CONTROL OF GAS PRODUCTION IN CHEESE

Under conventional Cheddar cheesemaking conditions, using only lactococcal cultures, the four main substrates used by microorganisms for gas production are lactate, lactose, citrate and urea. Note gas can be formed from other substrates and these may be involved in particular incidents; e.g. when cultures containing Str. thermophilus are used.

In the incidents of early gas production studied by the author and summarised here no evidence was found to suggest that lactate or urea was involved in gas production. The volume of gas potentially available from lactose and citrate is shown in Table 5. Note that CO₂ is very soluble in water and that 'normal' cheese can contain significant concentrations of this gas. Clearly the volume of gas potentially available from low levels of these substrates is substantial.

There are a number of strategies that can be used to limit undesirable gas production in cheese. These include:

Choice of starter. Leuconostocs and Lc. lactis ssp. lactis biovar. diacetylactis can produce gas from lactose and citrate, and citrate, respectively. One method of control is to use cultures containing only strains of Lc. lactis ssp. cremoris and/or lactis. However, using only lactococci which do not produce gas from citrate may not always solve gas production problems. Some homofermentative lactobacilli e.g. L. casei and L. plantarum can produce gas from citrate. Due to the presence of high concentrations of citrate in cheese made with Lc. lactis ssp. cremoris and/or Lc. lactis ssp lactis, gas may be produced if citrate-utilising NSLAB such as L. casei or L. plantarum reach high levels. Since NSLAB may constitute a significant part of the indigenous microbial flora of a factory, it may take some time to to reduce them to low levels.

Apart from controlling gas production from citrate-utilising NSLAB by good hygiene, there is also the option of producing curd of low citrate concentration. Curd of low citrate-content can be produced in a number of ways including the controlled use of starters containing citrate-utilisers.

Cheese produced using starters containing Streptococcus thermophilus has the potential to gas if there is significant residual galactose in the cheese. This possibility combined with the urease activity of most strains needs to be considered carefully when plants are experiencing gas production problems including 'slitty' cheese.

Cheese-making conditions. To ensure that lactose levels in cheese after packing are low and that it is metabolised rapidly during the first 24 hours of curing, the acidity of curd at salting should be as high as possible. S/M levels should be between 4.5-5.5% since marked inhibition of lactose utilisation occurs at S/M levels >5.8%. Phage levels in curd at pressing must be controlled to prevent starter cell lysis that could result in 'sweet cheese' containing high concentrations of residual lactose⁶.

There is also an option of producing curd of low citrate concentration as discussed previously.

Curing temperatures and pressing systems. The length of time required for the temperature at the centre of cheese blocks to reach <10°C is influenced by type of pressing system used, whether pre-cooling was used and the method of handling and storing the packaged cheese. Results indicated that, in some cheese plants it took some 3-6 days from the start of pressing for the temperature at the centre of cheese blocks to reach <10°C. These conditions are ideal for the rapid growth of NSLAB.

Cheddar cheese should be cooled rapidly to <10°C and reach a core temperature of 7°C as soon as possible. Consideration should be given to pre-chilling cheese to 7ºC, using a blast-chilling system, before placing cheese in ripening rooms. Low temperatures will retard the growth of NSLAB, while the starter lactococci utilise the residual lactose thus reducing the potential for gas production from lactose.

Barrier bags permeable to carbon dioxide. Barrier bags of increased permeability to CO₂, but of only slightly increased permeability to O₂, can be purchased and may be useful in some instances. Because of their greater permeability to oxygen, the potential for mould growth on the packaged cheese is increased. They are also more expensive than conventional bags, and because of the cost difference, it may be cheaper to repack blown cheese.

Biological control . Some forms of gas production, e.g. that caused by clostridia, can be controlled by the use of nisin-producing starter bacteria.

Hygiene. It is important to minimise the numbers of gas producing NSLAB in cheese. Good cleaning and sterilising procedures are required. There may also be a requirement for aerosol disinfection and/or fumigation of the cheese-making area.

Pasteuriser operation. Coliform contamination can sometimes be linked with pasteurisation faults including incorrect pasteuriser operation. Mixing of pasteurised and non-pasteurised milk due to holes in plates or leaky gaskets occurs occasionally and should be considered in studies of "gassy" cheese. In situations where long pasteurisation runs occur, there may be merit in testing milk from the regeneration section of pasteurisers for the presence of Streptococcus thermophilus.

QUALITY ASSURANCE PROCEDURES THAT MAY BE HELPFUL IN FACTORIES EXPERIENCING GASSY CHEESE PROBLEMS

Note the information presented here is in summary form for commercial reasons and additional information will generally be required to investigate many incidents of gas production including those causing cracks/slits in the cheese.

Starter testing. If it is decided to eliminate starters, that produce gas from citrate, then those that produce gas in citrated-milk should not be used. Starters containing high levels of heterofermentative lactic acid bacteria capable of growth in cheese can be detected based on agar seal displacement using Rogosa broth or RSM supplemented with yeast extract. More sophisticated testing methods will be required when the starter contains only low concentrations of potentially fault-causing microorganisms.

pH and salt in moisture levels. pH and S/M values in 24-h old Cheddar cheese produced from pasteurised milk should be within the range 4.9 -5.2 and 4 - 5.8% (w/w), respectively.

Counts on Rogosa agar. Counts on this medium should be low in 1-week old cheese. The author has observed NSLAB counts on this medium of <1x10⁵/g after 5 months storage for cheeses from some plants. Note while this medium can be useful modifications may be required to aid the successful investigation of some incidents of unwanted gas production.

CONCLUSIONS

While it has been possible to ascribe some incidents of early gas production in Cheddar cheese to the activity of citrate-utilising and/or heterofermentative strains in mixed strain cultures, we have not been able to establish a definitive causal relationship between starter type and early gas production in Cheddar cheese.

Many of the commercial mixed-strain cultures studied over a number of years contained low levels of heterofermentative gas-producing, non-starter lactic acid bacteria capable of growth at 6ºC and at high S/M levels, conditions found during the maturation of cheese. Evidence for the involvement of heterofermentative, psychrotrophic, salt-insensitive organisms capable of growth on Rogosa agar in early gas production was found.

Heterofermentative lactic acid bacteria have been isolated from raw milk and tanker samples of pasteurised skim. It is probable that the gas-producing NSLAB in cheese are derived from contamination of the factory environment by bacteria present in raw milk, tanker skim-milk and/or mixed-strain cultures. If it can be shown that small numbers of gas-producing NSLAB survive pasteurisation, then there may be a direct raw milk quality or a tanker skim-milk quality dimension in early gas production in cheese.

Contamination of cheese milk with low levels of gas-producing lactic acid bacteria may not always result in blown cheese.

Gas production in cheese of normal chemical composition and pH results from a number of interacting factors, including the starter used, lactose and citrate levels in the curd, temperatures of curd/cheese during pressing and curing, the salt sensitivity of the starter, the S/M level in the cheese, the levels of gas-producing NSLAB bacteria in the cheese, and the level of phage-induced cell lysis in the curd at pressing and during early cheese maturation.

How to cite this article

Mullan, W.M.A. (2003). [On-line]. Available from: http://www.dairyscience.info/cheese-quality/67-causes-of-early-gas-production-in-cheddar-cheese.html?start=2 . Accessed: 10 September, 2010. Revised 25th June 2008; 6th July 2008;8th July 2008

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