A few days ago I published an article on early blowing defects in cheese making. So it makes sense to follow up that post with an article on the other form of blowing – late blowing. This is another common form of contamination encountered by cheese makers. Its presence indicates an issue with your milk source or production practices. In this post I’m going to do a dive into its causes, the underlying microbiology, and how to prevent it.
Like early blowing, late blowing is characterized by the appearance of large, unwanted gas bubbles in the paste of cheese. As the name suggests, late blowing occurs later in the cheesemaking process – specifically, it occurs during the aging of the cheese. Early blowing, in comparison, occurs during the produciton, pressing, or brining stages of cheese making. As with early blowing, late blowing is purely a product of contaminating microorganisms which produce gaseous metabolic products. And, as with early blowing, it is unsafe to consume cheese which has undergone late blowing as some of the organisms which cause late blowing are pathogens or produce toxic metabolites.
Late blowing often has a different appearance than early blowing. Early blowing produces a spongy interior of smooth shiny holes. In comparison, late blowing produces irregular-shaped and -sized holes, and often produces large horizontal cracks and pockets within the paste.
What are the signs of late blowing?
Because it develops as cheese ages, late blowing is not always as obvious as early blowing. In extreme cases, your cheese may bulge, or even crack, due to the expanding gasses. But signs are usually more subtle than this, as the gasses are often released through small defects in the rind before large changes occur to the surface of the cheese. Late blowing is often more obvious in cheeses which have been waxed or vacuum-packed. In the case of waxed cheeses, even modest expansion of the cheese can crack the wax. Likewise, escaping gas will become trapped between the cheese and wax. This can cause a section of the wax lifting off of the cheese.
This is even more obvious in vac-packed cheeses, as the normally tight bag will loosen. In some cases, it may even inflate like a balloon. That said, loosening of the vac-pack is not a sure-fire sign of blowing, as a pinhole in the bag or damaged seal may simply have released the vacuum.
The most obvious sign of late blowing is, of course, when you cut into the cheese. The presence of horizontal cracks and irregular bubbles is a sure-fire indication that late blowing has occurred.
What do you do with a blown cheese?
It is not safe to consume a cheese that has undergone late blowing. While some organisms that cause this form of blowing are harmless, some of the others are pathogens. This includes Clostridium botulinum, which produces the single most toxic substance known to humankind. There is no test you can perform at home to determine whether a blown cheese is safe to consume. As with early-blown cheeses, you can try feeding these to poultry or pigs, but be aware that there is some risk to them.
Causes of Late Blowing
Late blowing is due to contamination of cheese by certain bacteria. These microbes are quite different from those which cause early blowing. The bacteria and yeast that cause early blowing are lactose fermenters – organisms which eat the lactose sugar in cheese. However, lactose is consumed as cheeses age, leaving little behind. As such, the bacteria which cause late blowing are generally species that are proteolytic (eat proteins), lipolytic (eat fats), or which can get buy on trace amounts of lactose.
The primary species of bacteria that cause late blowing are three members of the Clostridia genus – Clostridium tyrobutyricum, C. butyricum, and C. sporogenes. The former is the dominant cause of late blowing due to its ability to eat lactic acid – the primary product of lactose fermentation. In contrast, C. sporogenes tends to cause blowing early in aging, as it requires some lactose for its metabolism. C. butyricum is happy to consume proteins and lipids, but is unable to consume lactic acid. All of these species will metabolize their preferred “food” into a range of compounds, including hydrogen gas and carbon dioxide – the gasses which then cause the cheese to blow.
These bacteria produce a number of noxious compounds which can build to unpleasant levels in a cheese. If you cut into a cheese and get a whiff of vomit, rotten butter, or faeces, you almost certainly have Clostridia growing in your cheese. Even if visible signs of late blowing are not present, cheeses with these aromas should be discarded.
The pH of the cheese is a major factor in whether these bacteria cause blowing. All three species are readily inhibited at pH’s below 5.5, and become increasingly active as pH increases. Early in aging most cheeses have a pH above 5.5, with this dropping as lactose is consumed and lactic acid is produced. Later, lactic acid (and other organic acids) can be broken down, leading to an increase in the cheeses pH. As such, blowing by C. sporogenes can be prevented by ensuring proper acidification by your cultures while making cheese.
Because aging (and the subsequent increase in pH) are an important part of developing the organoleptic qualities of many cheeses, manipulating pH isn’t typically used to prevent late blowing. By extension, lower-acid cheeses such as washed-curd cheeses are at higher risk of late blowing than higher-acid cheeses. As such, preventing the initial contamination is the primary form of control, followed by proper salting, as salt will aid in the suppression of these bacteria.
In addition to the above three Clostridia species, two additional pathogenic species can grow in cheese – the aforementioned C. botulinum, as well as C. perfringens. These produce almost identical aromas and gas as the other Clostridia species, in addition to producing some highly toxic compounds.
