Does cheesemaking kill bacteria?

Cheesemaking reduces or eliminates some bacteria reliably and leaves others largely unaffected. Salt, lactic acid, and time collectively eliminate Campylobacter jejuni and the organisms the 60-day aging rule was originally designed for — Mycobacterium tuberculosis and Brucella abortus. However, E. coli O157:H7 and Listeria monocytogenes are not reliably eliminated by standard aging protocols across cheese styles. Salmonella reduction is load-dependent. The outcome depends on the pathogen species, the cheese style, and the starting contamination level.

What is the 60-day aging rule for raw milk cheese?

The 60-day aging rule, codified in 21 CFR Part 133, was established in 1949. It requires that raw milk cheese sold in the United States be aged at a temperature of at least 2°C (35°F) for a minimum of 60 days. The rule was based on research showing that Mycobacterium tuberculosis and Brucella abortus — the primary dairy pathogens of the era — were reliably eliminated during extended aging. E. coli O157:H7 was not identified as a human pathogen until 1982, thirty-three years after the rule was written. Research by Reitsma and Henning (1996) found viable O157:H7 in cheddar at 158 days, and D'Amico et al. (2010) recovered viable cells from raw milk cheddar and Gouda beyond 270 days using selective enrichment.

What is water activity and why does it matter for cheese safety?

Water activity (aw) is a measure of the free water available for microbial metabolism and reproduction. Pure water has an aw of 1.0. Most bacteria require an aw above 0.90 to grow. Hard aged cheeses such as cheddar, Parmesan, and aged Gouda typically reach aw values of 0.82 to 0.88, falling below the growth threshold for many organisms. Soft fresh cheeses have aw values of 0.97 to 0.99, remaining well within the range where pathogens can survive and multiply. Salt reduces water activity by drawing moisture from the curd through osmotic pressure.

Why is Listeria monocytogenes particularly difficult to control in aged cheese?

Listeria monocytogenes presents three specific challenges during cheese aging. First, it can grow at refrigeration temperatures as low as 0°C (32°F), making cold storage ineffective as a control measure. Second, it tolerates sodium chloride concentrations up to 10%, surviving the salt levels present in most cheese styles. Third, its biofilm-forming capacity allows it to persist on cheese surfaces and production equipment at concentrations 100 times or more above the lethal threshold for free-floating cells of the same organism. The 2015 Joint FDA/Health Canada Quantitative Assessment concluded that aging alone cannot reliably mitigate Listeria contamination in raw soft cheese.

Which cheese style has the highest bacterial risk?

Fresh soft cheeses such as ricotta and queso fresco present the highest bacterial risk profile, with water activity values of 0.97 to 0.99, minimal or absent aging, and lower salt content than hard aged styles. Surface-ripened cheeses including washed-rind and bloomy-rind styles present elevated Listeria risk because ripening cultures including Penicillium camemberti and Geotrichum candidum consume lactic acid, raising the surface pH from approximately 4.5 back to as high as 6.5 to 7.5 — nearly identical to fresh milk. Hard aged cheeses provide the most hostile microbial environment, though E. coli O157:H7 has been documented surviving in properly aged cheddar beyond 60 days.

Why does Campylobacter rarely appear in cheese outbreaks?

Campylobacter jejuni is among the most sensitive of the major foodborne pathogens to cheesemaking conditions. Its minimum growth temperature is 30°C (86°F), meaning it cannot reproduce at any point during aging or cold storage. It requires microaerophilic conditions of 3 to 5% oxygen and has a minimum water activity requirement of approximately 0.99, making it effectively inhibited by standard salt levels in cheese. Its minimum growth pH is 4.9, and it lacks the acid adaptation mechanisms present in E. coli O157:H7. Raw milk cheese outbreaks linked to Campylobacter are rare in surveillance data; the organism's risk is essentially confined to unpasteurized fluid milk.

What is log-reduction kinetics in the context of cheese aging?

