alpha-Keto acids. 1

The alpha-keto acids produced by aminotransferase activity are relatively unstable and do not accumulate in cheese but are rather degraded via a range of pathways. Taking tyrosine as an example, its alpha-keto acid (p-hydroxy phenylpyruvate) can be degraded by 2-hydroxyacid dehydrogenases to the corresponding hydroxy acid (p-hydroxy phenyl lactate). Other pathways of degradation of alpha-keto acids include decarboxylations and chemical degradations forming other volatile flavour compounds, which will be discussed in future posts.

Aminotransferases

The catabolism of amino acids to a wide range of volatile flavour compounds is amongst the most important series of reactions in the development of cheese flavour. The key enzymes in the degradation of free amino acids appear to be aminotransferases (ATases) from lactic acid bacteria. ATases are intracellular enzymes whose physiological role is in the interconversion of amino acids. These enzymes require pyridoxal-5'-phosphate (PLP) as a co-factor and catalyse the transfer of the amino group of a donor amino acid (leucine in the example below) to an acceptor molecule, usually alpha-ketoglutarate, forming a product alpha-keto acid corresponding to the donor amino acid (alpha-ketoisocaproate in this example) and glutamic acid. The catalytic mechanism of ATases involves two steps: firstly, the amino group of the donor amino acid is transferred to PLP to yield the product alpha-keto acid and enzyme-bound pyridoxamine-5'-phosphate. Secondly, the amino group is transferred from pyridoxamine-5'-phosphate to the acceptor alpha-keto acid, thus regenerating PLP. The alpha-keto acids formed by ATase action are unstable and degrade to a wide range of compounds via enzymatic and/or chemical pathways.

Syneresis- V

Stirring the curds-whey mixture facilitates heat transfer during cooking, prevents the curd pieces from fusing and promotes syneresis by encouraging collisions with other curd pieces and the vat wall. It is important to stir gently after cutting to avoid curd shattering (and thus yield losses); indeed, some cheesemakers leave a 5-10 min "healing time" after cutting before starting to stir.

In Cheddar-type cheeses, the curds-whey mixture is stirred and cooked to a desired pH (e.g., 6.1-6.2) but in many varieties (e.g., Emmental), the whey is drained at a target temperature. In a few varieties (e.g., stirred-curd Cheddar or Colby cheese), it is normal to stir the curd pieces after drainage ("dry stirring") which also promotes syneresis.

Note: Figure shows the effect of stirring (solid curves) and no stirring (lower broken line) on percentage syneresis as a function of time afer cutting.

Syneresis- IV

Cooking temperature is a major factor that determines the rate of syneresis. Cooking temperature varies from ~31C (Camembert) to 52-55C (Emmental or Parmigiano-Reggiano) and temperature must match the starter. Acid production by lactococci is slowed ~35C and many strains are killed >40C (which is very close to the Cheddar cooking temperature of ~38.5C). Thermophilic starters, while surviving high temperatures do not gro >~52C so syneresis in Swiss cheese (cooked 54-55C) is mainly due to heat; starter grows as curd cools.

Cooking is normally achieved using a jacketed vat although "curd washing" (removal of perhaps 30-40% of the whey and replacement with hot water) is used in Dutch-type cheeses. In addition to increasing the temperature, curd washing reduces the lactose levels and helps to control the final pH of the cheese.

The rate of cooking is important. If it is too fast in the early stages, case hardening can result.