Part 1. Essential stages in starch production.

Exercise 1. Read and translate the following words without a dictionary.

Million, ton, portion, centrifuge, hydrocyclone, transport, hydraulically, gravitational, separate – separator – separately,

Pre-reading task. Answer the following questions.

1. What agricultural raw materials are used for starch production?

2. Is the starch production process more difficult than flour production process? Explain your opinion.

Reading. Read the text and try to find answers to these questions.

World corn crop is 600 million ton per annum. Nearly 10% is made into starch or starch-derived sweeteners making corn starch the largest starch commodity in the world.

There are many stages in corn starch production. Corn is wet-milled to produce corn flour, or cornstarch, desirable for cooking. In wet milling, the grains are first dry-cleaned so that other cereals and some of the impurities are removed. The cleaning is normally done twice before wet processing. After the second cleaning the corn is divided into portions by weight and intermediately stored in hoppers. From a hopper it is hydraulically transported to the steeps.

Then the grains are steeped in warm water containing sulphur dioxide. This process softens the grains, and the outer skin and the germ are rendered removable. A proper steeping is essential for high yields and high starch quality. The steeping is carried out in a continuous counter-current process. The cleaned corn is filled into a battery of large steeping tanks (steeps), where the corn is soaked in hot water for up to fifty hours.

The soften kernels are broken up in attrition mills to loosen the hull and break the bonds between germ and endosperm. Water is added to assist the wet milling. A good steeping ensures that the germ will be freely released intact from the kernel by a gentle milling operation with no free oil.

Oil constitutes half the weight of the germ at this stage, and the germ is easy to separate by centrifugal force. The lightweight germs are separated from the ground slurry by hydrocyclones to remove the prime germ. To complete the separation the product stream with remaining germs is reground by a second milling operation followed by a second hydrocyclone separation, which effectively removes residual germ - the secondary germ. The germs are washed repeatedly counter-currently on a three-stage screen to remove starch. Process wash water is added at the last stage.

After germ separation the mill flow is finely ground in impact or attrition mills to release starch and fibres. The degerminating mill starch leaving the fine mill is pumped to the first stage of a fibre washing system, where starch is screened off. The fibres are washed free from adhering starch on screens in counter-current with process wash water added at the last stage. The last fibre washing stage has a slightly courser screen for pre-dewatering the fibre prior to a tapered screw press. The throughs (fine fibre) of that screen are separately dewatered on a rotating screen.

Washing with fresh clean water refines the crude starch milk. By using multistage hydrocyclones all soluble materials and fine cell residues are removed in a water saving process. Increasing the number of hydrocyclone refining steps may accomplish considerable water savings; which is one of the advantages of using hydrocyclones. The refined starch milk contains an almost 100% pure starch slurred in pure water.

The overflow of the first washing stage contains mainly protein, some starch and other impurities, which are recycled to the primary separator. In the strong gravitational fields of a hydrocyclone and a centrifuge, the starch settles quickly.

Exercise 2. Translate the following word-combinations.

High yields and high starch quality, a continuous counter-current process, a battery of large steeping tanks, to assist the wet milling, a gentle milling operation with no free oil, to complete the separation, a three-stage screen, the last fibre washing stage.

Exercise 3. Answer the following questions.

1. What are the essential stages in starch production?

2. What are the advantages of increasing the number of hydrocyclone refining steps? 3. Why is wet-milling more widely used than dry-milling to produce corn flour?

4. What is the objective of steeping?

5. Why is three-stage grinding usually practised?

Exercise 4. Complete each sentence with a word or phrase from the box.

hydrocyclones continuous counter-current process cleaning screens attrition mills

1. The ___ is normally done twice before wet processing. 2. The grains steeping is carried out in a ___.

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3. After germ separation the mill flow is finely ground in impact or ___ to release starch and fibres. 4. The ___ to remove all soluble materials and fine cell residues.

5. The fibres are washed free from adhering starch on ___.

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