External factors - product handling

2.4.1 Harvesting
A crop should mature well. This will result in a relatively high dry matter content and a minimal reducing sugars content. This combination provides a relatively good frying quality. During harvesting, severe loss of quality may occur when potatoes are damaged during lifting. This causes subcutaneous discoloration, which should be prevented. The likelihood of internal bruising is greatest when the potatoes are lifted under dry conditions in cloddy soils.

To a certain extent damage can be rectified during harvesting by means of light irrigation just before the crop is lifted. Furthermore, the grower should try to keep soil on the web of the harvester for as long as possible, and the speed of the elevator chain should be adjusted to the driving speed (approximately a ratio one to one). In dry conditions it is useful to cover the spindles of the elevator chains with synthetic material or rubber. The dropping height of the potatoes should not exceed 40 cm during harvesting, unloading in the store and during grading. In places where potatoes drop down, soft materials and shockabsorbing mats can be used to prevent damage.

Because of their weight, the largest tubers are most liable to suffer damage. The risk of bruising increases when soil temperatures at harvest time are low. To prevent internal injury caused by harvesting, potatoes should not be lifted at soil temperatures below 12ºC. But if it is late in the season, attempts may have to be made to get the potatoes lifted, anyway. As a rule, harvesting at soil temperatures lower than 8ºC should be avoided whenever possible.

2.4.2 Handling
Bruises occur during storage as a result of the pressure that is put by one tuber on the other, causing the potatoes to flatten and be bruised. When the store is emptied, black spots in the tuber flesh can develop under these bruises. The incidence of bruising increases after dry seasons or as a result of too much ventilation. In ware potatoes, black spot incidence often increases after a prolonged storage (six months or more). To prevent bruising in this situation, the storage height of the potatoes should be limited to 3.5 - 4 metres, and equipment where potatoes strike hard surfaces should be cushioned. However, when potatoes are stored in boxes, bruising usually does not occur, as the dropping height is minimal. 

2.4.3 Storage
The potato is a living organism and, under favourable storage conditions, can be kept for long periods of time (7 - 9 months). However, during storage loss of weight, but also loss of quality may occur. Limiting these losses is a prerequisite for the production of potatoes for processing. When potatoes destined for the processing industry are stored, the following measures affecting quality, merit special attention:

• Ventilation
• Temperature control
• Application of sprout inhibitors
• Conditioning before delivery

2.4.3.1 Ventilation
To prevent weight loss, and consequently an increased susceptibility to black spot, the store should be ventilated as little as possible, and preferably at a high air humidity (RH >=92%). When this cannot be done, for instance in areas with very low outside temperatures or in very dry areas, artificial air humidification during ventilation can help to solve the problem.

2.4.3.2 Temperature
Storage losses caused by respiration and the occurrence of fungal or bacterial infection are smallest at storage temperatures of 3 - 5ºC. However at such low temperatures the development of reducing sugars, which negatively influences the fry colour, is stimulated. Therefore it is advisable to store potatoes that are intended for the processing industry at relatively high temperatures. For crisps production this should be at 7 - 10ºC and for the French fries and flakes industry, 6 - 7ºC. Potatoes with a reducing sugar content that is too high (due to storage temperatures that are too low) can be reconditioned. To do this, they can be kept at a temperature of about 15ºC for a period of two to three weeks. This has the effect of breaking down the surplus of sugars by converting them to starch and by respiration. The results of this reconditioning can vary, particularly after prolonged storage time. For example, prolonged storage decreases the effect of a reconditioning period, and senescent sweetening may occur. The effect of reconditioning is then even worse than if it had not been carried out, and should be avoided.

2.4.3.3 Sprout inhibitors
Sprout formation is minimal at a storage temperature of 3 - 4ºC. However, potatoes for the processing industry should be stored at higher temperatures to limit the formation of reducing sugars. Therefore measures to inhibit sprouting merit particular attention. The application of CIPC/IPC (Isopropyl 3-chlorophenyl carbamate) is the method used most for this purpose. CIPC is effective when potatoes are stored at temperatures up to about 12ºC. A new development is the application of Carvone (a component of caraway seed) as a biological sprout inhibitor. Carvone is not only a sprout inhibitor, but it also has a positive effect on controlling various storage diseases and it even kills slugs.

2.4.3.4 Conditioning before delivery
Potatoes with a high dry matter content are more susceptible to black spot than potatoes with a low dry matter content. Heating the potatoes before size grading or delivering them will considerably decrease black spot susceptibility. Therefore, potatoes intended for the processing industry should be heated to at least 15ºC, before unloading out of the store. Potatoes with a high susceptibility to black spot should be heated to 18ºC. During the heating process the air temperature should not exceed 20ºC. It is wise to allocate at least three days to the heating process.