Early gas production in Cheddar cheese is a sporadic and recurrent problem. In this article the major causal agents of unwanted gas production are discussed and potential gas producing organisms in Cheddar cheese are identified. Many of these agents will also cause problems in other cheese varieties. Early gas production in Cheddar cheese results from a number of interacting factors including lactose and citrate levels in the curd, the temperature of curd/cheese during pressing and curing, the salt in moisture level in the cheese and the levels of gas-producing, non-starter lactic acid bacteria in the cheese.

Early gas production in 18-kg blocks of commercial Cheddar cheese is a well known, if not fully understood, phenomenon¹. Incidents of gas production tend to be sporadic and recurrent and have probably been experienced at most cheese-making plants. Below the cheese on the left displays a "loose" bag due to gas production. The extent of gas production can be such that the 'slats' can buckle or even break due to the pressure generated.



Although some incidents of unwanted gas production can be explained by poor cheese-making practices, including bad hygiene, or starter failure, this is not universally true. The author has investigated incidents of early gas production in commercial cheese plants (within 3-8 weeks of manufacture) over many years in which the cheese graded normally, and was within acceptable limits for salt, moisture and pH. The cheeses studied were also free from significant levels of gas-producing non-lactic acid bacteria.

Note open texture including unwanted slits/cracks in cheese can be caused by unwanted gas production.While this contribution does not specifically deal with these problems there are many similarities. Note the cheese on the right displays cracks and fissures due to unwanted gas production. Another image showing more extensive slits/ fissures is also shown.



MAJOR CAUSAL AGENTS OF UNWANTED GAS PRODUCTION IN CHEESE

Products of the lactate fermentation, combined with control of the moisture and salt levels in the final cheese, good hygiene, and the use of good quality pasteurised milk effectively limit the range of bacteria which can produce gas in Cheddar cheese ⁶. Although gas can be produced from a wide range of compounds present in cheese, lactose, lactate, citrate and urea are the major substrates involved (Table 1).

The involvement of Clostridium tyrobutyricum in gas production in brined-cheeses such as Gouda and Grana Padano has been well established. However, this clostridium would not normally be expected to cause problems in Cheddar cheese of satisfactory salt, acid and moisture content⁷. Some homofermentative bacteria e.g. Lactobacillus casei, and Lactococcus lactis ssp. lactis biovar. diacetylactis can produce CO₂ from citrate and have been implicated in the blowing of film-wrapped cheese⁸. The potential for gas production by heterofermentative lactobacilli has been known since the work of Sherwood⁹. Coliforms are usually only a problem when the starter fails, due to bacteriophage infection or antibiotic residues in milk. Under certain conditions, urease producing strains of Streptococcus thermophilus can produce gas in Cheddar cheese ¹⁰. Since Streptococcus thermophilus can grow in the regeneration section of pasteurisers relatively high levels of this thermophile may occasionally occur in pasteurised milk. Incidentially the galactose produced by Streptococcus thermophilus can be used as a substrate for gas production ny non starter lactic acid bacteria. Note Sherwood⁹ first established that the addition of leuconostocs to milk for Cheddar cheese manufacture gave rise to gas and open texture in the cheese. Because of their sensitivity to salt and high acidity propionibacteria would not be expected to produce gas in Cheddar cheese. Examination of the gas produced by the major gas-producing organisms (Table 1) indicates that the identification of the gas present in blown cheese may have value in identifying the gas producing species.

PROBLEMS ASSOCIATED WITH MICROBIOLOGICAL STUDIES OF "GASSY" CHEESE

Determination of the causal agent(s) of gas production in cheese can be difficult. The numbers of the micro-organism concerned may have declined to a low, apparently insignificant level at sampling. This can be overcome by serial sampling during maturation. Some micro-organisms are not distributed uniformly in cheese, for example lactobacilli may be found in fissures or curd junctions. Sampling schemes should be designed to take account of the potential for non-random distribution of the gas producing agent

The selective agar media used may be inadequate; e.g., some leuconostocs and pediococci will form colonies on Rogosa agar originally developed for work with lactobacilli³. Certain media may allow the growth of starter lactococci.

Microscopic studies of 'gassy' cheese can also yield inconclusive results. Leuconostocs, for example, may appear as small rods, cocco-bacilli or cocci. Heterofermentative lactobacilli may also appear as cocco-bacilli

Where a particular micro-organism is suspected of being the causal agent, current molecular biology techniques involving polymerase chain reaction (PCR) amplification of DNA sequences combined with species-specific DNA probes can be used to confirm the identify of the micro-organism. Klijn et al.¹² have used PCR and a species specific-DNA probe to confirm Cl. tyrobutyricum as the causative agent of late blowing in experimental cheese.