Recent advances in milk clotting enzymes

Coagulating enzymes are an absolute necessity for the production of ripened cheese varieties. The objective of this review is to summarise and interpret the latest findings for the most important types of enzymes, which are animal rennet, genetically engineered chymosin, coagulants of microbial origin, and plant‐derived clotting enzymes. Special emphasis has been placed on aspects of enzyme chemistry and technology, selected methods for the analysis of coagulants, and the impact of the enzymes on proteolysis, cheese yield and cheese quality.     

TRANSGLUTAMINASE IN DAIRY PRODUCTS: CHEMISTRY, PHYSICS, APPLICATIONS

Literature on the effects of microbial transglutaminase on various dairy‐based systems is discussed. Beginning with a short synopsis on the development of microbial transglutaminase as a functional tool for modifying foods, the principles of reactions catalyzed by transglutaminase and their structural implications, as well as the mechanisms of formation and cleavage of isopeptide bonds are reviewed. After summarizing the present knowledge on the specificity of microbial transglutaminase towards milk proteins, including reactions determined by individual lysine and glutamine residues, emphasis is placed on the effects of enzymatic cross‐linking on physicochemical properties in foods and, particularly, dairy‐based systems. Discussed are implications of cross‐linking on acidified milk gels including yogurt and effects on single milk protein fractions, with respect to several physicochemical properties including rheology and mechanical properties of these systems, but also syneresis, and emulsification behaviour.     

RELATIONSHIP BETWEEN WATER ACTIVITY AND CRISPNESS OF EXTRUDED RICE CRISPS

Extruded rice crisps were adjusted to water activities in the range of 0.05–0.65, and texture measurements were carried out by single compression or compression of a layer of 10 specimens. Sensory crispness ranking was performed by a panel familiar with texture evaluation. Several parameters were derived from the force/displacement curves, and a crispness indicator was calculated on the basis of the relationship between the texture parameters and water activity. This crispness indicator showed best correspondence sensory results, especially when layer compression was performed.     

The sampling interval in systematic activity sampling

This article deals with the effect of the sampling interval on the precision of systematic activity sampling, and models for medium and large sampling intervals are developed. The problem of poor precision due to possible cyclic properties of the process is also analysed, and a necessary criterion for non-interference is derived.     

Using near infrared spectroscopy for the determination of total solids and protein content in cheese curd

In-line or on-line monitoring requires fast and reliable methods for determining composition or processing variables. A near infrared spectroscopic method for the determination of solids and protein content in cheese curd has been developed. Based on approximately 250 curd samples, calibration models were built using partial least squares regression. The calibration models resulted in coefficients of determination of R² = 0.994 for total solids and R² = 0.985 for protein content for unhomogenized curd, which improved only slightly after subjecting the curd to homogenization. As confirmed by results that were obtained from a validation set, this suggests that near infrared spectroscopy is a potential and powerful method for the rapid monitoring of curd composition.     

STIFFNESS OF COMPRESSION TESTING MACHINES

A ring test with six participating laboratories and rubber stoppers as reference material was performed to monitor the performance of commercial compression testing instruments. Testing conditions were chosen to be in the range of regular compression testing. Small, single screw instruments with a cantilever exhibited a significant deviation in the force/deformation-response compared with double-screw instruments with a crosshead. Additional tests made with an external device for displacement recording revealed that these differences have to be attributed to insufficient stiffness and some compliance in the cantilever of the tested single-screw instruments. Some consequences of the test results for experiments on soft, semi-soft and rigid food materials are discussed and an equation is developed to correct for the flexing of the cantilever beam in the single screw machine.