Proteolytic Activity of Edible Spruce Morchella esculenta

This study focused on the determination of non-specific proteolytic activity of edible spruce Morchella esculenta in water extract, phosphate-buffered saline (PBS) solution (pH = 7.5) extract and a suspension prepared from 200 mg DW (dry weight) of edible spruce in PBS solution (pH 7.5). A clear casein solution was used as a substrate. The absorbances were measured in quartz cuvettes at the wavelength of 280 nm against a blank with zero concentration of trypsin. Non-specific proteolytic activity was expressed as trypsin equivalents per kilogram of mushroom dry weight (mg.kg−1 DW). All of the extracts demonstrated non-specific enzymatic activity. The highest activity was observed in the PBS suspension and the lowest enzymatic activity was measured in the water extract of the Morchella esculenta fungi. The non-specific proteolytic activity decreased in the following order: PBS suspension extract (pH 7.5; 22.9 mg.kg−1 DW), followed by PBS extract (pH 7.5; 13.6 mg.kg−1 DW) and finally the water extract (10.94 mg.kg−1 DW).

Changes of pH inβ-Lactoglobulin andβ-Casein Solutions during High Pressure Treatment

The pH changes in the milk systems,β-lactoglobulin B,β-casein, and mixture ofβ-lactoglobulin andβ-casein (pH 7 and ionic strength 0.08 M) were measuredin situduring increasing pressure up to 500 MPa. An initial decrease to pH 6.7 was observed from 0.1 to 150 MPa forβ-lactoglobulin, followed by an increase to pH 7.3 at 500 MPa. The initial decrease is suggested to be caused by the deprotonation of histidine, while the increase is suggested to result from an increase of hydroxide ions due to the loss of tertiary structure. The change in pH of theβ-casein solution displayed an almost linear increasing pressure dependency up to a pH of 7.7 at 500 MPa. The limited tertiary structure ofβ-casein could allow exposure of all amino acids; thus the increase of pH can be caused by binding of water protons resulting in an increase of hydroxide ions. Addition ofβ-casein toβ-lactoglobulin (1:1) was found to suppress the initial pH decrease found for theβ-lactoglobulin solution. The pH change of the mixture did not suggest any intermolecular interaction, and a simple additive model is proposed to calculate the pH change of the mixture from the corresponding individual samples.

Influence of pressure release rate and protein concentration on the formation of pressure-induced casein structures

The formation of pressure-induced casein structures (600 MPa for 30 min at 30°C) was investigated for different pressure release rates (20 to 600 MPa min−1) and casein contents (1 to 15 g/100 ml). Structures from liquid (sol) to solid (gel) were observed. The higher the protein content and the pressure release rate, the higher was the dynamic viscosity. A firm gel was built up at a casein content of 7 g/100 ml for a pressure release rate of 600 MPa min−1, while lower release rates resulted in less firm gels (200 MPa min−1) or liquid structures (20 MPa min−1). In a 5 g/100 ml casein solution and at a pressure release rate of 600 MPa min−1, casein aggregates were generated which were built from smaller casein particles with a larger hydrodynamic diameter and higher voluminosity than in the untreated solution. After a slow release rate casein micelles had a smaller hydrodynamic diameter and a lower voluminosity, but were similar in shape and diameter as compared with the micelles in solution before high pressure treatment.

A 43Ca and 31P NMR study of the calcium and phosphate equilibria in heated milk solutions

SummaryCasein micelle suspensions, colloidal Ca phosphate-free casein solution and simulated milk ultrafiltrate (SMUF) were studied using high-resolution 43Ca and 31P NMR in the temperature range 4–64 °C. Only one 43Ca and one 31P signal was obtained for each solution. Signal intensities and line widths varied with the environment and were affected by the redistribution of Ca and P during heating and cooling. Based upon the temperature-dependent broadening of the 43Ca signal, five different Ca environments were discerned in the heated milk fractions. The Ca state, induced by heating of a casein-containing milk salt solution, differed from both the state of Ca present in micellar colloidal Ca phosphate and from the state of Ca induced by the heating of SMUF.