Milk Model Systems Research Articles

Aldehyde-induced xanthine oxidase activity in raw milk.

In the present study, the aldehyde-induced pro-oxidative activity of xanthine oxidase was followed in an accelerated raw milk system using spin-trap electron spin resonance (ESR) spectroscopy. The aldehydes acetaldehyde, propanal, hexanal, trans-2-hexenal, trans-2-heptenal, trans-2-nonenal, and 3-methyl-2-butenal were all found to initiate radical reactions when added to milk. Formation of superoxide through aldehyde-induced xanthine oxidase activity is suggested as the initial reaction, as all tested aldehydes were shown to trigger superoxide formation in an ultrahigh temperature (UHT) milk model system with added xanthine oxidase. It was found that addition of aldehydes to milk initially increased the ascorbyl radical concentration with a subsequent decay due to ascorbate depletion, which renders the formation of superoxide in milk with added aldehyde. The present study shows for the first time potential acceleration of oxidative events in milk through aldehyde-induced xanthine oxidase activity.

Effect of hydrocolloid type and concentration on flow behaviour and sensory properties of milk beverages model systems

Formulation of low-calorie flavoured milk beverages involves the use of hydrocolloids to obtain an acceptable mouthfeel. Sodium alginate and κ-carrageenan are the most commonly used. In this paper, the rheological behaviour of model solutions containing either of these two hydrocolloids, with or without the addition of sucrose, in water or milk, has been studied. All solutions fitted well to the Ostwald de Waele model. The analyses of variance showed that in κ-carrageenan solutions, the effect of the medium–hydrocolloid interaction on both flow index ( n) and apparent viscosity at 1 s −1 ( η 1) was significant. Milk-based solutions were more pseudoplastic and more viscous than the aqueous solutions, due to the well known κ-carrageenan–casein molecular interaction. In alginate solutions, the medium–hydrocolloid interaction was also significant but the differences in both n and η 1 values were of less entity. Sensory viscosity differences of more viscous chocolate milk model systems were well explained by apparent viscosity values at low shear rate (10 s −1), while for less viscous samples apparent viscosity at higher shear rate (300 s −1) values were appropriate. At similar viscosity, κ-carrageenan systems showed better flavour-releasing properties than alginate.

Thermal Resistance of Bacterial Spores in Milk-Based Beverages Supplemented with Nisin

The effect of nisin, added in the form of Nisaplin, on the thermal resistance of bacterial spores and the effects of medium composition, exposure time, and pH on nisin enhancement of heat sensitivity were evaluated. Nisin apparently required specific nutrients to sensitize spores to heat. For example, D130°C values of approximately 10 s were observed in sodium phosphate buffer with and without 6% sucrose with no significant (P ≥ 0.05) differences detected as a result of increased nisin concentration. In a nutrient-rich chocolate milk model system (CMMS), increasing either the time of exposure to nisin (5, 15, or 24 h) before heating or nisin concentration (0, 2,000, or 4,000 IU/ml) increased the sensitivity of Bacillus stearothermophilus spores to heat. In the CMMS with 10 to 12% fat cocoa powder, increasing nisin concentration (at 5 h of exposure) significantly (P ≤ 0.05) reduced D130°C values; D130°C values were 21.7, 17.2, and 17.8 s, respectively, for the 0-, 2,000-, and 4,000-IU/ml nisin treatments. Fifteen and 24 h of exposure further reduced D130°C values in the nisin-containing treatments compared to the control (0 IU of nisin per ml). A lower-fat CMMS (0 to 1% fat cocoa powder) had lower D130°C values (19.3, 15.8, and 14.7 s for the 0-, 2,000-, and 4,000-IU/ml nisin treatments, respectively). Nisin activity was enhanced by lowering pH in the CMMS (10 to 12% fat cocoa powder), with reductions in D130°C values across all pH values (ranging from 18.0% at pH 6.4 to 41.9% at pH 5.0). zD values were 9.6, 9.0, and 8.4°C for the 0-, 2,000-, and 4,000-IU/ml nisin treatments, respectively. Spores of B. licheniformis yielded results similar to those obtained with B. stearothermophilus. For example, decreasing CMMS (10 to 12% fat cocoa powder) pH values from 6.4 to 5.0 produced D100°C values of 3.3, 2.8, and 2.8 min (pH 6.4) and 1.0, 0.8, and 0.8 min (pH 5.0) for the 0-, 2,000-, and 4,000-IU/ml nisin treatments. This study clearly verified that the addition of Nisaplin to dairy-based beverages, such as a chocolate milk drink, or other foods intended to be heated reduces the thermal resistance of selected bacterial spores. Increased spore sensitivity to heat may provide food processors with an opportunity to reduce their thermal processes and expenses while maintaining product quality, functionality, and shelf stability.

Volatilities of Short‐Chain Fatty Acids in a Fermented Milk Model System as Affected by Stabilizers

ABSTRACTA fermented milk model system was employed to investigate the volatilities of butyric, caprylic, and caproic acids as affected by low, intermediate, and high levels of carrageenan, guar and xanthan gums. Carrageenan exhibited no effect on fatty acid volatility. Intermediate levels of guargum increased the volatility of caprylic acid. Low and high levels of guar gum had no effect on free fatty acid volatility. At low and high levels, xanthan gum had no effect on volatilities of fatty acids. The intermediate level of xanthan gum increased volatilities of caproic and caprylic acids. The effects of viscosity were potentiated with increased fatty acid chain length.

PERCEPTION OF FAT IN A MILK MODEL SYSTEM USING MULTIDIMENSIONAL SCALING

ABSTRACTThis study examined the contributions of stimulus fat content and flavor volatiles to the perception of fat in a milk model system. The model system was formulated by adding bland vegetable oil (0%, 5%, or 10% w/v) and natural cream flavor (0%, 0.5% or 1% w/v) to a skim milk base. Panelists judged pairs of samples for similarity on the basis of three attributes (fat content, mouthcoating and thickness) and the results were analyzed using a multidimensional scaling procedure. Two‐dimensional solutions best represented the data. The stimulus spaces for fat content and mouthcoating were visually similar to each other and provided reasonable separation of the samples. Instrumental measures helped to define the underlying dimensions of the stimulus space for fat content. Dimension 1 related to texture and included the contributions of viscosity, and fat particle size and number distribution; dimension 2 related to flavor perception. The sample with a moderate fat content (5%) and the highest concentration of added cream flavor (1%) was perceived to be similar to the 10% fat samples with added flavor. Thus, the added flavor provided the sensation of higher fat content. These data suggest that flavor plays an important role in the preception of fat in dairy foods. A psychophysical model of fat perception in dairy foods is proposed which includes the contribution of viscosity, fat particle size and number distribution, and volatile flavor perceptions.

Determination of free carbohydrates and Amadori compounds formed at the early stages of non-enzymic browning

α-N-acetyl, ϵ-N-(1-deoxy- d-lactulos-l-yl)- l-lysine (ALL) was prepared as a model of protein-bonded lactose, and characterized by NMR. An HPLC method was designed to determine it. Milk model systems containing lactose, α-N-acetyl- l-lysine and salts were heated at 120°C (in the liquid state) or freeze-dried and stored at 45°C and 0·44 a w. Free carbohydrates formed were determined by GC, and ALL by HPLC. Lactulose was formed at higher levels than ALL when heating, whereas the opposite was true during the storage of lyophilizates. It was shown that enolization is the main pathway of lactose degradation at high temperature, and that addition to amino groups predominates at low temperatures and a w.