Mechanical force and subcutaneous tissue discolouration in potato

Over the last decades rapid developments in potato production and mechanization occurred, aiming to increase capacity and to reduce costs. Concomitantly, potatoes can be damaged more easily. Subcutaneous tissue discolouration, a major quality problem, is a result of mechanical damage and reduces the market value of potatoes considerably. This study focused on the effect of mechanical force on subcutaneous tissue discolouration.

The study started with an analysis of representative Dutch ware-potato handling-chains in terms of utilization, produce-friendliness and the subsequent incidence of subcutaneous tissue discolouration. In parallel, forces acting in these chains were quantified. Using an instrumented sphere (IS100) a better understanding of the forces acting in the chain was obtained. It appeared that 78% of the measured impacts occurred during packaging and that 98% of the impacts did not exceed 150 g . The results of the chain analysis were the basis for experiments on dose-effect relations, focused on the effect of repetitive impacts and impact body shape on the occurrence of subcutaneous tissue discolouration. A new computer-controlled pendulum was developed to impact potato tubers at a desired rate. This pendulum proved to be an adequate tool to impact tubers precisely and reproducibly. In particular the effect of repetitive impacts on potato tubers and the effect of the shape (curvature) of both potato and impact body became clear. It appeared that splitting 0.6 J of impact energy over 9 equal impacts reduced the depth and volume of discoloured tissue by 33 and 64%, respectively. Spherical impact bodies, relative to a non-spherical one, doubled the volume and depth of discoloured tissue.

The results urge a reappraisal of some commonly accepted insights. For example the effect of low-energetic repetitive impacts on subcutaneous tissue discolouration cannot be neglected. The use of spherical impact bodies has to be minimized.

The research suggests important routes for the potato sector to make adequate changes in technique and technology. These changes make it possible to reduce both the number and the intensity of impacts and should result in a produce which can be offered to the consumer in a condition (almost) free of subcutaneous tissue discolouration. When the utilization of machinery is optimized damage reduction may be substantial. Some potential improvements are: better utilization of machinery, better planning of bunker storage, box handling and improved temperature control of the tubers.

To further optimize the whole potato handling-chain with respect to produce-friendliness additional research should focus on fundamental knowledge that is currently missing, and on developing an optimal logistic handling concept. Crucial in this concept are flexibility and produce-friendliness. Ideally, potatoes may be transported along different routes and the number and intensity of impacts are minimal. Also repetitive impacts on the same tuber site and the use of spherical impact bodies are avoided. The effect of the time interval between consecutive impacts has to be studied. It should also be elucidated at which curvature spherical impact bodies cause damage.

To create a more produce-friendly attitude of growers, and other actors in the potato handling-chain a better differentiation between 'good' and 'poor quality' potatoes is necessary. A financial incentive could have a substantial effect, resulting into potatoes with a high quality, i.e. free of tissue discolouration.

This thesis is also available as publication No. 99-14, ISBN 90-5406-180-4 of the Institute of Agricultural and Environmental Engineering (IMAG), P.O. Box 43, NL-6700 AA, Wageningen, The Netherlands