Log-reduction kinetics describes how pathogen populations decline during food processing. Each log reduction represents a 90% decrease in bacterial count. A 4-log reduction eliminates 99.99% of organisms present. The critical implication for cheese aging is that the same reduction process produces different safety outcomes depending on starting contamination levels. If a cheese begins with 100 CFU per gram of a pathogen, a 4-log reduction brings that to 0.01 CFU per gram. If the same cheese begins with 1,000,000 CFU per gram, the identical reduction leaves 100 CFU per gram — a clinically significant remainder for pathogens like E. coli O157:H7, which has an infectious dose as low as 10 to 100 cells.

Wedges of natural-rind aged cheese displayed at a USDA farmers market, showing the firm texture and pale interior typical of hard aged styles.

What Happens to Bacteria During Cheese Aging: Salt, Acid, and Time

Q: Why does E. coli O157:H7 survive cheese aging better than other bacteria?

E. coli O157:H7 can induce an acid tolerance response (ATR) when exposed to mildly acidic conditions in the range of pH 5.0 to 5.5 — a level typical of early-stage cheese curd. This adaptive mechanism, regulated in part by the RpoS sigma factor, upregulates acid shock proteins that allow the organism to survive pH levels that would kill non-adapted cells. Non-adapted O157:H7 has a minimum growth pH of approximately 4.4; ATR-adapted cells can survive exposure to pH as low as 3.5. The infectious dose for E. coli O157:H7 can be as low as 10 to 100 cells, meaning even a small surviving population is clinically significant.

Q: Do staphylococcal enterotoxins survive pasteurization and cheese aging?

Yes. Staphylococcal enterotoxins including enterotoxin A (SEA) and enterotoxin B (SEB) are heat-stable proteins that resist both pasteurization and extended aging. Enterotoxin A retains biological activity after exposure to 121°C (250°F) for 30 minutes — a temperature and duration that sterilizes essentially all other biological material in food. The infectious dose is approximately 1 microgram of enterotoxin, an amount that can be produced when Staphylococcus aureus populations reach roughly 100,000 CFU per gram of food. Once present in milk or curd, the toxins persist through all subsequent processing regardless of aging duration.

June 22, 2026 · 12 min read

Cheesemaking transforms milk through a sequence of chemical and biological changes that are hostile to many microorganisms. Salt draws moisture from the curd. Bacterial starter cultures acidify the environment. Time and controlled temperature continue both processes, progressively altering the conditions available to surviving cells. Together, these forces eliminate some pathogens reliably, reduce others significantly, and leave certain organisms largely unaffected.

Understanding which bacteria respond to which conditions, and why, requires examining each mechanism in detail.

Why Starting Contamination Levels Determine What Survives Cheese Aging

Raw milk contains a complex microbial community. Most organisms present are harmless or beneficial, but milk from any animal can carry pathogens including Escherichia coli O157:H7, Listeria monocytogenes, Salmonella spp., Campylobacter jejuni, and Staphylococcus aureus. The concentration of these organisms at the start of cheesemaking, called the initial load, has a direct bearing on what survives the process.

Pathogen reduction during aging follows log-reduction kinetics. Each log reduction represents a 90% decrease in bacterial population. A 4-log reduction eliminates 99.99% of organisms present. A 6-log reduction eliminates 99.9999%. If a cheese begins with 100 colony-forming units per gram (CFU/g) of a given pathogen, a 4-log reduction brings that figure to 0.01 CFU/g. If the same cheese begins with 1,000,000 CFU/g, the identical reduction leaves 100 CFU/g remaining. For pathogens like E. coli O157:H7, where the infectious dose can be as low as 10 to 100 cells, that remainder is clinically significant.

The same reduction process produces different safety outcomes depending on starting conditions. This relationship is central to understanding how contamination events can occur even when standard aging protocols are followed correctly.

Salt and Water Activity: How Moisture Control Shapes Bacterial Survival

Salt is added to cheese either by direct incorporation into the curd or by brining after pressing. Its primary antimicrobial mechanism is not chemical toxicity but physical: salt reduces water activity (aw), a measure of the free water available for microbial metabolism and reproduction.

Pure water has an aw of 1.0. Most bacteria require an aw above 0.90 to grow. As water activity falls below that threshold, cellular processes slow, membrane integrity degrades, and reproduction becomes impossible. Below approximately 0.85, bacterial growth ceases for most species.

