Jiří Blahovec

Potato Tuber Dynamic Mechanical Analysis at Temperatures of Starch Gelatinization

Dynamic mechanical analysis (DMA) was applied to potato cortex tissue in temperature scans in range 30–90°C and constant air humidity of 90%. The obtained scans indicate peaks in both storage and loss module of elasticity (SM and LM, respectively) at temperatures higher than 70°C. The peaks follow starch gelatinization processes in tissue detected by DSC at lower temperatures. The peak characteristic temperatures were determined in replicated experiments for seven potato varieties. It was shown that increasing tissue density leads to higher characteristic temperatures. The peak characteristic values (for both SM and LM) were more variable and cultivar role for them was not proved. The role of the observed module peaks in the potato cooking process is discussed.

Changes in carrot (Daucus carota) parenchyma at higher temperatures detected in vivo by dynamic mechanical (thermal) analysis

The outer parenchyma of carrot was tested using dynamic mechanical analysis (DMA), in air with 90% humidity between 30 and 90°C. Temperature plots of storage (SM i.e. elastic) and loss (LM i.e. inelastic) moduli were obtained. The SM and LM values were the basis for the calculation of the loss tangent (LT), the parameter expressing the ratio of inelastic to elastic parenchyma toughness. As expected, the tissue toughness decreased with increasing temperature. For both moduli, two characteristic temperature areas with temperature slope minima were observed--they were termed low (I) and high (II) temperature negative peaks on the temperature slope plots. It was shown that the negative peaks were related to an increase in the inelastic part of the tissue toughness. All plots were dependent on the temperature rate: increasing the temperature rate (from 0.5 to 2°C/min) resulted in a shift of both the negative peaks I and II to higher temperatures, thereby reducing I and increasing II. It was shown that the observed behaviour cannot be described by simple kinetic equations due to the time dependent and complicated character of the thermally induced changes. These changes were interpreted as a consequence of pore protein denaturation followed by changes of the stress inside the parenchyma cells.

Pulsed Electric Fields Pretreatments for the Cooking of Foods

Development of the concept of electroporation opened new perspectives for promising applications in food technology. Treatment of foods with pulsed electric fields (PEFs) allows facilitation of different food transformation operations (extraction, expression, osmotic treatment, drying, and freezing) with minimal energy consumptions and better retention of flavor, color, and preservation of nutritional properties of foods. This work shortly reviews the effects of PEF on the biological cells and food products and gives the examples of PEF-assisted techniques. The PEF protocol, power consumption, and existing small- and large-scale electroporation systems are presented. Some examples of PEF-assisted processing of meat, fish, and fat frying are discussed. The main principles of PEF-assisted cooking and kitchen operations are also discussed. The variants of PEF-assisted non-thermal cooker and PEF/ohmic thermal cooker are presented. It is speculated that PEF allows more homogeneous treatment of foods as compared to the conventional methods of thermal cooking. The PEF-assisted cooking can be faster and more effective for nutrient retention and sensory qualities of foods. Moreover, the PEF treatment can be used for producing the types of the products of fresh/natural quality and new tastes. The recent examples of PEF-assisted processing of meat and fish, assistance of frying, and commercial-scale processing are also presented and discussed.

Reciprocal slope transformation applied to physical problems

Abstract The reciprocal slope transformation (RST) for variables x (independent) and y (dependent) is given [ y ] = x / y where the transformed quantity is denoted by the square brackets. The simple rules for calculation with this transformation are developed and the use of the transformation is demonstrated on a simple physical example: analysis of sorption phenomena.

Stress Relaxation in Potato

Most part of vegetable flesh is formed by soft and juicy parenchyma. The big cells with thin cell walls belong to the important characteristics of its structure. The vegetable deformation has partly irreversible character (Mohsenin, 1970), even at low deformation levels. It means that the simple mechanical tests, originally developed for testing the solid products of elastic character, can be used for testing the vegetables only in some limited cases. Instead of elasticity the rheological methods, models and theories have to be used. They are based on an important role of time in deformation process.

