Edyth L. Malin

Chemistry of structure-function relationships in cheese

Proceedings of an American Chemical Society Symposium held in Chicago, August 1993. Contributors evaluate the current status of cheesemaking and highlight the information that will be essential for new developments. The papers are arranged within sections devoted to physical chemistry of cheese text

Rheology and microstructure of low-fat Mozzarella cheese

The contributions of fat and moisture content to Mozzarella cheese texture were investigated to provide a basis for developing low-fat cheese with consumer acceptability. The characteristics of low-fat high-moisture (LFHM) experimental Mozzarella cheeses before and after 6 weeks of refrigerated storage were compared with those of high-fat low-moisture controls. High levels of either moisture in nonfat substance or fat in dry matter (FDM) were accompanied by decreases in hardness, complex viscosity, and elastic modulus and increases in meltability during the storage time. Starter culture bacteria were observed at the surface of the fat droplets, the latter having a tendency to coalesce during storage. Development of texture and meltability in LFHM Mozzarella appeared to be directly related to increased proteolysis of αs1-casein observed during storage. These results show the feasibility of making Mozzarella cheese containing <25% FDM with textural properties similar to those of a full-fat cheese if the product contains enough moisture and is stored under refrigeration for several weeks.

Chromatographic and electrophoretic methods used for analysis of milk proteins

Current knowledge of milk proteins and their behavior in dairy foods is based on early applications of chromatography and electrophoresis. Electrophoretic identification of the number and genetic variety of milk proteins inaugurated a research effort in which chromatographic techniques were successfully applied to the isolation of each milk protein, thus facilitating the characterization and further study of milk and dairy products. This review focuses on recent applications of chromatography for separations and analysis and on analytical applications of electrophoresis.

Effects of Skim Milk Homogenization on Proteolysis and Rheology of Mozzarella Cheese

Abstract The influence of milk homogenization on proteolysis and functional properties of Mozzarella cheese was examined in an effort to develop low-fat Mozzarella with desirable textural and melting properties. Two-stage homogenization of skim milk at 6870/3430 kPa prior to the addition of nonhomogenized cream did not inhibit the meltability of full-fat or low-fat cheeses, when compared with cheeses made from nonhomogenized milk, but the meltability of full-fat cheese made from homogenized milk was greatly decreased. Homogenization of whole or skim milk had no significant effect on other rheological parameters or on proteolysis of αs1-casein in full-fat and low-fat cheeses. Refrigerated storage at 4 °C caused meltability and proteolysis to increase and the other parameters to decrease. Differences in fat content caused significant changes in hardness, springiness and meltability. Fat globule size and homogenization of protein affected proteolysis and rheology much less than fat content and storage time.

Reorganization of casein submicelles in Mozzarella cheese during storage

Abstract To gain a better understanding of the factors involved in textural changes in Mozzarella cheese during storage, electron microscopic imaging was used to compare the ultrastructure of the protein matrix on the day of stretching (0 weeks) and after 6 weeks of storage at 4 °C. Low-fat and full-fat cheeses were prepared from nonhomogenized milk and from milk homogenized at 10.3 and 17.2 MPa. Analysis of the stained electron density distribution in image transforms of the protein matrix revealed that spacing patterns of regular structures with the dimensions of casein submicelles changed during storage, in most cases forming larger aggregates at longer average spacings, but there were no significant changes in total electron density between fresh and stored cheeses. These effects were related to proteolysis. Submicelles in low-fat cheeses homogenized at 17.2 MPa reorganized differently, a likely result of decreased proteolysis. Submicelles formed part of a new membrane around fat globules in cheeses made from homogenized milk, as previously observed. Changes in submicelle size and distribution could explain some of the differences in texture properties between fresh and stored Mozzarella cheeses, and between products made from homogenized and nonhomogenized milk.

Solution Structures of Casein Peptides: NMR, FTIR, CD, and Molecular Modeling Studies of αs1-Casein, 1–23

To determine its potential for interacting with other components of the casein micelle, the N-terminal section of bovine αs1-casein-B, residues 1-23, was investigated with nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopies, and molecular modeling. NMR data were not consistent with conventional α-helical or β-sheet structures, but changes in N-H proton chemical shifts suggested thermostable structures. Both CD and FTIR predicted a range of secondary structures for the peptide (30–40% turns, 25–30% extended) that were highly stable from 5°C to 25°C. Other conformational elements, such as loops and polyproline II helix, were indicated by FTIR only. Molecular dynamics simulation of the peptide predicted 32% turns and 27% extended, in agreement with FTIR and CD predictions and consistent with NMR data. This information is interpreted in accord with recent spectroscopic evidence regarding the nature of unordered conformations, leading to a possible role of αs1-casein (1–23) in facilitating casein-casein interactions.

