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Preheating potatoes at 50 to 80 degrees C has a firming effect on the cooked potato tissue. This effect is particularly pronounced at a preheating temperature of 60 to 70 degrees C followed by cooling. Several theories have been presented in the literature to explain this firming effect: retrogradation of starch, leaching of amylose, stabilization of the middle lamellae and cell walls by the activation of the pectin methylesterase (PME) enzyme, and by the release of calcium from gelatinized starch and the formation of calcium bridges between pectin molecules. Most probably, none of these theories alone can explain the phenomenon and more than one mechanism seems to be involved. Some of these mechanisms seem to be interdependent. As an example, calcium could be considered as a link all the way through release after starch gelatinization to cross-linking pectin substances in the cell wall and the middle lamellae, which has been demethylated by the PME enzyme. More research and "clear cut" experiments are needed in order to elucidate the role of each mechanism, especially which of them is the main contributor to the process of firming. Most probably, the calcium-pectin-PME mechanism plays a secondary role, that is, it only retards the collapse of the tissue structure that would otherwise occur during the final heating without preheating, and it is not the main factor of firmness.

The current study denotes the prediction of activity coefficients of fifteen natural phenols (tyrosol, hydroxytyrosol, oleuropein, caffeic, cinnamic, p-coumaric, ferulic, gallic, p-hydroxybenzoic, p-hydroxyphenyl acetic, protocatechuic, rosmarinic, sinapic, syringic, and vanillic acid) in seven solvents (water, ethanol, methanol, acetone, dichloromethane, ethyl acetate, and diethyl ether), and three extraction temperatures (298.15, 313.15, and 333.15 K), using the universal functional-group activity coefficient model. Solvents were classified for their ability to dissolve phenols and were compared with experimental data of the literature in order to observe if the solvent extraction of phenols in practice matches with the authors' theoretical approach. Results indicated the superiority of alcohols and acetone for the recovery of phenols in line with experimental data of previous studies. Furthermore, activity coefficients' values were found to increase with the increase of temperature. This study provided a knowledge base for the selection of the most appropriate solvents for a given phenolic compound.

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Orphanides A., Goulas V., Gekas V. (2013): Effect of drying method on the phenolic content and antioxidant capacity of spearmint. Czech J. Food Sci., 31: 509-513.The changes in total phenolics, hydroxycinammic acid derivatives, and antioxidant properties of spearmint after five drying treatments (convection oven drying, freeze-drying, microwave drying, and air drying with the sun exposure and without the sun exposure) were investigated. Phenolic composition of dried spearmint was analysed by spectrophotometric assays, while DPPH radical scavenging activity and Ferric reducing/Antioxidant power (FRAP) assay was used to measure the antioxidant properties. The results showed that freeze drying produced dried spearmint that had the highest total phenolics (34.6 ± 1.9 mg/g) content and the most potent antioxidant capacity (126.2 ± 0.4 mg/g for FRAP and 88.1 ± 5.9 mg/g for DPPH, respectively). On the other hand, spearmint that was dried by convection oven and microwave drying presented the lowest amount of phenolic compounds (12.0 ± 0.5 mg/g) and antioxidant potency (49.3 ± 0.7 mg/g for FRAP and 26.9 ± 1.6 mg/g for DPPH, respectively). This might be attributed to the fact that heat-sensitive phenolics were degraded or biotransformed at high temperatures. The loss of phenolic compounds and antioxidant activity reached up to 60% compared to freeze drying.

The objective of this work was to study the dependence of the heat transfer coecient (h) on the water loss rate of potato during frying. An indirect method was used where a metal piece with the same geometry of the potato pieces was placed on top of various potato samples at dierent frying times, and its temperature was recorded for 20±30 s. Another method consisted of direct recording of the temperature within a potato slice, close to the surface. Water loss rate was estimated by image analysis of bubbles. After immersion in hot oil, the potato temperature increases and water starts vapourising, leaving the surface in the form of bubbles that ow through the oil. The water loss rate increases until complete drying of the potato surface and then decreases till the end of frying. The h value showed the same behaviour increasing up by two times in relation to the values measured in the absence of bubbling, with maximum values depending on the oil temperature and potato geometry (443±750 W m À2 K À1 ). The percentage of heat transferred to the potato that is used for water evaporation showed an increase with time up to complete surface drying.

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