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Organisation: Sorghum Institute of LAAS
Author: Jianqiu Zou & Yuxue Shi
Edited by AGSI/FAO: Danilo Mejia (Technical), Beverly Lewis (Language&Style), Carolin Bothe (HTML transfer)

CHAPTER VIII: Industrial use of sorghum


2.1 Pre-harvest operations

2.2 Harvesting

2.3 Threshing

2.4 Drying

2.6 Cleaning

2.8 Storage


2. Post-Production Operations

When sorghum grains are ripe, it is time to harvest. Using appropriate methods to harvest, thresh, dry and store at right moment, are the best guarantee of getting a high yield and good quality.

2.1 Pre-harvest operations

If the sorghum pollinates and fertilises normally, grains will form rapidly. Usually, it takes about 30-50 days from fertilising to ripening. The period includes three stages--milk stage, dough stage and ripening stage. During milk stage, sorghum grain is green or light green in colour, filled with milky liquid. At this the moment, embryo has the ability to germinate. In the dough stage the grain is slightly yellow with (almost solid and waxy) fillings. If you press the grain, it is not easy to crush which signals its ripening stage. During this stage, the head and grain appear in its original shape and colour with moisture content of 15-20 percentage. The highest accumulation of dry substance is in the dough stage, which contains about 20 percentage moisture. It is the optimum harvest time (5,8). If harvest takes place too early, the filling stage will be interrupted causing low 1,000-kernel weight, which corresponds to lower grain yield. If harvesting is too late, there will be moisture loss, natural grain falling, sprouting on the head, drought or respiration. The 1,000-kernel weight will drop, thus causing a decrease of grain yield. (Table 3) (5). Sweet sorghum, forage sorghum or ensilage sorghum, can be harvested at any time from heading stage to milk stage. The optimum time to harvest ensilage sorghum is at the end of the milk stage to obtain the highest biomass and nutrition. If grains are intended to be used as food and the stalks and leaves as ensilage, then the optimum harvest is at the end of the dough stage (4).

In addition, different harvest time will affect grain quality and viability. Premature harvesting will result in protein content that is a little higher while starch content and unit weight is lower. By slightly delaying harvesting time, starch content and unit weight are almost same, but protein and soluble sugar content drop. Late harvested sorghum measures lower for all indices (Table 3). In addition low temperatures decrease germination rate. Grains for seed must be harvested at the right time.

Table 3. The Influence of Harvest Time to Sorghum Grain Quality of Xiongyue 253

Harvesting time
Grain yield
(%)
Unit weight
(g/L)
Moisture
(%)
Starch
(%)
Soluble sugar
(%)
Protein
(%)
1,000 Kernel
weight (g)

Early milk stage

100.0

700

66.6

66.6

1.41

8.63

7.5

Late milk stage

123.9

727

37.5

70.2

1.50

7.79

26.0

Middle dough stage

144.6

744

33.3

72.0

1.62

7.86

29.0

End of dough stage

162.1

741

20.0

72.9

1.50

7.74

29.0

Ripening stage

160.9

739

13.0

68.4

1.41

7.76

29.5

2.2 Harvesting

In China, there exist two harvest methods--traditional hand cut and mechanical harvest. At present, the dominant way is manual harvest:

a. The harvest tradition of the Chinese;

b. Reflects the economic situation, as many Chinese farmers are still poor;

c. Most of planting fields for sorghum are on hillsides and fields are too small to harvest with machines;

d. Too many sorghum cultivars are planted, and sometimes plants are not in good condition;

e. There are not enough satisfied harvesters.

Manual harvesting cuts sorghum with a sickle. The first method is harvesting the plants with their heads on. The method is to cut plants first, then bind 20-30 plants together. After binding all the plants, a vertical rafter is made using 25 bundles for drying the sorghum in the field. If rainfall is not too great, the farmers put sorghum bundles on the ridge of the field for drying. The height of stubble is diverse. If the stubble is used as fuel, the stubble height will be 30 cm, otherwise, it will be 10 cm. After drying about 10 days, the heads are cut (about 50 cm long) and bound. Then the heads are sent to threshing site. This method can clear the fields allowing farmers to prepare fields for the next season. It is the most popular production method.

In the second method the sorghum heads are cut first tied up and dried. The straws are cut last. The method is primarily used in the coastland, low-lying land and southern China. Sometimes, the method is also used to harvest the short-stalked variety.

In some areas of Henan, Shanxi and Shandong province, the farmers dig the whole sorghum plants with roots, and then cut the heads in order to get more fuel,

Two kinds of mechanical harvesters are used in China. One is designed for short-stalked sorghum, the other for long-stalked sorghum. The advantages of mechanical harvest are efficiency and fewer losses.

The Dongfeng combine harvester is for short-stalked sorghum demanding sorghum less than 100 cm. However most Chinese sorghum cultivars have medium-long-stalks of 180-250 cm high. This machine is seldom used.