Electron-Density Patterns in Low-Fat Mozzarella Cheeses During Refrigerated Storage

Consumer demand for palatable low-fat foods has prompted the development and production of a wide variety of new foods. Consumer interest in low-fat dairy products has increased dramatically, and among the many new and improved dairy foods are some Mozzarella cheeses that have suitable textural properties yet contain less fat than part-skim varieties (Tunick et al., 1991). We further improved the textural properties of low-fat Mozzarella cheeses by adjusting the proportions of ingredients and processing conditions (Tunick et al., 1993). High levels of moisture in the non-fat substance significantly decreased the hardness, complex viscosity and elastic modulus, and the meltability increased during storage. These improvements in textural properties appeared to be most directly related to increased proteolysis under conditions of high moisture.

Alkaline phosphatase in the lactating bovine mammary gland and the milk fat globule membrane. Release by phosphatidylinositol-specific phospholipase C

1. Alkaline phosphatase is covalently bound to bovine mammary microsomal membranes and milk fat globule membranes through linkage to phosphatidylinositol as demonstrated by the release of alkaline phosphatase following treatment with phosphatidylinositol-specific phospholipase C. 2. The release of alkaline phosphatase from the pellet to the supernatant was demonstrated by enzyme assays and electrophoresis. 3. Electrophoresis of the solubilized enzymes showed that the alkaline phosphatase of the microsomal membranes contained several isozymes, while only one band with alkaline phosphatase activity was seen in the fat globule membrane. 4. Levamisole and homoarginine were potent inhibitors of the alkaline phosphatase activities in both membrane preparations and in bovine liver alkaline phosphatase, but not in calf intestinal alkaline phosphatase.

Reactions of hydroxyamino acids during hydrochloric acid hydrolysis

Chlorosubstitution reactions occur readily during HCl hydrolysis of delta- and epsilon-hydroxynorleucines (Hnle), the products of deamination of poly-L-lysine by nitrite at low pH. During amino acid analysis, chloronorleucines elute as new peaks after delta- and epsilon-Hnle. To determine if other hydroxyamino acids undergo similar changes during hydrolysis, they were subjected individually to HCl hydrolysis conditions with and without added phenol. Amino acid analyses indicated that terminal hydroxy groups on linear side chains undergo reactions during HCl hydrolysis; the products appear as new peaks which may be chloroderivatives. In contrast, no new peaks are observed in HCl hydrolysates of delta-hydroxylysine or amino acids with beta-hydroxy groups (beta-hydroxynorvaline, serine, and threonine). Phenol did not protect linear amino acids from reactions during HCl hydrolysis but did prevent loss of the cyclic amino acids tyrosine, hydroxyproline, and 3,4-dihydroxyphenylalanine. Although the gamma-hydroxy group of homoserine would be expected to undergo reaction, HCl catalyzes its cyclization to form homoserine lactone instead.

Influence of casein peptide conformations on textural properties of cheese.

An important relationship exists between casein proteolysis and the development of texture in cheese (Fox, 1989). Breakdown of caseins may begin as the curd is formed and continues through the ripening stage; it also occurs during refrigerated storage of non-ripened cheeses such as low-fat, high-moisture Mozzarella (Tunick et al., 1991; Malin et al., 1993). Renneting enzymes and plasmin provide casein fragments that eventually become substrates for internal proteases and peptidases that starter culture microorganisms use for metabolic benefit (Kok, 1990). Earlier investigations of proteolysis focused on ripened cheeses such as Cheddar or Gouda because the resulting peptides are necessary for texture and flavor attributes. Casein proteolysis has now proved to be a significant factor in development of texture and meltability in low-fat, high-moisture Mozzarella cheese (<10% fat) because of the correlation of these characteristics with breakdown of α s1-casein (Tunick et al., 1993a, 1993b; Malin et al., 1994; see also Chapters 2 and 14).