M. Karel

Oxidation at intermediate moisture contents

In the study of the oxidation rate of methyl linoleate in protein and cellulose systems, a prooxidant effect was found at intermediate moisture contents. At low water content, water hydrates metals and hydrogen bonds with peroxides, and an overall decrease in the rate of lipid oxidation results. With an increase in the water content to the region with a water activity of 0.6 to 0.7, the water predominantly acts as a solvent to dissolve and mobilize previously unavailable trace metals with the result of increased oxidation rates. Use of chelating agents such as ethylenediaminetetraacetic acid and citric acid reduced oxidation significantly although some antioxidant activity was also observed for butylated hydroxyanisole. These results have important implications in the preparation of intermediate moisture foods.

Effect of water-binding agents on the catalyzed oxidation of methyl linoleate

Autoxidation of methyl linoleate was studied in model systems containing added metals and different water-binding agents including cellulose, dextran and glycerol. In all systems, addition of water was increasingly antioxidant up to a critical level of water activity, above which further additions promote oxidation. The specific level of water activity at which oxidation rates are minimal and the water contents at this critical activity depend on the composition of the system. Results of the present study and much of the conflicting data in the literature can be explained as follows. A small amount of water is tightly bound to polysaccharides and does not affect lipid oxidation. Additional amounts of water are antioxidant because of their ability to hydrate metallic catalysts and to form hydrogen bonds with hydroperoxides. At high water contents the waters solvent action mobilizes catalysts, thereby overcoming the antioxidant effects. The present study indicates that water bound to glycerol is still capable of mobilizing catalysts. Depression of water activity by addition of agents such as glycerol has different effects on oxidation than does depression of water activity by other means. Interaction between the effects by system composition and water activity must be considered when either is varied to maximize storage stability of foods

Metal-catalyzed oxidation in the presence of water in foods

Methyl linoleate was oxidized in model systems consisting of either cellulose or casein with which the lipid was dispersed with water containing cobalt salts. The dispersion was extruded into Warburg flasks, frozen and freeze-dried at 100 µ Hg and with platen temperatures of 80 F. The samples were then humidified over saturated salt solutions to give moisture contents from less than 1 g H2O/100 g solids up to 30 g H2O/100 g solids. The higher moisture contents were obtained by addition of glycerol to the model system during preparation and humidification at 60–75% RH. Chelating agents including EDTA and citric acid in concentrations of 1 to 10 moles per mole of cobalt ion were used in some experiments. Oxidation was followed manometrically and by peroxide analysis. At low water contents, water acts as an antioxidant through hydration of metallic catalysts. As the moisture content increases, water promotes oxidation through its solvent activity. In the region of capillary condensation, antioxidant effect of metal hydration is overshadowed by the prooxidant effect of metal solubilization. The water soluble chelating agents such as EDTA act on metals in aqueous solution and their activity is promoted by increased moisture content.

Effects of humidification on activity of catalysts and antioxidants in model systems

Studies on freeze-dried model systems containing methyl linoleate and various additives have shown that water content plays an important role in controlling the activity of antioxidants and catalysts; water can counteract the catalytic activity of certain metals, and can enhance the antioxidant effect of some chelating agents. Different additives, however, respond differently to the effects of humidification, especially at high water activities. Data are presented which show that complexes of managanese and histidine show enhanced prooxidant activity at high water contents, while those of cobalt and histidine become more antioxidant. The effectiveness of EDTA as a chelating agent is also enhanced as the moisture content is increased. However, the effects of humidification are dependent on other components of the system, including the nature of the hydrophilic support.

Potential for utilization of algal biomass for components of the diet in CELSS

Techniques which eliminate or reduce the undesirable cell components of algae and enhance the potential nutritional and organoleptic acceptability of algae products are studied. The cell walls, nucleic acids, and pigments and lipids of the green algae Scenedesmus obiliquus need to be removed. The procedures for determining the composition of proteins, pigments and lipids, and moisture and ash are described. Chemical, enzymatic, and physical methods of removing the cell wall to make the algae digestable are analyzed; a homogenization technique is utilized. The problems encountered if algae nucleic acids are ingested directly are discussed; the reduction of DNA and RNA by applying extracellular DNase and RNase to the nucleic acids is examined. The color and flavor of the algae are enhanced with the extraction of pigments and lipids from the algae protein concentration.