Early Blowing in Cheese Making

Anyone who starts making cheese will inevitably encounter early blowing. This is one of the more common forms of contamination, and its presence can indicate 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.

Note: an article on Late Blowing is also available.

Background

First-things-first, what exactly is “blowing”? Blowing is the development of unwanted gas bubbles in the cheese. Early blowing is blowing that occurs within a few days of the day you make cheese. Most cheese cultures produce little to no gas. As such, most cheese has either a solid paste (e.g. cheddar), or a paste with small and infrequent holes (e.g. havarti). The exception to this is some types of Swiss cheese, which due to the inclusion of Propionibacterium shermani, develop large bubbles.

What are the signs of early blowing?

Cheeses with early blowing will look like the image above – sponge-like, with many round bubbles of varying sizes throughout the paste. For some cheese makers, the first sign that their cheese has blown is when they go to eat it. When they cut the cheese, they find the paste is riddled with bubbles and their cheese resembles a sponge. But often, signs appear much earlier. In extreme cases, bubbles will appear during the cheese making process itself.

I didn’t take a picture, but last year I had a shropshire blue that had bubbles appear during the renneting tage. This represents a worst-case scenario, and is not typical. Early blowing most often appears during the press, or while the cheese is being brined. Even though the bubbles are hidden beneath the rind of the cheese, you can sometimes determine that they are there. These cheeses will often be soft in texture, and may bulge on the sides or on top.

What do you do with a blown cheese?

It is not safe to consume a cheese that has blown. While some organisms that cause blowing are harmless, others are pathogens. There is no test you can perform at home to determine whether a blown cheese is safe to consume. If you raise pigs or poultry, it may be possible to feed it to them. I would recommend only feeding a small amount at a time if you go this route. While these animals are somewhat resistant to these organisms, they’re not completely immune.

Causes of Early Blowing

The cause of all forms of blowing is contamination. The major difference between early and late blowing are the specific microorganisms involved. Early blowing most occurs due to contamination by coliform bacteria or by certain species of yeast. These organisms ferment the lactose in the cheese, forming carbon dioxide and/or hydrogen gas. It is these gases which form the bubbles that “blow” the cheese.

Coliforms

Coliforms are a group of bacteria that commonly found in feces, soil, and throughout much of the environment. These include dozens of species from the Klebsiella, Serratia, Aeromonas, Citrobacter,
Escherichia and Enterobacter genera (note: there are multiple species within a single genus). A key feature of these bacteria is their ability to consume lactose. These bacteria can grow very quickly under typical cheese making conditions – for example. E. coli can divide every 20 to 30 minutes at the temperatures typical during the culturing and renneting stages of cheese making⁠—far faster than the “good” lactobacilli that we want in our cheese.

Coliforms represent the major risk of early blowing. While some coliforms are harmless, many are pathogens and can make you quite ill if consumed. Unfortunately, there are no indicators that can tell whether a blow was caused by a harmless versus harmful coliform.

Yeasts

A couple of species of yeasts also can cause early blowing. Now when I say “yeast” I don’t mean it in the conventional sense – e.g. the bread/beer yeast Saccharomyces cerevisiae. Rather, I mean it in the technical mycological use, where yeasts are defined as single-celled members of the fungus kingdom. There are literally thousands of species of yeast, of which our friend S. cerevisiae is but one member.

S. cerevisiae is unable to ferment lactose, and as such is safe to use while cheese making. The confusion of the lay versus scientific meaning of “yeast”, however, has led to a myth that you cannot make bread and cheese at the same time, or in the same kitchen. This is a myth, and one I plan to tackle in a future article. There are several yeasts which can ferment lactose, and it is these species which can cause early blowing.

Most common are Kluyveromyces marxianus (previously known as Candida kefir) and Kluyveromyces lactis. But there are species in the Debaryomyces and Pichia genera which can also cause blowing. Unlike the coliforms, these yeasts are generally safe for human consumption. In fact, some are common in other fermented milk products. As its old name suggests, C. kefir is found in kefir, as is K. lactis. Debaryomyces hansneii (formerly named Candida famata) are often found in cheese brines and on the rinds of some cheeses.

The good news is that these yeasts are generally considered to be safe, and are only really a concern for people who are immunosuppressed. As always, if that is you, consult with your physician on whether home cheese making is something you can do safely. The bad news is that – without a microscope – there is no practical way to determine whether early blowing is due to yeast or coliforms.

Where does this contamination come from?