A second group of organisms that can cause late blowing are the heterofermentative lactobacilli. Most cheese-making bacteria are homofermentative, meaning that they strictly produce lactic acid from lactose. Heterofermentative lactobacilli such as Lactobacillus helveticus and Streptococcus thermophilus use a different biochemical process to consume lactose, and in doing so, produce gas. Homofermentative species will produce four lactic acid molecules per molecule of lactose consumed, with no other major products made. From the same lactose molecule, heterofermentative species will produce two molecules of lactic acid, two molecules of another metabolite (often ethanol or acetic acid), and two molecules of carbon dioxide. The carbon dioxide (a gas) is what causes the blowing of the cheese.
The good news is that, unlike the clostridia, there are no known pathogens among the heterofermentative lactobacilli. In fact, S. thermophilus is a critical organism in the production of yoghurt. L. helveticus is sometimes used in cheese making, and some strains may have probiotic properties. In cheese making, L. helveticus is added in minute amounts to prevent bitterness and to add nutty flavours. L. helveticus is only really an issue if cheeses are aged at too warm a temperature (15C or higher), as at these temperatures it can outgrow the “good” lactobacilli and blow the cheese. Under proper aging conditions the “good” strains will consume most of the lactose, leaving just enough behind for L. helveticus to do its job, but not so much that it blows the cheese.
Unfortunately, there are no indicators that allow you to determine whether a blow was caused by a Clostridia or Lactobacilli. Any cheese with late blowing should not be consumed.
Where does this contamination come from?
The contamination that leads to late blowing largely comes from the same sources as the contamination which causes early blowing – Milk, Equipment, and You. To save time I’m not going to rehash this in detail, and rather I’ll direct you to my article on early blowing where this is covered in detail. There is, however, one additional source of contamination specific to late blowing:
Animal Feed: The type of feed fed to milk animals can be a major factor in late blowing. Silage (fermented hay), in particular, is a major source of the Clostridia bacteria that can cause late blowing. Silage is made by containing fresh hay in a low-oxygen environment. Here, the hay undergoes a lactic acid fermentation – not unlike that of kimchi or sauerkraut. This preserves the hay and improves its digestibility. But unlike kimchi and sauerkraut, salt is not added to silage. As such, Clostridia can also grow during the early stages of silage production.
As fermentation progresses the silage becomes acidic, causing the Clostridia form endospores. These endospores are incredibly resilient, can last for years in silage, and easily survive the passage through the animal’s digestive tract. The spores can then be deposited on the udder, from where they can get into the milk. Pasteurization cannot kill these spores, meaning that even pasteurization is not protective against this source of contamination.
Preventing Late Blowing
As with early blowing, late blowing can largely be prevented through proper milk collection and storage, pasteurization (with the caveats from above), good sanitation approaches, and careful control of temperature and the use of brine (and nitrates, if you are so inclined). These were discussed in depth in my early blowing article, so I’m not going to repeat that information here. But, in addition to these approaches, we can also control salting, animal feed, and use preventative cultures to help address late blowing.
Clostridia are sensitive to modest concentrations of salt – this is why salt is used to preserve many different foods. Ensuring that your cheeses receive the correct amount of salt, quickly enough, is important for preventing late blowing. Under-salting, or salting in a way which leads to slow salt buildup in the cheese, can lead to late blowing. Following the salting guidelines for your recipe is usually sufficient. But if you are increasing the size of your recipe you will need to change your salting schedule.
Salted Curd Cheeses: Generally speaking, if your cheese is salted by mixing salt to the curd before pressing, nothing needs to be changed with the salting schedule. Simply scale the weight of salt you use with the volume of milk you use, then proceed as per usual. E.G. If you double the volume of milk you use to make the curd, double the weight of salt you add before pressing.
Dry-Salted Cheeses: If you salting by rubbing dry salt onto the outside of the cheese, keep the size of the cheese the same. Meaning, if you want to double the batch size, you should make 2 regular-sized cheeses instead of 1 large cheese. This is the best practice as the surface area of the cheese (e.g. the amount of space you have to smear salt onto) grows more slowly than the volume of your cheese. The volume of your cheese increases linearly with your cheeses weight – double the weight, double the volume. The change in surface area depends on the shape of your cheese, but as an example, let’s look at the common 22 cm hard-cheese moulds:
I have a 4 kg version of the 22 cm mould. In this I can press cheeses as small as 1 kg, or upto nearly 4.5 kg. The diameter of the cheeses pressed in this mould is always the same (22 cm), meaning that the thickness of the cheese is what increases when I scale up a recipe. In this mould a 1 kg cheese will be a 22 cm cylinder that is ~5 cm thick. This gives the 1 kg cheese a surface area of ~1100 cm2. If I make a 4 kg cheese, I end up with a cheese 22 cm in diameter and ~20 cm thick, giving it a surface area of ~2100 cm2. So in this mould, scaling the cheese recipe four times its original size increased the cheese volume (and thickness) by 4 times, but the surface area only increased 1.9 times.
