To make Gruyère, raw cow's milk is heated to in a copper vat, and then
curdled by the addition of liquid
rennet. The
curd is cut up into pea-sized pieces and stirred, releasing
whey. The curd is cooked at , and raised quickly to . The whey is strained, and the curds placed into moulds to be pressed. After salting in
brine and
smearing with bacteria, the cheese is ripened for two months at room temperature, generally on wooden boards, turning every couple of days to ensure even moisture distribution. Gruyère can be cured for 3 to 10 months, with long curing producing a cheese of intense flavour.
Sérac, a whey cheese, is a historical by-product of the manufacture of Gruyère.
Natural holes in cheese The presence of holes in cheeses such as Gruyère and Emmental was long thought to be due to bacterial activity in the mass of the cheese. In the 21st century, researchers discovered that it also was due in part to microscopic hay dust that got into the milk during milking. Bacteria formed around the dust particles and produced gases, which caused the characteristic holes. In addition to their traditionally favoured appearance, the holes, if not too small, have the beneficial effect of preventing gaps and cracks in the block of cheese from forming. Some suggestion in the past indicated a variable effect; Emmental cheese made from summer milk had smaller holes than winter cheese—the cows ate hay in winter, but grass with less dust in summer. Modern production techniques and milking machines are less susceptible to hay dust, and indeed 21st-century Swiss cheeses have fewer and smaller holes. Holes can be restored to their former size and prevalence by adding hay flower powder to the milk; this is done in Germany and France, but such additives are forbidden in Swiss cheeses.
Affinage An important and the longest part of the production of Gruyère in Switzerland is the (French for "maturation"). According to the AOC, the cellars to mature a Swiss Gruyère must have a climate close to a natural cave. This means that the
humidity should be between 94 and 98%. If the humidity is lower, the cheese dries out. If the humidity is too high, the cheese does not mature and becomes smeary and gluey. The temperature of the caves should be between . This relatively high temperature is required for excellent-quality cheese. Lower-quality cheeses result from temperatures between . The lower the temperature, the less the cheese matures, resulting in a harder and more crumbly texture. , marked by
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