External factors influencing quality
When producing potatoes for processing the grower has three major aims: high yields, high quality and the lowest costs. While yields are expressed in tonnes per hectare, quality is expressed in terms of recovery, that is: the number of kg of processed product that are produced from 100 kg fresh potatoes. High yields are ensured when proper varieties are subjected to suitable growing conditions and are supplied with adequate amounts of inputs. The highest yields do not necessarily lead to the best quality because recovery depends on high dry matter concentrations, homogeneous and relatively large tuber sizes, low amounts of black spot due to bruising, low reducing sugars and few defects. Wellmatured crops have higher recoveries than crops harvested before maturity. Cost reduction results from the efficient use of land, labour and inputs such as water, fertilisers and crop protection chemicals. Crop management continuously takesthese three factors into consideration to optimise the benefits of the crop.
2.1 Climatic conditions
The production of dry matter and the dry matter concentration of potatoes are a result of photosynthesis and respiration. These processes are not only influenced by the physiologically determined factors of the plant, but also to a large extent by climatic conditions: radiation, day-length, and temperature. Daily radiation determines the growth and production (photosynthesis) of the crop. The temperature influences the efficiency of photosynthesis during the day and losses by respiration at night. At higher temperatures a greater proportion of the dry matter produced is allocated to the leaves. Temperature also determines the length of the growing season (ideally frost free but not too hot) and, as a consequence, the attainable yields. Daylength determines tuber initiation. The short day species of Solanum forms tubers earlier and matures earlier at shorter day-lengths. Both high temperatures and short days lead to lower tuber dry matter concentrations.
It is generally assumed that for the production of potatoes the optimum temperature is around 20ºC during the daytime and around 15ºC at night. Long days combined with relatively high temperatures tend to delay tuber initiation and stimulate haulm growth. This will result in an increased reducing sugar content. Experiments have shown that with day-lengths of 10, 14 and 18 hours the colour of French fries becomes progressively darker. This means that an increase in day-length corresponds with an increase in reducing sugar. This is in accordance with the evidence that varieties developed under long-day conditions improve in processing quality when grown under shorter-day conditions. When the potato crop for processing is grown with high yields and high recovery in mind, the management should aim to match the available growing season and inputs with the requirement of the variety. When the growing season is too short to lead to high yields and high dry matter concentrations, or the variety is too late or too early, it is likely that profits will be reduced.
2.2 Variety
In determining the suitability of potatoes for the processing industry, internal and external quality plays an important role. Only varieties that meet special quality standards and can be processed on a cost-effective basis will qualify for the processing industry.
The ‘Netherlands Catalogue of Potato Varieties’ (a four-yearly publication by NIVAP) gives an indication of the suitability for processing of the varieties grown under Dutch conditions. In the catalogue, a selection is given of the most important Dutch potato varieties for the global potato sector. The various properties and qualities of each variety are stated clearly, for instance: dry matter content, shape of tubers, shallowness of eyes, size of tubers and internal bruising.
When selecting a variety for a particular environment it should be kept in mind that most varieties respond differently to temperature, day-length, availability of water and nutrients, planting patterns and the timing of haulm killing or harvest. Therefore it is advisable to test a number of varieties for a few years under local conditions to find out how to optimise management.