Jan Pokorný

Physical refining of edible oils

Crude oils obtained by oilseed processing have to be refined before the consumption in order to remove undesirable accompanying substances. The traditional alkali refining is often replaced by physical refining in which the use of chemicals is reduced. The most widely used method is steam refining. The crude oil quality is very important in order to obtain high quality refined oil. Furthermore, the oil should be efficiently degummed to remove phospholipids as well as heavy metals and bleached to remove pigments. The most important step consists of the application of superheated steam under low pressure and at temperatures higher than 220 °C. Both free fatty acids and objectionable volatiles, formed by cleavage of lipid oxidation products, are removed. A disadvantage is the partial loss of tocopherols. Side reactions, particularly isomerization of polyunsaturated fatty acids, should be minimized. The quality of physically refined oil is close to that of alkali refined oils, but losses of neutral oil are lower and the environment is less polluted. Among other methods of physical refining the application of selective membranes is promising.

Effects of plant-scale alkali refining and physical refining on the quality of rapeseed oil

The physical refining of soybean oil was introduced as an energy saving and environmentally friendly procedure alternative to the traditional alkali refining, and the process was successfully applied to other vegetable oils. We had compared the two procedures in industrial refining under conditions, which enable a clear comparison. In nine plant-scale experiments, crude rapeseed oil, taken from the same tank of crude oil, was processed on the same day both by alkali refining and by physical refining. Quality changes (free fatty acids, peroxide value, conjugated fatty acids, polar lipids, minor constituents) were determined, and also their stability against oxidation (Rancimat and Schaal Oven Test), and the fatty acid composition. In refined oils, the sensory acceptabilities and the sensory profiles were assayed. Finally deodorized oils, produced by the two methods, did not appreciably differ in their sensory characteristics and chemical composition, excepting slightly higher concentration of isomeric polyunsaturated fatty acids in physically refined oils. During storage for one year in commercial packagings at 15 °C, oxidative and sensory changes were negligible.

Rheological properties and sensory texture of mayonnaise

Mayonnaises are viscoplastic oil-in-water emulsions, in which rheological properties have important influences on functional properties and the sensory quality. A set of 17 samples of both traditional and light mayonnaises were tested at 10 and 25 °C, using a rheoviscometer Rotovisco RT 10. Nonlinear relations were observed between the yield value and the apparent viscosity. The yield value was correlated with several sensory characteristics rated after manipulation with a spoon, but not in oral testing. No relations were obtained between the apparent viscosity and sensory characteristics. The yield value and the texture acceptability were related significantly to the flavour acceptability. Multivariate statistical techniques were found advantageous for improving the prediction of the texture acceptance.

Role of oxidized lipids in nonenzymic browning reactions

Abstract Different lipid oxidation products, such as free radicals, hydroperoxides and aldehydes, react with free amine groups of proteins and with free amino acids, forming yellow intermediary products which polymerize into dark brown macromolecules. The intermediary products are imines, pyridine derivatives and even aldolization products with low nitrogen content. The nutritional value of brown products is very low.

Effect of the unsaturation degree on browning reactions of peanut oil and other edible oils with proteins under storage and frying conditions

Abstract Modified peanut oil cultivars with 3–6% linoleic acid such as SunOleic, are much more stable against oxidation, and thus against browning discolouration than traditional peanut cultivars. Interaction products using SunOleic peanuts are much less coloured, which is important for the production of snacks, crackers and peanut butter.

Interactions between phospholipids, saccharides and oxidized lipids in non-enzymic browning reactions

Abstract In addition to browning products analogous to those occurring in oxidizing lipids, other types of browning reactions were observed that produced by intermolecular or intramolecular interactions between amine groups of phospholipids and hydroperoxides or their carbonylic degradation products. Primary amine groups of phosphatidylethanolamine, phosphatidylserine and the respective lysophospholipids react with oxidized acyl chains of phospholipids, forming yellow or red oxidation products. Polyunsaturated imines (Schiff bases) are formed, which rapidly turn brown. The presence of amine groups accelerates browning reactions, slowly on storage, rapidly on heating.

Psychorheology of food dispersions

Psychorheology of food dispersions Food dispersions are the most frequent and most important food components. Useful information was obtained in the investigations of liquid emulsions. The results obtained in their study were compared with tomato ketchups. They were chosen as an example of food dispersions with low lipid content. A set of 20 samples of tomato ketchups purchased on the native market were analyzed by several rheological procedures (RheoStress 300) and by sensory profiling both of textural and flavour characteristics. The psychorheology was used for the determination of relationships between results of instrumental and sensory measurements. The data base obtained was processed by statistical methods. The temperature affects the results differently, but it is not useful to study temperatures below room temperature. The rheological parameters agree with the Herschel-Bulkley equation. Therefore, the psychorheology was applied as a suitable complex method. About a third of plots between two variables were significantly related. Several statistically significant relationships were obtained between rheological attributes, sensory attributes, and the combinations between rheological and a sensory attributes. Great variance in all characteristics was observed in all samples variables. Similarly as in case of lipid dispersions, the sensory texture acceptability was significantly related with the overall flavour acceptability, similarly as in case of other food dispersions. It proves the importance of texture in consumption of tomato ketchups. Psychoreologie potravinářských disperzí Potravinářské disperze patří mezi nejrozšířenější potravinářské produkty. Potřebné informace k prezentovaným výsledkům předloženého studia byly získány předchozím studiem potravinářských emulzí. Kečupy byly vybrány jako příklad potravinářských disperzí s nízkým obsahem tuku. Bylo hodnoceno celkem 20 vzorků komerčních výrobků tomatových kečupů, zakoupených v české obchodní síti. Reologické charakteristiky byly získány pomocí rotačního reometru RheoStress 300 a texturní vlastnosti pomocí senzorické analýzy. Jako nejvhodnější metoda byla zvolena psychoreologie, která zkoumá funkční vztahy mezi reologickými parametry, stanovenými instrumentální analýzou a senzorickými charakteristikami, stanovenými senzorickou analýzou. Znalost těchto vztahů je důležitá ke kvantitativní charakterizaci tokových a deformačních dějů, probíhajících při senzorické zkoušce. Reologické parametry vyhovují stavové rovnici Herschel-Bulkley. Korelací reologických a senzorických dat byly získány v třetině případů těsné funkční závislosti s vysokými hodnotami korelačních koeficientů. Analýzy prokázaly velké rozdíly mezi jednotlivými vzorky kečupů. Při nižších teplotách by se patrně projevily zřetelněji vlivy optimální textury a získaly se bližší hodnoty mezi vzorky. Stejně jako u dříve hodnocených potravinářských produktů bylo prokázáno, že textura výrazně ovlivňuje chuťové vlastnosti, což zdůrazňuje význam textury při konzumaci rajčatového protlaku.