Different cheese styles reach different water activity levels during and after aging:

Fresh soft cheeses (ricotta, queso fresco): aw 0.97 to 0.99
Semi-soft cheeses (Havarti, Fontina): aw 0.92 to 0.96
Hard aged cheeses (cheddar, Parmesan, aged Gouda): aw 0.82 to 0.88

Hard aged cheeses fall at or below the lower limit of bacterial growth for many organisms. Soft fresh cheeses remain well within the range where pathogens can not only survive but multiply after production.

Salt tolerance varies significantly across pathogen species. Listeria monocytogenes can grow in sodium chloride concentrations up to 10% and maintains metabolic activity at aw levels as low as 0.92. Salmonella and E. coli O157:H7 are moderately salt-sensitive but can persist through the salt concentrations present in most commercial cheese styles. Campylobacter jejuni is among the most salt-sensitive of the major foodborne pathogens, with a minimum aw requirement of approximately 0.99, making it effectively inhibited by even modest salt levels in cheese.

How Lactic Acid Lowers Cheese pH and Why Some Pathogens Resist Acidification

Cheesemaking depends on lactic acid bacteria (LAB) as starter cultures, primarily species of Lactococcus, Lactobacillus, and Streptococcus thermophilus. These organisms ferment lactose into lactic acid, driving the pH of the curd from the near-neutral range of fresh milk (pH 6.5 to 6.7) down to the acidic range of aged cheese (pH 4.5 to 5.2, depending on style).

This acidification creates an environment hostile to many microorganisms. Most bacteria have a minimum growth pH between 4.5 and 5.0. Below those thresholds, enzyme systems fail and cells die or enter a non-reproductive state.

Beyond direct acidification, LAB exert competitive pressure through two additional mechanisms. First, they consume available nutrients rapidly, reducing the resources accessible to pathogens. Second, some LAB strains produce bacteriocins, which are antimicrobial peptides that act against competing bacteria. Nisin, produced by certain Lactococcus lactis strains, is specifically active against Listeria monocytogenes and is used in some commercial cheese applications for this reason.

The critical limitation of lactic acid as a safety mechanism is that acid tolerance is not uniformly distributed across pathogen species. Some organisms have developed adaptive responses to acidic environments that substantially alter their survival profile.

E. coli O157:H7 can induce an acid tolerance response (ATR) when exposed to mildly acidic conditions in the range of pH 5.0 to 5.5, a level typical of early-stage curd. This adaptive mechanism, regulated in part by the RpoS sigma factor, upregulates acid shock proteins that allow the organism to survive pH levels that would kill non-adapted cells. Non-adapted O157:H7 has a minimum growth pH of approximately 4.4. ATR-adapted cells can survive exposure to pH as low as 3.5. Pre-exposure to mild acidity during early curd formation therefore prepares O157:H7 to withstand the more extreme acid conditions that develop as aging progresses.

Listeria monocytogenes has a minimum growth pH of approximately 4.4. While it cannot reproduce at the pH of most finished cheeses, it can survive in a dormant state and resume growth if conditions shift, including during temperature fluctuation in storage or in higher-pH zones within inhomogeneous curd.

Campylobacter jejuni has a minimum growth pH of approximately 4.9 and lacks the acid adaptation mechanisms present in E. coli O157:H7. Combined with its requirement for microaerophilic conditions of 3 to 5% oxygen, its sensitivity to acid makes it highly susceptible to standard cheesemaking conditions.

The 60-Day Aging Rule: Origins, Limitations, and What the Science Shows

The 60-day aging requirement for raw milk cheese sold in the United States was established in 1949 and codified in 21 CFR Part 133. Its scientific basis was research demonstrating that Mycobacterium tuberculosis and Brucella abortus, the pathogens then considered the primary threats in raw dairy, were reliably eliminated during extended aging through the combined effects of salt, acid, and time.

For those organisms, the standard accomplishes what it was designed to accomplish. Both M. tuberculosis and Brucellaare sensitive to acidic conditions and low water activity and do not persist through a properly executed aging period.

