Tuber characteristics determining quality

1.1 Size and shape of tubers
The external quality of potatoes is extremely important in the processing industry. Characteristics of particular interest are the shape, the occurrence of diseases on the skin, and the extent of external damages. Size, shape and shallow eyes are important with regard to the appearance of the product and the influence on wastage during peeling.

• French fries producers prefer long-oval or long tubers with a size of at least 50 mm.
• For the production of crisps, round tubers are required with a size range of 40-60 mm.

1.2 Injuries and defects
Rough handling of the potatoes during harvesting and transportation causes internal bruising. The internal tissue cells crack and brown stains are formed in the tuber, which is an impediment to processing. Internal defects such as hollow or black heart are also undesirable.

One of the most important problems the industry often has to deal with is black spot. This blue to greyish-black discoloration in the tubers is more inclined to occur during transport and grading if the potatoes are not handled and stored in the proper way. As a result of tissue injury, chemical conversions take place, which after a day or two cause a dark discoloration. Therefore it is necessary to handle potatoes as carefully as possible to prevent black spot. In addition, potatoes should be heated to about 15¨¬C before grading. Some varieties are far more susceptible to black spot than others.

Almost all potato varieties are more or less susceptible to common scab. But this disease can be effectively controlled by keeping the soil moist, especially during the susceptible period of tuber initiation until about four weeks later.

1.3 Dry matter content
The term dry matter content means the mass fraction (%) that remains after the water fraction (%) has been removed by drying.The opposite term ¡®moisture content¡¯ is also used.

Both the processing efficiency and the quality of the finished product benefit from a high dry matter content. If the dry matter content is too low, the French fries or crisps will be too soft or too wet. In addition, more energy will be required, since more water must be evaporated. A high dry matter concentration results in a lower fat content. This lowers the processing costs and is better for the health of consumers. However, if the dry matter content is too high, the French fries will be too hard and dry and the crisps will be too brittle. The dry matter content also partly determines the texture of both the fresh and the processed potato.

The requirements in respect of the dry matter content are determined by the end product:

• For the production of French fries, potatoes with a dry matter content of 20 - 24% are preferred.
• For the production of crisps, preference is given to potatoes with a dry matter content of 22 - 24%.
• For the flakes industry, potatoes with a rather high dry matter content (higher than 21%) are required.

1.4 Colour
1.4.1 Reducing sugars
The frying colour is an important criterion for potatoes destined for the French fries and crisps industry. The frying colour of the fried products is determined to a large extent by the reducing sugar content in the potatoes. The higher the content of reducing sugars the darker the frying colour. A dark frying colour results in a bitter taste, which is unacceptable in the production of French fries and crisps.

The requirements with regard to the content of reducing sugars depend on the end product:

• Of all processing industries, the crisps industry makes the highest demands on the content of reducing sugars; the reducing sugar content may not exceed 0.2 - 0.3% of the fresh weight.
• For the French fries industry the standard is less than 0.5% of the fresh weight.
• For flakes and granules used in snack production, the reducing sugar content should not exceed 0.3% of the fresh weight.

Another important aspect is colour distribution. Unevenness in colour distribution results in French fries with a brown colour at one end. The causes of this phenomenon are senescence after long storage and secondary growth. Some varieties are prone to the so called ¡®sugar ends¡¯ as a result of senescence after long storage. In extreme cases of secondary growth, starch is abstracted from the primary tuber. The abstraction starts at the heel-end of the potatoes and may lead to glassiness. Glassiness is when the potato tissue of a re-sprouted tuber, or the top end of a long tuber, looks watery-translucent when cut. It may even have a spongy texture. Glassy tubers can be sorted out by dipping the potatoes in a bath with a salt solution with a SG of 1,060 (153g NaCl/l water). The glassy potatoes will float and can easily be skimmed off.

1.4.2. Darkening
1.4.2.1. Non-enzymatic
During the production of potatoes a greyish hue sometimes develops. This occurs when a compound of iron and chlorogenic acid is oxidised due to the oxygen in the air. Once the French fries have been fried this discoloration is hardly noticeable, if it is at all. Yet this characteristic, closely linked with certain potato varieties, is considered to be a serious quality defect.

1.4.2.2. Enzymatic
Enzymatic darkening occurs when cells are damaged and enzymes and substrate are mixed, initiating all kinds of reactions. There are also reactions that cause brown and grey discolorations, due to the formation of a type of enzyme that affects tissue colour.