DOSTÁLOVÁ J., HANZLÍK P., RÉBLOVÁ Z., POKORNÝ J. (2005): Oxidative changes of vegetable oils during microwave heating. Czech J. Food Sci., 23: 230-239.The oxidative stabilities of pork lard, sunflower, zero-erucic rapeseed, peanut and high-oleic peanut oils were tested under microwave heating conditions. Vegetable oils and lard were heated in a microwave oven for up to 40 min between 25°C and 200°C. The peroxide value, the contents of conjugated dienoic and trienoic acids, and polymers were used as markers of lipid degradation. Sunflower oil was found the least stable oil because of a high polyenoic acid content and a low content of γ-tocopherol. Rapeseed oil was more stable because of a lower polyenoic acid content and a high γ-tocopherol level. Conventional peanut oil was relatively stable, but substantially less stable than high-oleic peanut oil. Pork lard and high-oleic peanut oil formed only low levels of polymers due to a low polyenoic acid content.

Abstract:Oxidation changes of different types of vegetable oils were studied during microwave heating. Samples of vegetable oils (rapeseed, sunflower, soybean and corn oil), commercially available at the market in the Czech Republic, were heated in a microwave oven. Parameters as peroxide value, conjugated dienes and trienes levels were determined in oil samples before and after heating in the period from 3 to 30 minutes.

Methoxypyrazines are the principal aroma components responsible for the vegetative and herbaceous green bell-pepper flavour of Sauvignon blanc wines produced in Moravia. The aroma profiles of 8 samples of Moravian Sauvignon wines were analysed; the levels of 3-isobutyl-2-methoxypyrazine varied in the range of 4.7-17.0 ng/l. The commercial Sauvignon aroma preparation Aroma Fantasia S, available in the region, was analysed; the product contained the 3-isobutyl-2-methoxypyrazine, and in negligible concentrations also anethol and ethylbenzoate. The Moravian Sauvignon blanc wine samples spiked with different amounts of aroma preparation were evaluated by hedonic sensory analysis, to estimate the meaningfulness of such illegal improvement. The most preferred concentration ranged from 5 ng/l to 10 ng/l, which are the natural levels of MP in Moravian Sauvignon blanc wines, therefore the addition of aroma at this level, which can be detectable with difficulties, has no reason. The less sophisticated adulteration of wine, such as the production of artificial Sauvignon blanc wine by the addition of MP into less distinctive wines, is easily detectable according to the aroma profile.

Abstract:The oxidation stability of extra virgin and refined olive oils produced in different countries were studied under different conditions of microwave heating (microwave oven Electrolux, 2450 MHz, 500 W) and conventional heating (200°C). Oils were heated in a microwave oven and in a conventional oven for 0, 3, 6, 9, 12, 15, 20, 25, and 30 minutes. The evaluated parameters were peroxide value, content of conjugated dienes, conjugated trienes (determined by absorbance at 233 nm and 274 nm, respectively), and fatty acid composition by GC. During microwave and conventional heating peroxide values and contents of dienoic compounds differed significantly between control and the heated samples. The microwave treatment did not produce significantly greater amount of oxidation products than traditional heating.

A set of 20 samples of tomato ketchup purchased on the market were analysed by several rheological procedures (Rheo-Stress 300) and by sensory profiling of both textural and flavour characteristics. A great variance was observed of all characteristics in all sample variables. About a third of plots between two variables were significantly related in the case of two rheological attributes and two sensory attributes, and in that of combinations between rheological and a sensory attributes. Similarly as in the case of lipid dispersions, the sensory texture acceptability was significantly related to the overall flavour acceptability. This proves the importance of texture in the consumption of tomato ketchup.

Abstract:We used time-intensity studies in this work for evaluating the perception of sweet taste of sucralose, aspartame, and saccharose over time. In second part of this study the mixture of sweeteners and zinc sulphate, which modifies sweet taste perception, was used. We determined the following parameters from the assessor's charts of time profile: maximum intensity of sweet taste and the time which was needed to get to its maximum, the time period of perception of sweet taste and the area under the curve. For evaluation each assessor received 20 ml of a water solution with the sweetener followed by the mixture of the same sweetener with zinc sulphate. The intensity of sweet taste was measured before and after spitting out the solution. Then it was measured at intervals of 5 s for 70 seconds. It can be seen from the charts that sucralose has a more gentle sweet taste perception in time than the other sweeteners and that the zinc sulphate modifies the sweet taste but it does not inhibit it absolutely.

The influence of microwave heating (microwave oven Electrolux, 2450 MHz, 400 W) from 8 up to 24 min on the oxidation and fatty acid composition of lipids of common carp (<I>Cyprinus carpio</I>) and Atlantic mackerel <I>(Scomber scombrus</I>) minced fish flesh were studied. The heating treatment at all conditions reduced moisture and therefore, increased lipid and dry matter contents. The isolated lipids were subjected to the following analyses: peroxide value, acid value and content of conjugated dienes (by absorbance at 232 nm). The free fatty acid content in the lipid fraction of the fish flesh was significantly reduced by cooking. Conjugated diene levels in fish muscle increased and peroxide values decreased for all cooked samples. Changes in fatty acids composition were only small.

Abstract:The objective of this work was to choose an optimal rate of citric acid and sweeteners for lemon flavoured drops. Two model samples with defined concentrations of citric acid and two commercial samples were evaluated with using instrumental and sensory methods. Concentrations of sweetening agents and citric acid in the samples were determined using HPLC with a RI detector and isotachophoresis, respectively. The general pleasantness of taste and the intensity of acid and sweet tastes of four lemon drops were evaluated using sensory analysis. General pleasantness was evaluated using a ranking test. Intensities of acid and sweet tastes were evaluated using unstructured graphic scales. The best tasting lemon drops contained 11 g/kg of citric acid and 691 g/kg sweeteners related to the sweet potency of sucrose. The sample with an extremely acid taste was considered unpleasant for most assessors (Friedman, α = 0.05). Generally, the assessors preferred the lemon drops with well-balanced acid and sweet tastes.