When cheese suddenly begins to show early holes, unwanted eyes, swelling, or strange porosity in summer, many people immediately look for one single cause: a bad starter, contamination, or the wrong rennet. In practice the summer problem is usually broader. It begins before cheesemaking itself, in the milk, in cooling, in transport, in total bacterial load, and in the way warmth changes the behavior of the whole microflora. That is why trying to fight only the final symptom rarely works. You need to understand why summer milk lives differently from winter milk.
The trap is that summer cheese can already look wrong very early. Sometimes eyes are visible in the curd before pressing and before ripening. That feels alarming because the cheesemaker expects a later defect and instead finds it during the cut. Without a clear process view it is easy to blame a mysterious failure or one wrong ingredient. The real logic is more practical: more warmth, higher bacterial load, faster multiplication, and a better chance that gas-forming bacteria survive the journey from milking to the vat.
Why summer milk is riskier from the start
The issue is not only hot kitchen air, although that matters too. The key problem is that milk in warm months begins collecting and developing a heavier microbial load before you even start making cheese. During milking, transfer, holding, carrying, and transport, bacteria get a much friendlier environment than in winter. Even if the initial contamination is not wildly different, the warm season gives those bacteria time and conditions to multiply very rapidly.
In winter many bacteria that enter the milk simply do not have the same chance to explode in number before proper cooling begins. In summer the opposite happens. Temperatures above about 20°C are comfortable for the growth of many microorganisms, so even a short delay before chilling can produce a significant jump in microbial count. The difference is often not that summer milk receives ten times more bacteria at the start, but that the bacteria already present are given a much better opportunity to expand.
Why transport and cooling become critical
Even if the milk looked acceptable immediately after milking, summer logistics can damage it quickly. Not every producer has a truly cold chain. Tank cooling helps, but the milk still has to be moved, transported, unloaded, and held before processing. Every warm interval gives bacteria more time. That is why a cheesemaker in summer should not only ask whether the milk is fresh, but how exactly it was chilled, how long it traveled, and where it spent time warm.
The problem is not only the total number of microbes. Summer also increases the odds that gas-forming groups develop well along with the usual milk flora. That may include not only the textbook organisms people remember, but a broader background flora that becomes much stronger under warm conditions before pasteurization even starts. That is the flora that later creates the nasty surprise of early eyes and gas inside the body of the cheese.
Why gas appears instead of only faster souring
It is reasonable to ask why more bacteria do not simply mean faster souring. Why do we see holes and gas? The answer is that warmth does not selectively favor only harmless acid producers. It also helps the growth of organisms capable of producing gas. In the context of warm transport, a more active environment, and the biological realities of summer farming, the odds of that kind of flora increase substantially.
This does not always look dramatic in the raw milk itself. Milk can appear fairly usable and still contain enough unwanted flora to trigger early eyes after pasteurization and inoculation. The situation becomes especially unpleasant when some of that flora is thermophilic, because then it can survive heat more easily and remain active inside the cheese mass.
Why pasteurization does not automatically solve everything
One of the most common summer illusions is the belief that pasteurization will reset the system completely. In practice it is not a universal guarantee. First, if the initial bacterial load has already become high, part of the unwanted flora may still survive. Second, summer may favor organisms that are inherently more heat-tolerant. Third, a gentle winter habit of working at lower pasteurization settings may simply become insufficient once the seasonal microbial pressure rises.
That is why cheesemakers often say that a softer regime around 68°C may have behaved acceptably in winter, while a move closer to 72°C becomes more reasonable in summer. This is not a rigid law for every milk source in the world, but it is a strong practical orientation. Summer microbial load often demands a stricter heat step, because a softer one leaves too many living participants in the system. When the weather is hot, the transport was not ideal, and the milk spent time warm, being economical with pasteurization becomes risky.
Why the problem continues after starter addition
Even after pasteurization, the post-inoculation environment behaves differently in summer. The room is warmer, the milk cools more slowly, and bacteria therefore spend longer at temperatures that favor rapid division. In winter the culture often begins to lose heat together with the environment fairly quickly. In summer the milk may remain in a comfortable zone for microbes for much longer. By the time rennet is added, the biological activity inside the vat can already be more intense than usual.
This is one reason summer acidification often runs faster, and why background flora activity may increase at the same time. A cheesemaker may follow the same steps as always and still obtain a very different curd, because the milk and the microbial field have lived through a different day. At that stage you can no longer trust only the clock or a standard winter recipe. You have to look at the speed of acid development and ask whether the mass already seems suspiciously airy before cutting.
When eyes are visible already in the curd
One of the most characteristic summer warning signs is the appearance of eyes directly in the curd. That is a strong indication that the problem did not begin on the ripening shelf. If gas is already present in the curd, the unwanted flora developed early and aggressively. In that situation it makes little sense to argue only about pressing or aging. The origin is upstream: milk, chilling, heat treatment, and overall process tempo.
It is also important to understand that such early holes do not necessarily point to one single terrible bacterium. More often they reflect an overloaded system: warm milk, gas-forming background flora, gentle pasteurization, a warm room, and winter habits applied unchanged to summer. When those factors combine, cheese begins showing porosity much earlier than expected.
What to do in practice during summer
The first rule is to reduce the warm life of milk as much as possible. The faster it is chilled after milking and the more stable the cold chain remains until processing, the better. The second rule is not to transfer winter pasteurization habits into summer automatically. If a mild regime worked acceptably in the cold season, a move closer to 72°C may be much wiser in summer, especially if you do not fully control the transport chain.
The third rule is to watch the speed of acid development more carefully after inoculation. Summer conditions can make it run faster simply because the environment stays warmer, so familiar waiting times become less trustworthy. The fourth rule is not to ignore early signs such as eyes in the curd, unusual porosity, unexpectedly light structure, or visible gas activity. Those are not minor details that will disappear later. They are instructions to review the entire path of the milk from milking to vat.
Why this cannot be fixed only by changing starter or rennet
It is tempting to buy another starter, switch rennet, or add more calcium and hope the issue goes away. But early summer holes are usually a story about milk background and microbial load, not about one magic packet. Starter choice can influence the speed and style of acidification, but it cannot erase an already accumulated unwanted flora. Rennet certainly cannot solve a bacterial problem, because its job is coagulation, not sanitation.
The better question is therefore not what to buy, but where control over the microbial picture is being lost. Until logistics, cooling, and the summer heat regime are corrected, targeted substitutions often offer only the feeling of action. The system remains overloaded and will show itself again through early eyes, unstable curd, or acidification that runs too fast.
Practical conclusion
Early summer holes and gas formation in cheese usually begin long before ripening and even before molding. They are a story about how milk lives through the warm season: how quickly it is chilled, how it is transported, how many warm pauses it experiences, and how firmly the cheesemaker adapts pasteurization and timing to summer conditions. If you look only at the finished cheese, you usually miss the true origin of the defect.
The hotter the season and the weaker the cold chain, the more carefully you need to manage bacterial load, pasteurization, and the speed of the whole milk-handling process. Summer cheesemaking should not be winter cheesemaking with sweat added. It needs its own adaptation to a more active and more aggressive microbial environment. That is what sharply reduces the chance of seeing eyes already in the curd.
