Process-structure-function relations during the production of cut potato products

Texture is an important quality attribute of fruit and vegetable tissue based food products as it contributes to the sensorial appreciation by the consumer. It is mainly determined by the turgor pressure in the cell, which is maintained by an intact cell membrane, and the cell wall strength and intercellular adhesion, which are affected by the network strength of polysaccharides in the cell wall and middle lamella. On the one hand, plant tissues are thermally treated to obtain a softer texture and increase their palatability. The softer texture results from the loss of turgor pressure and the chemical depolymerisation and solubilisation of the polysaccharide pectin. On the other hand, thermal treatments such as pasteurisation, blanching, and sterilisation are performed to stabilise the tissues for storage. As the consumer prefers shelf-stable but fresh-like food products, several texture preserving tools are currently available to retard the chemical depolymerisation and solubilisation of pectin and, hence, prevent excessive softening during thermal processing. Mild thermal processing at 60 °C aims the thermal activation of the enzyme pectin methylesterase (PME) which reduces the pectin degree of methylesterification (DM). Pectin with a low DM is less susceptible to chemical depolymerisation during thermal processing, and facilitates the formation of pectin-calcium (Ca2+) crosslinks. The retardation of pectin depolymerisation and the stronger pectin network result in a better texture preservation during cooking.

Pulsed electric field (PEF) technology is currently used to soften raw plant tissues and reduce their cutting force. In this thesis the potential of a PEF pretreatment as texture preserving tool during thermal processing was investigated. More exactly, it was hypothesised that a PEF pretreatment facilitates the demethylesterification of pectin by PME and the ionic crosslinking with Ca2+. The permeabilisation of the cell membrane due to the PEF pretreatment may promote the transport of intracellular cations to the cell wall, promoting PME activity, and Ca2+ ions, promoting ionic crosslinking. The effect of PEF on the texture of raw plant tissue, on the texture degradation during subsequent thermal processing (i.e. cooking behaviour), and on pectin structure and organisation was studied in carrot and potato tissue, and compared to the effects of a mild thermal pretreatment and a combination of a PEF and mild thermal pretreatment.

The mild thermal pretreatment reduced the hardness of the raw vegetable tissues, but the PEF and the combination pretreatment resulted in the softest texture. The sensitivity of potato tissue to softening due to PEF increased with postharvest storage time, but the hardness after intense PEF treatments was independent of the postharvest storage time. The cooking behaviour after the different pretreatments was tissue-dependent. Carrot cortex showed a slower texture degradation during cooking after the mild thermal pretreatment and especially after the combination pretreatment, whereas carrot vascular tissue also showed a slower cooking behaviour after the PEF pretreatment as such. The effect of the different pretreatments on the cooking behaviour of potato tissue was less clear. The combination pretreatment resulted in the hardest texture during cooking, but the effects of the PEF and the mild thermal pretreatments were less pronounced than in the carrot tissues. The cooking behaviour of potato tissue was slower if treated in Ca2+ rich media, but was independent of the postharvest storage time.

In carrot cortex, the effect of the different pretreatments on the pectin DM and formation of pectin-Ca2+ crosslinks was unclear, suggesting that other mechanisms might be involved in the textural changes after the different pretreatments. In carrot vascular tissue, on the other hand, all types of pretreatment were able to reduce the pectin DM and to improve pectin-Ca2+ crosslinking, especially the PEF and combination pretreatment. These differences between carrot tissues may be explained by their ionic composition. In potato tissue, especially the combination pretreatment reduced the pectin DM, whereas the effect of the PEF and the mild thermal pretreatments was less clear. Furthermore, the formation of pectin-Ca2+ crosslinks increased after each type of pretreatment in the presence of Ca2+ ions.

From this study it is clear that PEF technology has potential as a texture preserving tool during thermal processing of plant tissues. However, the tissue-dependent effect may result in texture heterogeneity within a vegetable. Further research is necessary to completely understand the impact of the different pretreatments on the pectin structure and organisation.