17.24 Cheese and salami manufacture

Cheese is a solid or semisolid protein food product manufactured from milk and before the advent of modern methods of food preservation, like refrigeration, pasteurisation and canning; cheese manufacture was the only method of preserving milk. Although basic cheese making is a bacterial fermentation, there are two important processes to which fungi make a crucial contribution. These are the provision of enzymes for milk coagulation right at the start of the process, and mould-ripening to change the flavour and/or consistency of the product.

Cheese production relies on the action of enzymes which coagulate the proteins in milk, forming solid curds (from which the cheese is made) and liquid whey. Making cheese involves the following basic steps:

  • adjusting the milk to a pH between 5.8 and 6.4 and adding a source of calcium (for example, calcium chloride)
  • mixing the milk coagulating enzyme into the milk in a cheese making vat, at this stage the milk temperature is about 45ºC
  • allowing the enzyme to react with the casein for sufficient time (around 10 minutes) to start solidifying the milk in the vat
  • the initial coagulum is cut into segments, heated (for example, by direct steam injection or through steam-jacketing of the vat) to about 60ºC, and stirred for about 15 min, which causes the fluid whey to start separating from the solid curds. Extraction or expulsion of a liquid from a gel like this is called syneresis. Enzymatic digestion of casein molecules enables calcium bonds to form and hydrophobic regions develop that force water molecules to leave the structure. The curd is cooked, or scalded, to expel the whey.
  • The curds are cooled and separated from the whey by draining and pressing
  • cheese curds can then be made into the product the cheese maker requires (Fig. 40), which is where the lactobacterial fermentation comes into prominence, though other factors (like temperature, pH and additives) can be varied for particular recipes.
  • Salt is added to the cheese to slow down bacterial activity and to enhance the taste of cheese. Salt also affects enzymatic activity in cheese during cheese ripening (Farkye, 2004).

A preparation of animal enzymes (called rennet or chymosin) extracted from the stomach membranes of unweaned ruminants has traditionally been the primary coagulant of milk protein in the manufacture of cheese. Rapid expansion of the production and consumption of cheese caused attention to shift to alternative sources of such enzymes. Stomachs from older animals were unsuitable because their high content of pepsin resulted in a less effective coagulation and more proteolysis, causing lower cheese yields and development of off-flavours. Moulds like Aspergillus and Mucor were identified as potential sources and aspartyl proteinases from Rhizomucor pusillus, R. miehei and Cryphonectria parasitica were found to be more or less satisfactory for at least some cheeses, so fungal enzymes supplied the market to an extent. Eventually, the gene for calf chymosin (or prochymosin) was cloned, first in prokaryotic and then eukaryotic microorganisms and in recent years animal enzymes produced by genetically modified microbes have come to dominate the market.

The cheese spectrum
Fig. 40. The cheese spectrum. There is a wide range of cheeses on the market; they vary from extremely soft to extremely hard, a factor which is controlled by the extent of the proteolysis which is allowed, the cooking temperatures and the pH. They vary in flavour according to the microbes the milk originally contained and/or according to the microbes and other materials introduced into the cheese during fermentation or later processing. They also vary in flavour, of course, according to the origin of their substrate. Milk is not a uniform commodity; cow’s milk differs between breeds of cattle, and cheese is also made from ewe’s, goat’s, buffalo’s and camel’s milk (redrawn after Farkye, 2004).

Indeed, chymosin, the milk-clotting enzyme used to make cheese and other dairy products, was the first enzyme from a genetically-modified source to gain approval for use in food manufacture. In March 1990, the US Food and Drug Administration (FDA) issued the first regulation in the US for the use in food of any substance produced by recombinant DNA technology by affirming that such chymosin was ‘generally recognised as safe’ (GRAS). The importance of this was that it exempted the product from the premarket approval requirements that apply to new food additives.

At that time, the source of the new enzyme was a bovine pro-chymosin gene expressed in the bacterium Escherichia coli. Subsequently, chymosin preparations produced commercially from the yeast Kluyveromyces lactis and the filamentous Aspergillus awamori (both Ascomycota) genetically modified to include bovine pro-chymosin were also given GRAS status. The FDA concluded that fermentation-derived chymosin (FDC) was purer than traditional calf rennet and was identical to its natural counterpart. It was verified that the yield, texture, and quality of cheese made with FDC was comparable to that made with calf chymosin and that FDC gave superior yield to other coagulants.

Microbial rennets have been improved in a number of ways including reducing their nonspecific proteolysis (which improves yield) and thermal stability (which makes it easier to control the elasticity of the product; keeping soft cheeses soft in other words). Today, about 90% of cheese production depends on enzymes from genetically modified microbes (mainly yeasts, but including A. awamori) for the coagulation step. Commercial microbial proteinases derived from Bacillus, Aspergillus spp. or from Rhizomucor niveus, and nonspecific aminopeptidases from A. oryzae are used for a range of other food modifications including casein and whey protein hydrolysates with reduced allergenicity and rich in bioactive peptides, debittering and flavour generation in protein hydrolysates, and enzyme-accelerated cheese ripening and production of enzyme-modified cheeses (Stepaniak, 2004).