The Liaoning 4G-4 sorghum harvester is used for long-stalked sorghum varieties. Its efficiency is not as high as the combine, but it has only 15 kg grain loss per hectare. Today, the small holders and subsistence farmers use the traditional hand cut method. Harvesters are used on large farms.

2.3 Threshing

After being fully dried in the sun, sorghum needs to be threshed. The threshing methods can be divided into artificial threshing, animal threshing and mechanical threshing. Along with the rise of mechanisation, more and more farmers use mechanical threshing machines. Table 4 details different threshing methods (4).

2.4 Drying

Grain drying is the key to ensure good storage quality. Without drying, the moisture content of sorghum grains remains too high. The grains generate heat because of its respiration and a great amount of nutrition is wasted, so the edible and seed grain qualities decrease.

Natural drying is usually used for sorghum production, in the case of small holders and subsistence farmers. They spread sorghum grains out about 10 cm thick. To increase the irradiated area of grains, some small ridges can be made from south to north. Then they turn the grain over again and again to decrease moisture more rapidly. After 3~4 days when the moisture content is about 13 percentage, the grain is ready to store.

More and more grain depots and seed companies use mechanical drying. There are 8 kinds of dryers shown in Table 5.

Different sorghum cultivars should be dried separately. Also to speed up the drying process, the grains with different moisture content should be treated separately.

Table 4. Difference of Three Sorghum Threshing Methods

Threshing
method
Threshing
tools

Procedure
Threshing output
(kg/person)

       

Artificial
threshing
flail
wooden fork
Spread sorghum heads in the threshing field, dry in the sun for half a day, then beat sorghum heads with flail. After most of grains have been threshed, turn over sorghum heads, beat sorghum heads again. Repeat these steps until all the grains are threshed. The method is inefficient and labour intensive, so it is seldom used now.
250

       

Animal
threshing

livestock
stone roller
wooden fork

Spread sorghum heads on threshing field (about 25-35 cm thick). Then let livestock pull the stone roller (rolling and pressing). When most of grains are threshed, turn over sorghum heads. Roll stone roller again, until all of grains are shelled. This is the most popular method in China.

1000

       

Mechanical threshing

drum thresher

large cereal-
thresher

Place sorghum heads into thresher. Inspect the heads frequently to see if all the grains have been threshed. Monitor if there are crushed grains. The advantages of the method are high efficiency, clean threshing and low glumaceous rate.

4000

       

2.6 Cleaning

In the course of harvest, storage, and transportation, many kinds of impurities can easily enter the crop. Cleaning becomes a necessary step to ensure grain quality and machine safety.

In sieve cleaning separating grains from impurities by size is the principle of the method. The most popular equipment types are the vibrating screen and holding sieve (4).

The vibrating screen is composed of a deed hopper, sieves, dust remover, cleaner and drive equipment. The sieves lie on three layers with different slopes. The slope of the first layer is 3-5 degrees for removing large foreign matter. The slope of the second layer is 4-6 degrees for screening medium-sized impurities. The slope of the third layer is 6-9 degrees for cleaning impurities smaller than the grains.

The holding sieve is suitable to unprocessed grains with more glumaceous and obstinate grains. The components are the holding sieve, rubber mill and specific gravity separator. The use of the holding sieve is to separate good grains from glumaceous or obstinate grains. The rubber mill is for dehusking. The specific gravity separator can separate glumaceous grains from obstinate grains.

The principle of pneumatic cleaning is to separate impurities using the various specific gravity values of grains and impurities. The common equipment of pneumatic cleaning is the wooden windmill, aspirating separator and model 600 double board aspirating bellows.

2.8 Storage

Sorghum grains are not only an important food crop, but also provides animal feed and major industrial material. Maintaining good sorghum grain quality for food, industry, feed and seed are the significant objectives of storage. Over the period of storage, the grains' physical and chemical changes will have major impact.

During sorghum storage, air temperature, humidity, storage microbes, storage pest and moisture content of grains are key factors which affect storage quality. Among these factors, temperature and moisture content are the most important ones. Lower temperature and moisture content are the most efficient to restrain mould growth. The lower the moisture content, the longer keeping the grains from mildewing at high temperature. For example, if grain moisture content is 13 percentage, the grain temperature should be lower than 30 0C, but when grain moisture content is 14 percentage, the highest temperature should be 25 0C.

In order to safely store sorghum, according to grain quality, utilisation, moisture content, climate, and storage equipment, different storage methods can be used. The common storage methods in China are: drying storage, low-temperature storage, sealing storage, lack oxygen-free storage, ventilating storage and chemical storage. Table 6 details these methods. (4)

Generally, small holders and subsistence farmers use natural storage methods, while large producers and grain depots use natural and mechanical storage methods.

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