Ripening processes: chemical and physical changes
Cheese ripening is basically about the breakdown of proteins, lipids and carbohydrates (acids and sugars) which releases flavour compounds and modifies cheese texture. The biochemical and biophysical processes involved have only partly been elucidated. Here we include only a few practical principles of ripening.
- Ripening varies from nil for fresh cheese to 5 years for some hard ripened cheese. Like a good wine, a good aged cheese should get better and better with age.
- Ripening processes are broadly classified as interior and surface ripened.
- Cheese which depend mainly on interior ripening (most hard ripened cheese such as Cheddar and Italian types) may be ripened with rind formation or may be film wrapped before curing. Having said that, I hasten to add, that traditional Italian types are always rind ripened. Cheddar and American varieties are the only ripened cheeses which (in my view) are not drastically altered by film wrapped curing.
- Cheese which depend mainly on surface ripening include smear ripened and mould ripened
- In the broadest terms there are three sources of cheese flavour:
- Flavours present in the original cheese milk, such as natural butter fat flavour and feed flavour.
- Breakdown products of milk proteins, fats and sugars which are released by microbial enzymes, enzymes endogenous to milk, and enzyme additives.
- Metabolites of starter bacteria and other microorganisms. These include products from catabolism of proteins, fats and sugars.
- Flavour and texture development are strongly dependent on:
- pH profile
- As a general rule factors which increase the rate of ripening increase the risk of off flavour development, and reduce the period of time when the cheese is saleable.
Protein Breakdown (Proteolysis)
Natural degradation of protein is called 'putrefaction' and results in 'rotten potato' type odours, especially if high quality proteins such as animal proteins are involved. That's because animal proteins contain the essential sulfur amino acids. These 'putrefactive' components are also the stuff of which good flavours are made. Protein degradation during cheese curing is a directed process resulting in protein fragments with desirable flavours.
- Some off flavours associated with undesirable or excessive protein breakdown in cheese are bitter, stringent, putrid and brothy.
- Protein breakdown causes shorter body which is less rubbery, less elastic, more meltable. For example, flavour and texture development in Cheddar are mainly dependent on protein breakdown and much less dependent on fat breakdown.
- Protein breakdown involves three general types of processes:
- Proteases break proteins into smaller peptides, some of which are flavour compounds. For example, bitter and brothy flavoured peptides are well known to occur in cheese.
- Peptidases further break down peptides to amino acids.
- Further catabolism of amino acids by cheese microorganisms produces aldehydes, alchohols, carboxlic acids and sulfur compounds, many of which are flavourful.
- The amino acid, tyrosine, forms crystals in aged cheese such as Parmaggiano regiano, which are readily detected on the palate.
Fat Breakdown (Lipolysis)
Dairy fat is a wonderfully rich source of flavours, because it contains an extremely diverse selection of fatty acids. In particular, butter fat is the only natural fat which is rich in short chain fatty acids. Butyric acid for example is a potent flavour compound. As with all potent flavours the trick is to add just the right amounts in balance with other flavours. Here are a few principles:
- Dairy fat without any ripening during cheese making is an important contributor to cheese flavour and texture:
- Fresh dairy fat has the well known 'buttery' flavour associated with extremely low levels of free fatty acids.
- Fat also acts as a flavour reservoir, so hydrophobic (fat soluble) flavours derived from protein breakdown are stored in the fat and released during mastication in the mouth.
- Finally, fat is an important component of cheese softening and melting.
- The fat derived flavours associated with cheese ripening result from the release of fatty acids by lipolysis and further modification of fatty acids by microorganisms to other compounds.
- Varieties traditionally made from goats' milk have higher levels of lipolysis.
- Blue moulds are generally quite lipolytic
Milk contains no starch or fibre or any sugar other than lactose so all carbohydrate compounds in cheese are derived from lactose or produced by microorganisms. Relative to fat and protein lactose contributions to flavour are minimal. Here's a few principles:
- At Day 1 following cheese manufacture most of the milk sugar has been removed in the whey by or by fermentation, that is converted to lactic acid by the cultures.
- Residual lactose depends on the type of cheese and other factors. For examples:
- High salt in the moisture phase of Cheddar slows lactose metabolism so lactose content is .3 to .7% at one day after manufacture and slowly declines to less than 0.1%.
- Residual lactose in Camembert cheese is used by Penicillium camemberti so it decreases quickly, especially on the surface, when the mould begins to grow.
- In well drained cheese such as Swiss types, lactose is completely used up in a few hours.
- In washed cheese varieties, lactose not leached by washing is quickly used up by the culture, especially for Dutch type cheese where salting is delayed. In Colby, early vat salting reduces the rate of utilization of residual lactose.
- Many organisms, including yeasts and moulds in mould and smear ripened cheeses utilize lactate and produce various flavourful compounds.
- Calcium salts of lactic acid may form white precipitates on the surface of aged cheese.