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During the storage season changes in the chemical composition of the pectin moiety of the cell walls of raw potatoes were studied. This compositional analysis was performed for the cultivars Nicola and Irene, which represent two extremes with regard to sensory-perceived texture. Both cultivars were divided into three size categories. From each size category a dry matter (DM) distribution was made. From these distributions potatoes at the low and high ends of this distribution were selected for further analysis. In total 12 different samples were analyzed three times during the storage season. The analysis comprised a pectin fractionation study. Pectic fractions were extracted from the cell wall material (CWM) by increasing the harshness of the extraction procedure. This resulted in a calcium-complexed pectic fraction, two pectic fractions weakly bound to the CWM, and a residue fraction, respectively. It was shown that no statistically significant differences (p > or = 0.95), either in yield or in chemical composition, could be observed between the two cultivars studied (Nicola and Irene), between sizes (large, medium, and small), and between potatoes with either high or low DM contents. However, statistically significant effects of storage both on the yield and on the chemical composition of the pectic moiety of the CWM could be observed, irrespective of cultivar, size, and DM content. Despite the substantial changes in the composition of the pectic moiety of the CWM of the raw material, no to minimal changes in the sensory-perceived texture of the cooked potatoes were observed upon storage. This suggests that the observed changes in pectin composition upon storage are overruled by other aspects that contribute more importantly to the sensory-perceived texture of steam-cooked potatoes.

Two potato cultivars representing extremes with regard to the texture of the cooked product were divided into subcategories based on size and dry matter (DM) content. The effects of the preheating temperature and time on both the instrumentally determined firmness and the sensory-perceived firmness were measured and compared. Increasing the preheating time at 60 degrees C followed by cooking resulted in greater force required to fracture the tissue, an increase in perceived firmness, and a less mashable product. A principal component analysis showed that with higher DM contents of the potato samples, preheating resulted in a larger force required to fracture the tissue and a firmer product. The changes in fracture force were not linearly related with the changes in perceived firmness. The effects of preheating on the pectin methylesterase (PME) activity, the enzyme assumed to be responsible for the firming effect upon preheating, showed that the activity of this enzyme remained rather constant during preheating at 60 degrees C for 1 h. Preheating at 78 degrees C for 10 min abolished virtually all PME activity. To obtain insight into the consequences of preheating and preheating followed by steam cooking on the yield and composition of the cell wall material (CWM) of potatoes, a cell wall isolation followed by a pectin fractionation study was performed. Attention was also paid to the consequences of the processing conditions applied on the chemical composition of the CWM and the sequentially extracted pectic fractions. Preheating resulted in an increase in yield of the CWM of cooked potatoes and, as a consequence, all of the sequentially extracted fractions, including the residue. Preheating did not have a pronounced effect on the composition of the pectin of the sequentially extracted fractions. This altogether strongly indicates that preheating causes a PME-based firming effect, resulting in an decrease in pectin degradation and, as a consequence, a larger yield of CWM. It seems reasonable to assume that this increase in amount of CWM results in a firmer texture. The contribution of starch-based degradation products to the texture after preheating can, however, not be excluded.

Properties of fresh potatoes, including dry matter (DM) content, starch content, and near-infrared (NIR) spectra, were determined and related to the sensory-perceived texture of the steam-cooked samples. To quantify these relationships, three potato cultivars, respectively representing a firm cooking potato (cv. Nicola), a mealy cooking potato (cv. Irene), and a cultivar (cv. Bintje) with intermediate cooking properties, were classified on the basis of three size categories. For each size category and cultivar a DM distribution was determined on the basis of the underwater weight of the individual potatoes. Each DM distribution was divided into three subcategories based on low, medium, and high DM contents. This categorization was performed for freshly harvested potatoes and for potatoes stored for 3 and 6 months, respectively. In total, this resulted in 27 DM distributions, of which 16 were non-normally distributed, and 81 samples. Linear relationships were established between the DM content, as determined by either underwater weight analysis or oven-drying, and the starch content. On the basis of partial least-squares regression (PLSR), statistical models were developed relating sensory-based texture descriptors with the DM matter content of the samples. It was also shown, by applying PLSR, that the NIR spectra, originating from the potato samples, could be related to the DM content and to the sensory-perceived texture. From the relationships between DM content and sensory-perceived texture, on the one hand, and from the DM content and NIR spectra, on the other, it was concluded that the DM content rather than the cultivar determines the sensory-perceived texture of steam-cooked potatoes. Cultivar-specific elements may also contribute to the perceived texture but are overruled by the DM content. Storage did not affect the mutual relationships between the DM content, the sensory properties, and the NIR spectra.

SummaryEight mechanical properties of three potato cultivars were measured during low-temperature blanching (55, 65 and 75 °C) and subsequent cooling and cooking treatments. The mechanical properties were determined with a ring tensile test. Force-based properties showed a decrease in value during blanching and cooking, whereas deformation-based properties showed an initial increase during blanching, before decreasing, during cooking or after longer blanching times. This behaviour was attributed to the loss of turgor of the fresh potato tissue. Rates of change of mechanical properties during cooking depended on the pre-blanching treatments: longer blanching times resulted in a slower decrease of the mechanical properties. The overall behaviour during blanching and cooking of the three cultivars was similar. A principal component analysis on the measured mechanical properties, for each blanching temperature separately, was used to study the blanching temperature effect and to segregate potato cultivars on their cooking behaviour. [ABSTRACT FROM AUTHOR]