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Methods of quality assessment

Only varieties that meet particular quality requirements are suitable for the potato processing industry. These requirements are comprehensive, varying from external size and shape, which is dependant on the destiny of the product (French fries or crisps), to the internal composition of the potato (e.g. dry matter or sugar content). A wide range of testing methods is available to measure these properties. These methods vary from simple visual inspections to advanced instrumental methods. Quality assessments will be done on raw material as well as on the end product.

For the raw material, particular aspects such as size, shape and the lack of defects like diseases, rot, bruising and dry matter and sugar content are of importance. Assessment of defects has to be done by visual quality inspection at the start of the production line. For the determination of the dry matter and sugar content a method is described under 3.1 and 3.2, respectively. The quality of the end product depends, among other things, on the buyer’s specifications. The most important quality characteristics of the end product are described under 3.3.

3.1 Dry matter content
For the assessment of the dry matter content of raw material there is a simple and quick method based on the density of the potato tubers. This density can be obtained by determination of the tuber weight in tap water (W.I.W). The assessment of the dry matter content is based on the principle that there is a close relationship between the density of the potato tuber and the dry matter content of the potato tissue. In table 2, the figures of dry matter content corresponding with W.I.W. are given. 

The W.I.W. of potatoes can be determined by the following procedure:

  • A representative sample of potatoes of over 5000 gram is taken from the lot. Scurfy and diseased tubers should not be included in the sample.
  • The sample is washed with water, drained well and eventually dried with a cloth.
  • In the air 5000 gram dry or 5050 gram wet potatoes (a gram) are weighed precisely to within one gram.
  • The weight (b gram) of the potato sample is determined in tap water at a temperature of 8 - 12ºC.
  • The W.I.W. can now be calculated with a formula, which is for dry potatoes: (5000 : a) x b, and for wet potatoes: (5050 : a) x b.

(Source: European Association for Potato Research, Methods of Assessment for Potatoes and Potato Products)

3.2 Reducing sugar content
The reducing sugar content of the tuber at harvest time depends on the maturity of the crop at the time of the haulm destruction. The reducing sugar content is higher as the potato crop has matured less. The reducing sugar content in potatoes and the colour of fried products are closely connected. The higher the reducing sugar content, the browner the colour.

To get a quick indication of the content of reducing sugars, glucose strips can be used. The potato is cut in two, and a glucose strip is placed on the freshly cut surfaces. The colour of the strip can be compared with the colour on the packing of the strips. This gives a quick visual indication of the glucose level. In addition, the strip can be put in a glucose indicator, thus giving a more exact indication of the dextrose content. The glucose strip only indicates the glucose content of the potato, not the fructose content. Reducing sugar content is often about twice the amount of the glucose content. A more accurate assessment of reducing sugar content in relation to the frying colour can be obtained from frying sticks of French fries or slices from crisps. The sample sticks should be cut from the heart of the tubers. These sticks and slices are then fried for three minutes at 180ºC. After that, the colour of the fried product is compared with the standard colour charts. Rising values represent an increasingly darker colour and an inferior quality.

3.3 Endproduct
The assessment of the quality of the end product serves two tasks. Firstly, the information is necessary to check if the end product meets the quality specifications and, secondly, it performs a feed back control on the running process. The quality of the end product consists of health and food safety aspects and of properties concerning consumer appreciation, such as appearance, texture and taste. Measurements of health and food safety properties are merely instrumental methods, such as the measurements of the concentration on nutrients or the quantity of micro-organism. Standardised methods have to be used for these health and safety properties.

Aspects related to the appreciation of the product are tested merely by sensory analyses. Some aspects can also be measured instrumentally, but a straight relationship between the instrumental result and the human observation is often poor. Mostly the relationship is restricted to some correlation between the instrument and the human sensation. The main aspects related to the appreciation are:

The appearance consists of colour, size and length of the French fried sticks, also cracks, bubbles on the surface, defects and grey colour. Mostly, all these aspects have to tested by visual judgement. The accuracy of these determinations depends on the way these judgements are organised. They can be quite objective and accurate if the judgements are standardised. The colour of the end product, mainly the brown colour, depends on the sugar content of the potato. The colour of the fried product is compared with standard colour cards (see 3.2).

The taste of the end product will be tested by human observations. The taste of potato products is merely the determination of the lack of presence of off-flavours. Other taste aspects are strongly related to the texture. An aspect of high importance relating to taste is the fat content, which has to be measured instrumentally. In principle the fat content is measured by means of extraction of the fat out of the product with a solvent like petrol-ether. 

Texture is a comprehensive property that consists of several aspects concerning different parts of the stick, such as the core or the crust of the stick. Terms used in the texture qualification of French fries are, for example, crispiness, mealiness, hardness, hollowness and heterogeneity. The crispiness of the French fries is determined by the characteristics of the crust, which is formed by the evaporation of water during the frying process. This makes water evaporation one of the most important texture-determining factors in the production process.

Principally texture can be measured instrumentally, but it is difficult to relate the forces for deformation of the product with the sensory qualifications. Only for some aspects, and with particular purposes, are instrumental texture measurements effective.