Moisture induced changes of volume and density of some cereal seeds

The effect of wetting in the density and volume of seeds of barley, rye and two cultivars of wheat was determined. Two levels of wetting were used: (i) 6 h wetting close to the end of imbibition; (ii) 24 h wetting close to the beginning of germination. The experimental results show that the variability of the seeds’ volume and density in all tested states can be well described by the Gaussian distribution. The changes of the seed’s volume and the seed’s density caused by wetting can be then easily expressed via the changes of the distribution parameters. The increase in the seed’s density and the seed’s volume was the main effect of the wetting, but the level of the observed changes was variable for different crops and different degrees of wetting. Density increase was observed mainly in the case of imbibition, whereas longer wetting was connected mainly with changes of the seed’s volume.

Starch behaviour in potato at temperatures close to cellular membrane breakdown

Abstract A combined thermal analysis (including a dynamic mechanical analysis and a dielectric thermal analysis) was applied to the cortex tissues of long-stored potatoes in the temperature range of 30-90°C, with the constant air relative humidity of 90%. Two heating rates, i.e. 1°C min−1 and 0.5°C min−1, were used for temperature scanning. The obtained results were found to differ in the inspected temperature range, indicating either a different mode of drying or differences in the starch gelatinization mechanism, the latter being usually controlled by water availability in contact with starch in both tests. The observed results indicate new possibilities in the cooking strategies that are usually applied to potatoes.

The modified CPEM (cooked potato effective mass) method: an instrumental assessment of potato sloughing.

The cooked potato effective mass (CPEM) method for potato sloughing assessment involves cooking the potato flakes on the sieve in a stirred water bath and periodically determining their effective mass during cooking. The final cooking curve divided into the cooking and breaking parts provides two parameters: the cooking time (CT) is the time required for starting disintegration, while the slope of the breaking part (SBP) describes the disintegration rate. The method enables a detailed analysis of the cooking properties in relation to the tuber density. The modified analysis of the cooking curve is based on polynomial approximation of the breaking part. It provides the time of cooking (CT maX ) required to reach the maximal disintegration rate (MDR). These new parameters represent an alternative to the existing ones, their values are easier to obtain from the individual cooking curves, and therefore they can serve as a base for further development of the CPEM tests.

Pistachio deterioration detected by X-ray absorption.

Proshlyakov A., Yanniotis S., Blahovec J. (2013): Pistachio deterioration detected by X-ray absorption. Czech J. Food Sci., 31: 126–131. The measurement of the absorption coefficient of X-rays in healthy and fungus infected pistachio kernels was the main objective of this work aimed at detecting the potential injury by insects and/or fungi in the kernel. It was found that the absorption coefficient in the injured parts is about half of that in the healthy parts. The absorption coefficient is also affected by the position in the kernel due to the variations in cell dimensions and the oil and moisture contents in the kernels.

Crop Prices in the Austrian Monarchy (1770–1816): The Role of Crop Failures and Money Inflation

Abstract The paper is based on data about local cereal prices in Bohemia recorded in the course of 46 years (1770-1816) by F.J. Vavák, Czech farmer living close to Prague. This information appeared in a series of his memoirs published within 1907-1938 and in 2009. The data analysis shows that annual means of prices of wheat, rye, and barley are relatively well correlated and can be used for illustration of the state monetary situation. A higher variation of the annual mean values was observed only in times of crop failure and was also related to the new harvest. The data obtained for pea differ from the data mentioned above. It was shown that the market prices were influenced mainly by crop failures caused by weather and climate and by the state monetary policies. The money purchasing power was influenced either by objective causes, like crop failures of reversible character, or by subjective changes caused mainly by inflationary monetary policies which in the case of Austria had conduced to a state bankruptcy.