Indeed, for what may be seen as a traditional farming industry, there have been many major biotechnological developments in recent years (Johnson & Lucey, 2006), but then it is a big industry, and it has been growing rapidly across the start of the 21st century. Using 2002 statistics, the overall value of cheese production in the US was US$13.3 billion, US$2.6 billion in the UK, US$6.9 billion in France, and US$5.6 billion in Germany; the growth in sales in these countries between 1998 and 2002 varying between 10 and 30% (Farkye, 2004). In the United States, the annual per capita consumption of natural (nonprocessed) cheese rose from just over 7.7 kg in 1980 to over 13.6 kg in 2004 and total cheese production rose from 1.8 million metric tons in 1980 to 3.9 million tons in 2003.

Although the number of cheese production plants in the US dropped from 737 in 1980 to 399 in 2003, production increases are matched by (due to?) greatly increased capacities in the remaining cheese manufacturing plants. The largest plants can handle more than 4 million litres of milk per day, and cheese production vats have capacities of 30,000 litres of milk compared to the 15,000 to 23,000 litre capacities that were common in the standard sized vats of 1980 (Johnson & Lucey, 2006).

Cheese ripening depends on a host of metabolic processes and involves a complex of interrelated events. The biochemical pathways through which lactose, lactate, milk fat and caseins are converted to flavour compounds are now known in general terms. More than 300 different volatile and nonvolatile compounds have been implicated in cheese flavours. These flavour compounds originate from biochemical pathways like proteolysis, lipolysis, glycolysis, citrate and lactate metabolism (McSweeney, 2004; Stepaniak, 2004); the relative contribution of these processes depends on the variety of cheese and its characteristic microbial flora.

Mould ripening is a traditional method of finishing cheeses which has been in use for at least two thousand years. Blue cheeses, like Roquefort, Gorgonzola, Stilton, Danish Blue, and Blue Cheshire, all use Penicillium roquefortii which is inoculated into the cheese prior to storage at controlled temperature and humidity by having its spores forced into the new cheese on the tines of a metal comb. The inocula come from starter cultures of proprietary strains of Penicillium roquefortii, P. camembertii (for cheeses) and P. nalgiovense (for salami) produced predominantly by solid state fermentation because this process gives better yields of homogeneous and pure spores.

After inoculation the fungus grows throughout the cheese and into the voids between curd particles, producing flavour and odour compounds. The holes and tracks made by the inoculation device are usually evident at point of sale (Fig. 41).

Samples of blue cheeses
Fig. 41. Samples of blue cheeses (gorgonzola on the left, Danish blue on the right) bought from a Stockport supermarket which show (top) holes in the outer ‘rind’ made by the tines of the inoculating device when the Penicillium roquefortii spores were injected into the newly pressed cheese, and (bottom) spore production within the cheese revealing the tracks made by the tines during inoculation and revealing growth of the fungus into the voids between curd particles. Photographs by David Moore.

Camembert and Brie are ripened by a mould called Penicillium camembertii, which changes the texture of the cheese more than its flavour. This fungus grows on the surface of the cheese extruding enzymes which digest the curds to a softer consistency from the outside towards the centre (Fig. 42).

A mature Camembert cheese ready for eating
Fig. 42. A mature Camembert cheese ready for eating (top). The sketch below is a schematic representation of the changes that occur during ripening of such a cheese as a consequence of the growth of Penicillium camembertii over the surface of the cheese.  Photograph by David Moore, schematic modified and redrawn from McSweeney, 2004.

Penicillium nalgiovense is a filamentous fungus (Ascomycota) which is the most widely used as a starter culture for cured and fermented meat products. Use of moulds in the production of fermented sausages is known from 18th century Italy where fermented and air-dried sausage was a popular peasant food because it could be stored safely at room temperature for long periods. Today salami is widely produced in southern-European countries using a range of meats, including beef, goat, horse, lamb, pork, poultry, and/or venison. The chopped meat is mixed with minced animal fat, cereals, herbs and spices, and salt and allowed to ferment for a day. This mixture gives the salami sausage its typical marbled appearance when cut. The mixture is then stuffed into a casing, treated by dipping or spraying with a suspension of the P. nalgiovense starter culture at a concentration of about 106 spores ml-1, and finally hung to cure.

The fungus does a number of jobs as it grows over the sausage and into the meat and the rest of the mixture. Basically, it imparts flavour and prevents spoilage during the curing process, but this is achieved by:

  • proteolysis, lactate oxidation, amino acid degradation, lipolysis, and fatty acid metabolism during the maturing process creating the desirable flavour;
  • with the surface of the sausage colonised by a specific, usually proprietary, mould the air-exposed sausage is protected against spoilage by other undesirable species of yeasts, moulds and bacteria;
  • the surface covering of the mould mycelium controls the drying process and ensures a smooth and uniform surface appearance of the product.

Updated December 17, 2016