Sensory Changes in Umami Taste of Inosine 5'-Monophosphate Solution after Heating

Discrimination in umami taste of inosine 5′-monophosphate (IMP) solution caused by thermal degradation was investigated by sensory evaluation. The difference threshold of umami taste of 0.005% IMP solution in the presence of 0.05% monosodium glutamate (MSG) was 0.002%. The difference threshold of a 0.005% IMP solution decreased by about one half when heated at 95°C for 15 h. Inosine, one of the main products of the thermal degradation of IMP, had a bitter taste. The detection threshold of inosine varied widely among panelists. Heating a 0.005% IMP solution at 95°C for 15 h formed inosine at about one tenth of its lowest detection threshold.     

Protein Content in Milk by Near-Infrared Spectroscopy

A corrected equation for the determination of protein content in oil/ water emulsions, developed previously, was applied for determination of protein content in milk and standard errors were satisfactory for the determination. This corrected equation can be used for determination of protein content in milk.     

n-Hexanol Formation from n-Hexanal by Enzyme Action in Soybean Extracts

n-Hexanal in soybean homogenates decreased during incubation at alkaline pH. When soybean extracts were dialyzed and then incubated with n-hexanal and cofactors [nicotinamide adenine dinucleotide (NAD⁺), the reduced form (NADH), nicotinamide adenine dinucleotide phosphate (NADP+), and the reduced form (NADPH)], NADH and NADPH stimulated the enzymatic reduction of n-hexanal with NADH being more effective than NADPH. When undialyzed preparations were incubated, all the cofactors stimulated enzymatic activity, with NADH being the most effective. The reaction product in all the incubation mixtures was n-hexanol; n-hexanal was reduced to n-hexanol stoichiometrically. It was suggested that alcohol dehydrogenase was responsible for the decrease of n-hexanal in soybean homogenate at alkaline pH.     

Near Infrared Spectroscopic Measurement of Protein Content in Oil/Water Emulsions

The regression coefficient (slope) of the calibration for determination of the protein content decreased as oil content increased. A corrected equation for the effect of oil was derived using optical intensities at 2170 nm (protein) and 2306 nm (oil). The accuracy of the corrected calibration was tested against separate data sets with each oil content. Standard errors of the corrected calibration were comparable to the standared regression equation over a wide range of oil content, suggesting that this corrected calibration is useful for determination of the protein content in oil/water emulsions.     

Cross-Linking of αs-1 Casein by Sodium Hypochlorite

α_s-1Casein was polymerized in the presence of sodium hypochlorite (NaOCl). The polymerization was suppressed by succinylation of the protein. Dityrosine and α-aminoadipic acid were detected in the hydrolysate of the reacted proteins. A carbonyl group was detected in the reaction product of acetyl lysine methylester and NaOCl. The Schiff base formation between lysine and a-aminoadipic δ-semialdehyde residues, and dityrosine formation may clarify the mechanism of polymerization. The chemical modification of proteins by NaOCl is expected to be useful for improving the functional properties of food proteins, since polymerization by NaOCl could occur under mild conditions (NaOCl cone, <0.05%; time, <5 min; at 37°C and pH 7-9).     

Thermal Degradation of Flavor Enhancers, Inosine 5'-monophosphate, and Guanosine 5'-monophosphate in Aqueous Solution

The mechanism of thermal degradation of inosine 5′-monophosphate (IMP) and guanosine (GMP) was investigated kinetically in aqueous solution as a function of pH and temperature. The degradation of IMP and GMP followed first order kinetics. The rate constants were considerably affected by pH and temperature. The half-life times at 100°C were: IMP, 8.7 hr (pH 4.0), 13.1 hr (pH 7.0), 46.2 hr (pH 9.0); GMP, 6.4 hr (pH 4.0), 8.2 hr (pH 7.0), 38.5 hr (pH 9.0). These times were shortened to about one-third by raising the temperature 10°C. The predominant degradation products were nucleosides and phosphoric acid, indicating that the main reaction of the thermal degradation was the hydrolysis of the phosphoric ester bond in the nucleotides.     

Damage of Amino Acid Residues of Proteins after Reaction with Oxidizing Lipids: Estimation by Proteolytic Enzymes

A mixture of casein and methyl linoleate was stored at 50°C and 80% relative humidity for 0 - 14 days and damage to amino acid residues assessed. The damage was estimated by determining the amino acid composition of the hydrolysate by using proteolytic enzymes (pepsin-pancreatin digestion, followed by aminopeptidase-prolidase hydrolysis). The damage to amino acid residues was the most extensive in methionine, followed by tryptophan, histidine, and lysine. The degree of damage to these amino acids was also determined by chemical methods without using proteolytic enzymes. The efficiency of detecting damage by the present enzymatic method was close to that of the chemical methods.