The organisms which cause early blowing can be introduced into cheese through a few different routes:

Milk

The most common source of these organisms is from the milk itself. Coliforms are found throughout the guts of ruminants like cows, sheep, and goats. Coliforms also make up part of the microbiome of the udder. Similarly, the yeasts which cause blowing are commonly found in the guts of ruminants, in soil, on udders, and can even be carried by insects. Unpasteurized milk inevitably contains microorganisms from the animals udder. If you’re milking into an open container, your milk will also contain some hair and other debris. Try pasteurizing your milk if you are having issues with early blowing and are using raw milk. If pasteurization fixes your issue, that is a sign that the milk producers collection and/or storage practices need to be improved.

Equipment

Because these yeasts and bacteria are normal components of our environment, it is not uncommon for equipment to become contaminated through now fault of your own. Dust, insects, pets, kids, even your hands, can all transfer these organisms from one location to another. For equipment, your best bet is a mixture of prevention and being proactive. To prevent contamination, always clean your equipment as soon as you are done with it. This ensures that there is no “food” for these organisms to eat between cheese making sessions. Likewise, storing your equipment in a cupboard, sealed container, or other location where animals, children, insects, dust, etc, cannot get at it can help to limit incidental contamination.

To be proactive, make sure that you clean and sanitize your equipment immediately before making cheese. Yes, even though you did all that before putting it away. Make sure your tools are kept on something sanitary while your work (e.g. a sanitized cutting board or plate). And clean and sanitize anything that you think may have contacted a potential source of contamination, or which has contacted milk and then sat for a period of time.

You!

As much as we don’t like to admit it, we are essentially a public transportation system used by an army of microorganisms to get around. Our guts and skin house many of the same coliforms and yeast as do ruminants – the same ones which can cause early blowing. Washing your hands frequently during cheesemaking is important. Wash with soap, and vigorously scrub. Likewise, you should avoid activities which could contaminate you while making cheese. Giving your goat a good scratch, or playing fetch with fido, a few minutes before you add rennet to your cheese is probably not a good idea.

This latter point is worth emphasizing, especially for those of you who raise your own milk animals. My wife struggled with early blowing over this winter. It seemed that no matter how well she cleaned the equipment, and cleaned/resanitized equipment between cheese making steps, every cheese blew during pressing. To make things even more frustrating, I made a series of cheeses successfully at the same time, using milk from the same animals that had been collected, pasteurized, and stored in the same way. It took a while, but through a process of elimination we were able to figure out that my wife was making cheese immediately after milking our goats, and dust or debris stuck to her sweater was contaminating the milk. Simply changing her shirt and doing a really good “surgeon-style” hand scrub solved the contamination issue!

Preventing Early Blowing

The only way to deal with early blowing is to prevent it from happening in the first place. There are four major tools we can use to prevent blowing. There are two additional methods which may have some utility for some cheese makers.

Milk collection and storage

I’ve written previously on how to limit contamination when collecting and storing milk, so I’m not going to rehash that in detail here. In short: 1) Make sure you’re milking into clean equipment. 2) If possible, use a closed milk collection system. 3) Clean the udder and strip the first milk before collection. These practices greatly reduce the amount of contaminants being transferred from your animals to the milk, and therefore reduce the risk of blowing.

If you don’t keep your own animals, you’re somewhat at the mercy of whomever you buy your milk from. When buying from a farmer, make sure they are taking good care of their animals and are milking properly. If issues persist, try switching providers (or use store-bought, pasteurized milk), to see if your provider is the issue.

Milk also needs to be stored properly. It should be cooled immediately after collection (or after pasteurization) and kept below 4C until used. Milk should be used as quickly as possible after collection, typically within a week.

Pasteurization

Pasteurization reduces (but does no eliminate) the number of microorganisms present in milk. The exception to this is ultra/HLT pasteurization, which completely sterilizes the milk, but also renders it unusable for cheese making. Reducing the number of microorganisms in the milk reduces both the risk of developing foodborne illness (again, I’ve got an older post that dives deep into this), and reduces the likelihood of developing contamination in cheesemaking. This includes reducing the risk of contamination that leads to early blowing.

Your other ingredients also need to be treated in a manner which reduces their risk of contamination. Water used for washing curd, dissolving rennet/calcium chloride, or making brine should be boiled (and cooled) before use. Boiling has the added advantage of driving off any chlorine in the water. Ingredients you cannot boil (rennet, lipase, cultures) should be stored in closed containers in the fridge or freezer, and handled in a way which minimizes their risk of being contaminated. Spices and other flavourings should be heat-treated (dry or steam, depending on the ingredient) to lower the levels of the bacteria that are commonly found on these foods.

However, pasteurization is not a panacea. You still need to take care during the other steps of cheese making. Contamination can be introduced at later time points through equipment, other ingredients, or through yourself.

