opinion that, based on his findings, dryers should not be designed for farm level use, but rather for traders and processors who can take advantage of economies of scale as the target users. He argues correctly that the harvest has to be dried anyway for storage and milling.

The drying strategy that suggests traders and processors as target users of dryers should recognise the fact that the marketing system is dealing with very high moisture grain that is unstable, that most of the irreversible deterioration occurs within the first 24 hours at the farm level, and that conditions for aggregating, purchasing, transporting from small farm lots over unpaved rural roads to drying plants can take more than 24 hours. The strategy can be operationalised. This has been done with corporate enterprises, but for small family-owned mills, which form the majority of processors in the region, the incentive for investment has to be proven. Point one in this discourse is that what is needed is not just a mechanical-artificial dryer, but a system for drying. Point two is that mechanised drying on the farm by farmers may not be feasible, but a low-cost, passive drying system may be what is needed in the small farms of Asia. Point three is the introduction and utilization of dryers must provide an opportunity for financial and economic gains to the investor/user.

What are the purported advantages of system dryers?

Drying is just one of the processes in the chain of operations in the food marketing system. This suggests that it has to be compatible with the operational requirements of the rest of the system. These requirements are for capacity-or volume dried per unit time, product quality assurance, for sanitation, and for dependability. Drying of the commodity in itself is merely in preparation for storage and further processing, and the value added to the product cannot cover the cost of drying. A system dryer linked or integrated with the rest of the system would allow the cost to be distributed to the various beneficiaries in the system.

The need for, and the advantage of having a system dryer are two sides of the same coin. The traditional practice of sun drying on mats on the highway, if and when the sun is shining, has not resulted in the best product, but people have survived. Inability to dry paddy on time only results in darkened and yellowed kernels, but the milled grain is still eaten by the local people. For maize, groundnut, and copra, inability to dry the grain on time and properly, results in aflatoxin contamination, which may not be physically evident. Aflatoxin is only circumstantially accused of being carcinogenic, but it has stifled free trade in favour of the American aflatoxin-free products. A system dryer can prevent this problem from occurring.

Rice processing and trading is big business. The investment costs and the operating costs to the businessmen are high, but the profits have to be made within narrow margins. The objective, therefore, of grain businessmen is to maximise profits, and a system dryer can allow an increase plant throughput.

Who are the potential users of system grain dryers?

A. The individual farmers

The rice and maize farmers of Southeast Ash cultivate anywhere from a third of a hectare to a few hectares. These are the poorer segments of society and are legitimate targets as beneficiaries of research and development. They may own the land or work as tenants. In most cases, they retain whatever paddy harvest they require for their own family requirements and sell the rest to traders. A three-hectare farmer producing 4 tonnes of paddy per hectare, or 12 tonnes in total, may take 1-2 weeks to harvest his crop. His drying load is anywhere from 1 to 2 tonnes per day for, at most, 2 weeks per season. If the weather is fair, this farmer has no drying problem. If it is rainy, he waits for the silver lining behind the dark clouds. If it is really bad weather, there is always a market for wet grain. How much does he lose for having to wait for the sun to shine, or for having to sell wet grain at a discount? Assuming a high 3% of the value of his crop, that works out to be US$56. These farmers would not even dream of acquiring a dryer.

The farmer may not have a serious drying problem, but somebody down the marketing line inherits the problem.

B. A group of farmers

Several farmers may be serviced by an irrigation system, and out of necessity have to work together. A group of farmers in a research project in the Philippines is reported by Lorenzana et al. (1987) as successfully collaborating in the use of harvesting machines like threshers, and even a small rice mill. The farmers, however, did not make full use of a flat-bed dryer's capacity. They only resorted to it when sun drying was not feasible, and then complained that it did not have the capacity to service the requirements of the group as they had to take turns on a first come first serve basis. The development of the market for their wet grain harvest might be more meaningful for them.

What system dryer would you recommend? Maybe a first-stage dryer, and pass the problem to the next stage.

C. A farmer based marketing cooperative; a family owned milling enterprise; a grain trader; a corporate enterprise

This group of potential users is in the rice processing business for profit. Its operations are centered around milling capability, which would be from 1 tonne of paddy per hour for the smaller plants up to 10 tonnes for the larger. They are motivated first and foremost by anything that can increase financial returns. How well are they doing? Many of the private millers tour the vacation spots abroad once a year. This group provides a service and I would like to review the opportunities for maximising profits to see how system dryers fit in with them. We need more farmer cooperatives to allow the farmers to participate in the rich proceeds of processing and trading.

Factors that can increase financial returns

High Cost Factors

1. Increase plant throughput.

2. Adopt bulk handling and storage and pest control measures.

Low Cost Factors

3. Increase milling recovery and improve milling quality.

4. Establish linkages between farm production, processing, and markets.

Factors that can decrease financial returns

The availability and proper use of a system dryer is heavily implicated in the financial well being of the grain businessman. To give meaning to this listing, a quantified contribution of each factor has to be estimated.

D. Government grain marketing agencies

These are government-subsidised service agencies equipped with dryers. Their problems centre on either lack of capacity or incompatibility of the different components of their systems during peak harvest periods, leading to bottlenecks To the flow of grain. They have, however, been the target of a lot of foreign research aid!

In summary, the problems are:

A. Drying technologies have not been designed as part of the system, resulting in the lack of supportive infrastructure, or incompatibility of capacities with requirements.

B. Farm drying, although the logical place for drying to minimise delays, provides insufficient incentive to farmers.

C. The vertical segmentation of the marketing channels, which are not supportive of each other.

D. The market failure to recognise the benefits of controlled drying and to provide the proper incentives for the production of good quality milled rice.

E. The non-structuring of payment for the cost of drying through a pricing system that distributes the cost among the different beneficiaries of better quality product produced.

What do we have now?

Every conceivable component drying technology has probably been introduced in the countries of the region. They include natural convection dryers such as the 'improved' African pit dryer, which is a traditional farm dryer for copra, flue curing/drying barns for tobacco adapted for other crops; the forced convection dryers: batch dryers of different configurations, recirculating dryers, continuous flow multistage dryers, in-store or deep-bin dryers; solar dryers galore; infrared radiation dryers; conduction heated dryers. Some of the donor organisations represented here are probably aware of their contribution. For some of the more utilitarian models, manufacturing with about 80% local content has been done.

Aside from the hardware, there is now a high level of native engineering capability for basic design work. Information on the thermophysical properties of local varieties, and constants in drying rate prediction equations have been generated, resistance to airflow for local products are being worked out. The psychrometrics involved in the heat and mass transfer in the drying process are very well understood

There is now a new breed of agricultural engineers that has the facility for analysing engineering systems using both engineering and economic parameters to determine optimal combinations. With desktop microcomputers, this is now a whiz.


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