The limitation is that E. coli O157:H7 was not identified as a human pathogen until 1982, thirty-three years after the rule was written. Reitsma and Henning, publishing in the Journal of Food Protection in 1996, found viable O157:H7 in cheddar cheese at 158 days, well past the 60-day threshold, though their study used pasteurized milk. Subsequent research in raw milk cheese confirmed and extended this finding: Schlesser et al. (2006) demonstrated less than a 1-log reduction of O157:H7 in raw milk cheddar after 60 days, and D’Amico et al. (2010) recovered viable O157:H7 from raw milk cheddar and Gouda beyond 270 days using selective enrichment.

Listeria monocytogenes presents a distinct challenge during aging. While low pH and reduced water activity inhibit its growth, Listeria remains viable at refrigeration temperatures down to 0°C (32°F), making it one of the few significant food pathogens capable of growth under standard cheese storage conditions. Its biofilm-forming capacity compounds this problem: Listeria biofilms on cheese surfaces and production equipment demonstrate resistance to sanitizers at concentrations 100 times or more above levels lethal to free-floating cells of the same organism. The 2015 Joint FDA/Health Canada Quantitative Assessment of the Risk of Listeriosis from Soft-Ripened Cheese Consumption concluded that aging alone cannot reliably mitigate the risk posed by Listeria contamination in raw soft cheese.

Temperature during aging interacts with all three mechanisms. Most cheese aging occurs between 10°C and 15°C (50°F to 59°F), well below the optimal growth temperatures of common LAB starter cultures, which range from 25°C to 45°C depending on species. At aging temperatures, LAB activity and acidification continue but at a reduced rate. Very cold aging slows pathogen metabolism as well, but the net effect can be a slower pH drop and less antimicrobial protection during the critical early weeks.

E. coli, Listeria, Salmonella, and Campylobacter: How Each Responds to Aging

The table below gives quantitative growth limits and survival characteristics for the five pathogens most relevant to aged cheese.

Pathogen	Min. Growth Temp	Min. Growth pH	Min. aw	Infectious Dose	Survives 60-Day Aging
E. coli O157:H7	8°C (46°F)	4.0 (ATR-adapted)	0.95	10-100 cells	Yes, documented
Listeria monocytogenes	0°C (32°F)	4.4	0.92	~1,000 cells	Yes
Salmonella spp.	5°C (41°F)	3.8	0.94	Highly variable; ~10^3 to 10^8 (serovar-dependent)	Variable
Campylobacter jejuni	30°C (86°F)	4.9	0.99	500-800 cells	No
Staphylococcus aureus	7°C (45°F)	4.0	0.83	1 µg enterotoxin	Organism no; toxins yes

Campylobacter stands apart as the pathogen most consistently controlled by cheesemaking. Its minimum growth temperature of 30°C means it cannot reproduce at any point during aging or cold storage. Its high minimum aw requirement of 0.99 means standard salt levels effectively immobilize it. Raw milk cheese outbreaks linked to Campylobacter are rare in surveillance data; the organism’s risk is essentially confined to unpasteurized fluid milk.

Salmonella occupies a middle position. It is more susceptible to acid and salt than either Listeria or O157:H7, and its reduction during aging is more reliably achieved, though not guaranteed when initial contamination loads are high or when low-acid cheese styles are involved.

E. coli O157:H7 and Listeria monocytogenes most consistently challenge the limits of aging as a control mechanism. O157:H7 does so through its acid adaptation capacity. Listeria does so through its cold tolerance, salt tolerance, and physical persistence as biofilm.

Why Staphylococcal Enterotoxins Survive Both Pasteurization and Aging

Staphylococcus aureus occupies a different category from the other pathogens discussed here, and its behavior during aging illustrates the limits of thermal and chemical controls across food safety broadly.

S. aureus itself can be inhibited by the conditions of hard cheese aging: salt, low aw, and pH reduction suppress its growth, and the organism is killed by pasteurization. The problem is that S. aureus produces heat-stable protein enterotoxins during its growth phase in milk or curd. These toxins, including staphylococcal enterotoxin A (SEA) and staphylococcal enterotoxin B (SEB), are resistant to both heat and acid. Enterotoxin A retains biological activity after exposure to 121°C (250°F) for 30 minutes, a temperature and duration that sterilizes essentially all other biological material in food. Neither pasteurization nor extended aging at any temperature degrades staphylococcal enterotoxins once they are present.

