Introduction to cheese science

Cheese is the most diverse group of dairy products and is arguably the most academically interesting and scientifically challenging of all foodstuffs. Unlike many other foods, which are relatively biologically and physically stable, cheeses are biologically and biochemically dynamic and hence are unstable. Cheesemakers often say that their product is “alive”; although this is obviously literally incorrect, this feeling contains a grain of truth, since cheese contains complex bacterial ecosystems and changes considerably during maturation due to a series of biochemical transformations. If these changes are balanced, they lead to the production of a very large number of flavour compounds (well in excess of 500 in Cheddar) and thus the highly desirable aromas and flavours of different cheeses. It is fascinating that such a wide range of flavours can be produced from the basic ingredients of cheese which are milk, starter cultures, salt and often rennet. A wide range of scientific disciplines are applied to cheese: protein chemistry, microbiology, enzymology, molecular genetics, flavour chemistry and rheology (the science of the flow and deformation of matter). Cheese research is on the cusp of major developments since the recent sequencing of the genomes of many lactic acid bacteria will enable future understanding of the molecular basis of cheese flavour. Although cheesemaking is an ancient art, modern industrial cheese production has relied heavily on the application of many branches of science and the most modern analytical techniques to ensure a consistently high quality product. The major variety produced in Ireland, Cheddar, is a long-ripened cheese requiring 9 months to 2 years for the development of flavour. The cost of cheese ripening stems primarily from its inventory cost and is a major expense to manufacturers, estimated at €1.25 per tonne per day; for a typical Cheddar cheese factory producing 12,000 tonnes of cheese per year, ripening costs translate into at least €5.4 million per annum. Because of this cost, research into cheese ripening has considerable practical significance and will lead to significantly improved competitiveness for the Irish cheese industry. All rennet-coagulated cheeses are ripened (matured) for a period ranging from 2 weeks (e.g., Mozzarella) to 2 or more years (e.g., extra-mature Cheddar) during which the flavour and texture characteristic of the variety develop. Cheese ripening involves changes to the microflora of the cheese, including death and lysis of the starter cells and the development of an adventitious non-starter microflora. Ripening also involves the softening of cheese texture, as a consequence of the hydrolysis of the casein matrix, changes in the water-binding ability of the curd and changes in pH (which may cause other changes such as the migration and precipitation of calcium phosphate). The flavour of cheese curd immediately after manufacture is rather bland and indeed it can be difficult to differentiate the flavours of certain different varieties at this stage. During ripening, cheese flavour develops due to the production of a wide range of sapid compounds by primary (lipolysis, proteolysis and metabolism of residual lactose and of lactate and citrate) and secondary (metabolism of fatty acids and of amino acids) biochemical pathways. Residual lactose is metabolised rapidly to lactate during the early stages of ripening. Lactate is an important precursor for a series of reactions including racemization, oxidation, or microbial metabolism. Citrate metabolism is of great importance in certain varieties, particularly Dutch-type cheeses. Lipolysis in cheese is catalyzed by lipases from various sources, particularly the milk and cheese microflora and, in varieties such as Provolone, traditional Greek Feta and the various Italian Pecorino cheeses, by enzymes from rennet paste which is used to coagulate the milk. Proteolysis, which is an important focus of our research, is the most complex biochemical event which occurs during ripening and is catalyzed by enzymes from residual coagulant, the milk (particularly plasmin) and proteinases and peptidases from lactic acid bacteria and, in certain varieties, other microorganisms which are encouraged to grow in or on the cheese. Secondary reactions lead to the production of volatile flavour compounds through the catabolism of fatty acids and amino acids.

Principal research interest is dairy biochemistry with particular reference to the cheese ripening process and includes the following:

Proteolysis and lipolysis in cheese during ripening. The principal biochemical events that occur in cheese during ripening are proteolysis (i.e., the hydrolysis of the caseins to peptides and free amino acids) and lipolysis (the liberation of fatty acids from triacylglycerides) and these processes are essential for the development of flavour in most cheeses.

Ripening of hybrid and non-Cheddar varieties. There is considerable interest in the Irish dairy industry to lessen our traditional dependence on Cheddar cheese. Research on hybrid cheeses and established European varieties derives from this interest but also has assisted the farmhouse cheese sector which produces many of these cheeses.

Role of the coagulant in proteolysis during cheese ripening. The principal proteolytic agents in most cheeses are enzymes from the rennet used to coagulate the milk which remain trapped in the cheese and the role of chymosin and other enzymes used to coagulate milk is one of our important research interests.

Specificity of proteinases on the caseins. Research on proteolysis in cheese during ripening is at the molecular level and is focussed on the specific peptide bonds cleaved by proteinases during ripening and the identification of the many hundred peptides found in cheese.

Role of non-starter lactic acid bacteria and smear microorganisms. In most hard cheeses, a wild microflora (“non-starter lactic acid bacteria”, principally facultatively heterofermentative lactobacilli) develops during ripening which is uncontrolled and contributes to variability of product quality. The control of NSLAB and their effect on cheese ripening has been amongst our major research interests. In certain cheeses (often referred to as “red smear cheeses”, e.g., Limburger or Tilsit), a complex Gram-positive bacterial flora develops on the surface during ripening.Characterization of microbial enzymes important to cheese ripening. The cheese ripening process is catalyzed by a wide range of enzymes most of which originate from microorganisms. Our group has isolated and characterized many of these enzymes (proteinases, peptidases, amino acid catabolic enzymes) from a number of cheese-related organisms.

Application of chemometrics to data analysis. Since cheese ripening is a complex process, large data sets are easily generated. An active area of research has been to apply multivariate statistical techniques (e.g., principal component analysis and multidimensional scaling) and hierarchical and non-hierarchical cluster analyses to the interpretation of chromatographic and electrophoretic data. This area has been developed in close collaboration with Prof E. Parente, Università degli Studi della Basilicata, Potenza, Italy.

Ingredient cheeses. Much cheese is now used for ingredient applications. Recently, we have worked on factors affecting the physical properties of processed cheese and much work on the pathways for flavour generation in cheese is relevant to the production of cheese flavour ingredients used as dustings on crisps and other savoury food products.

Indigenous enzymes in milk. Together with our colleague, Dr A.L. Kelly, we have been active in the study of some of the approximately 60 enzymes found naturally in milk. In particular, we have studied the proteinase, plasmin, which is a leakage protein from the cow’s blood and a number of enzymes from lysosomes of the somatic cells found in milk and our group discovered the presence in milk of the lysosomal thiol proteinase, cathepsin B.

Collaborative non-dairy projects. Many of the techniques developed for the study of cheese may also be transferred to other fermented foods. Together with colleagues in UCC and in universities in Italy, we have engaged in projects studying sourdough bread, meat and fermented sausages.