Because of this it takes a lot longer to “work” the salt into the larger cheese as you have half the area per weight of salt to work with. This means that salt absorption will be quite slow – as much as 4 times slower (absorption scales to the square of the surface area). You are also going to be more prone to having salt fall off the cheese, or having the surface brine that forms run off the cheese before it is absorbed.
Brined Cheeses: Cheeses which are brined are subject to the same physical limitations as those which are dry salted. As such. it takes longer for the salt to reach the middle when a cheese is made thicker. However, because the salt is pre-dissolved in water there are no concerns with loss of salt or brine from the cheese. Moreover, cheeses have a lot of micro-cracks in their paste. This allows brine to “flow” into the interior of the cheese (PDF), unlike dry salt whose absorption is limited largely to the cheeses surface.
This means that you can brine nearly any cheese. But you must scale your brining times relative to the thickness of the cheese. If using the same mould (or moulds of the same diameter), this means you need to increase your brining times roughly linearly with thickness. E.G. if you double the thickness of the cheese from your normal recipe, then double your brining time. On the scale of home cheese makers this is close enough, but be aware that absorption is not linear, and you will under-brine very large cheeses if you follow this approach. For example, a homemade ~1.5 kg parmesan (5-8 cm thick) needs around 12 hours of brining, and if you double that recipe you’ll want to brine for a full day. But the big commercial wheels (~30 cm thick) need to brine for around 21 days !
You will also need to use a lot more brine, as a larger cheese needs to absorb more salt. Since brine is usually saturated, this means you need to add more brine to ensure there is sufficient salt for the cheese. I aim for an equal volume of brine as cheese, assuming that 1 kg of cheese has a volume of 1L. If brining extremely large cheeses and you cannot manage that amount of brine, you may need to add additional salt to the brine mid-way through the brining process.
Dealing with silage
The easiest way to deal with silage is to simply avoid using it. This is only really an option for those raising their own animals. There has been some work into limiting Clostridia growth in silage, as Clostridium-fermented silage has poorer nutrition and animals don’t like to eat it due to its harsh aroma. Much of this involves controlling the starting moisture level of the hay. Some farmers even inoculate their hay with a starter culture. There have also been attempts to sterilize silage, although as far as I can tell, these approaches remain experimental.
If you are raising your own milk animals, avoiding silage is generally going to be easy – just don’t use it. Store-bought milk will be relatively free of Clostridia, as farmers work had to limit Clostridia as it can reduce milk production. Most milk plants won’t purchase Clostridia-positive milk either, giving farmers further motivation. If you’re purchasing directly from a farmer, you may need to look at finding a new farm if late-blowing is an issue in your cheese and the farmer is feeding silage.
Late blowing is where protective cultures really come into their own. I discussed these in my previous post, where I described three reasons why these cultures have limited use for preventing early blowing. Ironically, the very things that make protective cultures of limited use for early blowing make them excellent for controlling late blowing.
The major way in which protective cultures work is through the production of bacteriocins: antibiotic-like molecules that suppress or kill susceptible bacteria. Not all cheese-making bacteria produce bacteriocins, but there are a number of strains available to cheese makers which do. In addition, commercial producers can buy purified bacteriocins. This allows them to avoid the complexity of using two cultures in their cheese making. This can be desired as some protective cultures may change the flavour of some cheeses. Some home cheesemaking suppliers are beginning to sell protective cultures [E.G. cheeseneeds.com],but I’ve yet to see the purified bacteriocins offered to home cheesemakers. Perhaps that will come in the future.
Because home cheese makers use protective cultures and not purified bacteriocins, it will take time for the bacteriocins to reach levels where they are effective. This is part of why these cultures do not help much with early blowing, as the blowing is faster than the accumulation of bacteriocins. But bacteriocins are relatively stable, and will accumulate in the cheese to levels which will suppress the bacteria that cause late blowing.
The microbiology of late blowing is another reason why protective cultures are so effective. The bacteriocins made by protective cultures mostly target Gram positive bacteria. Most of the organisms which cause early blowing are Gram negative bacteria or yeast – organisms which are generally unaffected by these bacteriocins. In contrast, the Clostridia and homofermentative lactobacilli that cause late blowing are Gram positive. This makes them sensitive to these protective cultures, and they will be suppressed once the bacteriocins accumulate.
Late blowing is never OK
While some forms of early blowing are desired in some styles of cheese (Swiss cheese), late blowing is never desired. This is simply a factor of what causes late blowing. The “good” blowing of Swiss-style cheeses is caused by a harmless bacteria (Propionibacterium freudenreichii subspecies shermani). At best, late blowing is caused by an undesired heterofermentative lactobacillus. At worst, late blowing is caused by one of the most lethal bacteria known to humankind. There is no way to tell (outside of a microbiology lab) what bacteria caused a late blow, meaning there is no way to determine whether you blown cheese is safe to consume.