Sanitation

Sanitation is key to preventing contamination of your cheese. You can start with sterile milk and ingredients, and still end up with contaminated cheese due to the introduction of bacteria or yeasts on equipment, or from your food preparation area.

There are two main ways equipment can be sanitized. The first is heat – boiling equipment for 5-10 minutes will render it nearly sterile. Alternatively, you can heat in an oven at 170C (340F) for an hour. Obviously, this is limited to metal and silicone equipment. It is also limited in that it is difficult to re-sanitize equipment mid-way through cheese making.

The second option is chemical sanitation, using a food grade sanitizer. Note that while meant for kitchens, lysol is not food-safe. Lysol is fine for cleaning floors and counters, but food should not be placed onto any surface that may contain lysol residue. The two main food-safe sanitizers are iodiphore and starsan. The former is iodine-based, and consequently can stain clothes, cutting boards, and countertops. The latter is acid-based and therefore is much less likely to discolour things. Outside of that, they are similar in their efficacy, safety, and use. For either, spray, dip, or soak your equipment in them for a minimum of 30 seconds. Then allow to drip dry.

Start by creating a clean zone⁠—an area in your kitchen (or wherever you’re making your cheese) that you will continually keep clean and sanitized throughout cheese making. I like to use a sanitized dinner plate, but a cutting board, or a section of a countertop also works. This surface needs to be non-porous, so a wood cutting board is not ideal. Sanitize your spoons, measuring cups, curd knives, etc, and place them on this clean zone. Whenever you are working, make sure that no ingredients, milk, or other materials enter the clean zone. This is critical for preventing cross-contamination. I always rinse (and often re-spray with sanitizer) my equipment before returning it to the clean zone. The goal here is simple – to create an area where you keep your sanitized equipment, and from which you exclude potential sources of contamination.

You can also sanitize/re-sanitize equipment at any point in the cheese making process. For example, like most cheesemakers, I remove my cheese making spoon from the pot after I add rennet. Since the spoon is going to sit for 45 to 60 minutes before I use it again, I rinse off any milk, and then re-spray it with starsan before placing it in my “clean zone”. Rinsing off the milk ensures that there is no “food” for any contaminants that may fall onto the spoon. Sanitizing ensures anything already on the spoon is eliminated.

One final note – there a myth that vinegar can be used as a sanitizer. This is not the case. Conventional food/kitchen vinegar is 5% acetic acid. While this is acidic enough to slow the growth of most bacteria, it is not enough to kill them. Some bacteria and moulds are even happy to eat vinegar as a food source. Cleaning vinegar is sometimes available at a 10% concentration. At this concentration, vinegar is anti-microbrial. However, cleaning vinegar is rarely food-grade, and it requires long exposure times (hours) to have the same antimicrobial effect as a few minutes of boiling, or 30 seconds exposure to starsan or iodophore.

Temperature & Brine

Our final good tool is managing post-pressing temperatures and brines. Despite their high salt concentration, some yeasts and coliforms can survive in brine. As such, it is a good idea to boil your brine between uses to ensure it is sanitary. Also, to some extent early blowing is a race between the good cultures and contaminants to consume the lactose. By brineing and storing the cheese at cave temperatures (11-13C) we can promote the growth of the desired cultures and slow the contaminants, allowing the good cultures to consume the lactose before blowing occurs. Once the lactose is gone, there isn’t much left for the contaminants to eat, and the risk of early blowing goes away. In other words, sanitize your brine by boiling it after each use, and brine and store your cheese at cave temperatures.

So that’s the four good tools that we have access too – milk quality, pasteurization, sanitation, and temperature. But there are two other options that are employed by some cheese makers – protective cultures and nitrates.

Protective Cultures

There are protective cultures available that can prevent blowing. Generally, these work by including a strain of Lactobacillus that produces a bacteriocin – an antibiotic-like molecule that suppresses or kills contaminating organisms. Unfortunately, while these cultures work well for late blowing, their use for early blowing is minimal. Protective cultures work poorly for early blowing for three reasons:

Firstly, it takes a while for the bacteriocin to reach a concentration where it suppresses infection. As such, early blowing can occur before the protective effect takes hold. This isn’t an absolute truth – you can get suppressive levels of bacteriocins in the event of slow-growing contamination. But average or high levels of contamination will almost always lead to early blowing, even if a protective culture is present.