The practical implication is that milk containing S. aureus at sufficient density to produce enterotoxins before cheesemaking begins will carry those toxins into the finished cheese regardless of subsequent processing. The infectious dose for staphylococcal food poisoning is approximately 1 microgram of enterotoxin, an amount that can be produced when S. aureus populations reach roughly 100,000 CFU/g of food.

How Cheese Style Affects Bacterial Risk: Hard, Soft, and Surface-Ripened Compared

The interaction of salt, acid, and aging plays out differently across cheese categories. The table below summarizes water activity, salt content, and primary pathogen concerns by style.

Cheese Style	Typical aw	Typical NaCl %	Minimum Aging	Primary Pathogen Concern
Fresh soft (ricotta, queso fresco)	0.97-0.99	0.5-1.0%	None	Salmonella, Listeria, E. coli O157:H7
Bloomy rind (brie-style)	0.95-0.98	1.5-2.5%	2-4 weeks	Listeria (surface pH rebounds to 6.5-7.5)
Washed rind	0.95-0.97	2.0-3.0%	4-8 weeks	Listeria
Semi-hard (Havarti, Fontina)	0.92-0.96	1.5-2.5%	1-3 months	E. coli O157:H7, Listeria
Hard aged (cheddar, Parmesan)	0.82-0.88	1.5-2.0%	60 days minimum	E. coli O157:H7
Brined (feta, halloumi)	0.92-0.96	3.0-8.0%	Variable	Listeria

Fresh soft cheeses present the highest risk profile. High moisture keeps water activity elevated. Minimal or absent aging means acid development is incomplete at the point of consumption. Lower salt content further limits pathogen suppression.

Surface-ripened cheeses introduce a specific complication. The ripening cultures used in washed-rind and bloomy-rind styles, including Penicillium camemberti and Geotrichum candidum, consume lactic acid during metabolism, raising the surface pH from the acidified value of approximately 4.5 back to as high as 6.5 to 7.5 in fully ripened examples. That surface pH is nearly identical to fresh milk. The conditions this creates are specifically favorable to Listeria monocytogenes, which accounts for the disproportionate association between surface-ripened raw cheeses and Listeria in outbreak surveillance data.

Hard aged cheeses provide the most hostile microbial environment. Extended aging, low moisture, and significant salt concentration bring water activity into the range where bacterial activity is severely constrained for most organisms. The documented survival of E. coli O157:H7 in properly aged cheddar demonstrates that even hard cheese does not offer categorical protection with respect to that pathogen.

Brined cheeses such as feta and halloumi achieve significant salt concentrations through prolonged submersion. The salt tolerance of Listeria monocytogenes means brining alone does not reliably control that organism, though it is effective against less tolerant species.

What Cheesemaking Reliably Accomplishes

The combined effects of salt, acid, and aging accomplish the following with reasonable consistency:

Elimination of Campylobacter jejuni across all cheese styles
Elimination of Mycobacterium tuberculosis and Brucella abortus, the organisms the 60-day rule was designed to address
Significant reduction of Salmonella in hard and semi-hard styles when starting loads are not excessive
Inhibition of growth, though not elimination, of E. coli O157:H7 and Listeria monocytogenes in hard aged styles
Reduction or elimination of Staphylococcus aureus cells, though not the heat-stable enterotoxins produced prior to processing

The limits of the process are equally consistent. E. coli O157:H7 and Listeria monocytogenes are not reliably eliminated by standard aging protocols across cheese styles. Salmonella reduction is load-dependent. Staphylococcal enterotoxins, once present, are not degraded by any cheesemaking condition.

Cheesemaking as a microbial control system is real, meaningful, and well-documented in its effects. It is also bounded, with limits that depend on the pathogen species, the cheese style, the initial contamination level, and the consistency with which production conditions are maintained.