Secondly, bacteriocins are generally not effective against yeasts, and therefore protective cultures are unable to prevent yeast-induced blowing. Some yeasts do produce “killer factors” – the yeast equivalent of bacteriocins. These have the theoretical possibility of preventing early blowing by yeasts, but these yeasts are rarely included in protective cultures. This is because “factor-positive “killer” yeasts are often strains of the same species that cause blowing. So rather than preventing blowing, adding these to your cheese would simply lead to blowing by the “killer” yeast. That said, there have been a number of studies in using these killer yeasts on the rinds of natural-rind, washed-rind, and mould-inoculated cheeses to limit the growth of off-flavour producing yeasts. This involves applying these yeasts to the rind, not mixing it into the milk before setting, and as such does not affect (or cause) early blowing.

Lastly, protective cultures tend to include lactobacilli that make bacteriocins which target Gram-positive bacteria. Coliforms are Gram-negative, giving them a radically different cellular structure that is largely immune to the effect of these bacteriocins. That’s not to say that protective cultures have no value, but rather, that their value tends to be in preventing late blowing.

Nitrates

Another option is to use preservatives such as nitrates to prevent early blowing. Many cheese cultures are modestly nitrate resistant. This means that you can carefully dose in nitrates to prevent early blowing without affecting your cheese cultures. The use of nitrates in cheese making has a very long history, with many traditional European styles including nitrates as far back as the mid-1800’s. As with protective cultures, nitrates are best used for preventing late blowing, but they can reduce the risk of early blowing. So you may wonder why their use isn’t more common in home cheesemaking. This comes down to a few factors.

Firstly, the dose used needs to be very carefully controlled. A tiny amount extra could suppress your cheese culture, while a slight under-measurement won’t porevent contaminants from growing. Given the very small amount of nitrates needed, and the accuracy of dosing that is required, it simply isn’t practical to use nitrates at the scale of a home cheesemaker.

Secondly, when cooked nitrates (xNO3) decompose into nitrites (xNO2). Nitrites are carcinogenic (this is where the risk of cancer from BBQing and bacon comes from). While cheese is not often exposed to conditions where nitrite formation can occur (grilling and on pizza aside), nitrates themselves may be carcinogenic. That said, the data regarding the carcingenicity of nitrates is much less clear than with nitrites.

Lastly, many countries outside the EU ban or limit the use of nitrates in dairy products. For example, here in Canada nitrates can be used in milk used to make cheese. However, the nitrate levels in cheese must be reduced to below 50 PPM before it is sold. To have an effect, nitrates need to be in the range of 100-200 PPM, and there isn’t a good and consistent way to ensure the levels drop below 50 PPM once the cheese is made.

Sometimes early blowing is OK!

At this point I’ve written a lot about the risks of early blowing and how to prevent it. So to finish this article I want to talk about the exception to the rule. Yes, there are some types of cheese where early blowing is not only acceptable, but is a central and key feature of the style.

I, of course, am talking about Swiss-style (alpine) cheeses.

Many Swiss cheese styles are characterized by large, glossy bubbles spread throughout the paste of the cheese. This blowing is required to get the desired taste, texture, and appearance of these cheeses. But this isn’t any old early blowing defect. Rather, it is a carefully controlled early blowing driven by a specific species of bacteria.

What makes Swiss cheese”blow” ?

The “blowing” of swiss cheese is caused by Propionibacterium freudenreichii subspecies shermani. More often called P. shermani⁠, this bacterium ferments lactose and other components in the cheese to produce a variety of organic acids. This includes propanoic acid, from which it gets its name. These acids (and other compounds) provide the characteristic nutty flavour of these cheeses. During this fermentation, P. shermani also makes large amounts of carbon dioxide. This causes the “blowing” of the cheese. Critically, P. shermani is not a pathogen, nor does it make toxic compounds, making these cheeses perfectly safe (and absolutely delicious) to consume.

One thing I, and many other home cheesemakers, may encounter is the risk of cross-contamination of P. shermani from our Swiss-style cheeses. This is a very real risk, but unfortunately, is not something we can tell apart from other forms of early blowing. So even if you experience early blowing shortly after making a Swiss-style cheese, it is not a good idea to assume that the blowing is due to cross-contamination by P. shermani. To avoid this risk, simply follow the guidelines described above for contaminants from other sources. The bacteria and fungi we use in cheese making are every bit as susceptible to cleaning, sanitizers, and good practices, as are “wild” microorganisms.

One last thing

In my article I’ve provided links to many scientific studies, which are not always the most accessible thing for non-scientists to parse. During my research for this article I came across an amazing resource that is approachable for most readers, but which extensively cites the scientific literature. If you want to do an even deeper dive into early and late blowing, with more dicscussion of the hard-core microbiology, I recommend: Amanda Alvenäs (2015) Cheeses with blowing defect – Problematics and preventable methods. Bachelor Thesis. Swedish University of Agriculture, Department of Food Science.