The Science Behind Listeriosis in Cheese
Listeriosis is a serious infection caused by the bacterium Listeria monocytogenes. This resilient microbe can thrive in cold, moist environments, including refrigerated foods, and can cause severe illness, especially in pregnant women, newborns, the elderly, and people with weakened immune systems. The risk associated with cheese depends largely on a combination of factors:
- Moisture Content: Soft cheeses with higher moisture levels provide an ideal environment for Listeria to grow and multiply. Hard cheeses, with their lower moisture content, do not support bacterial growth as effectively.
- Acidity (pH): The acidity of cheese can inhibit bacterial growth. Cheeses that are more acidic are less hospitable to Listeria. The surface mold of certain semi-soft cheeses, like blue cheese, can increase the pH, creating a more favorable environment for the bacteria to flourish.
- Pasteurization: This heat treatment process kills harmful bacteria in milk. Cheeses made from unpasteurized (raw) milk inherently carry a higher risk of contamination. However, even cheese made with pasteurized milk can become contaminated after the heating process due to unsanitary conditions or cross-contamination in the facility.
- Aging Process: Longer-aged cheeses, particularly hard ones, tend to have lower moisture and higher salt content, which helps inhibit bacterial growth over time. Conversely, fresh soft cheeses that do not undergo a significant aging process can be more susceptible to contamination.
Comparing Listeria Risk: Soft vs. Hard Cheeses
To understand the disparity in risk, it's helpful to compare the two main categories of cheese. The CDC and FDA have issued specific warnings regarding soft cheeses due to their association with listeriosis outbreaks.
Soft Cheeses (Higher Risk)
- Characteristics: High moisture, low acidity, and often made with raw milk or contaminated post-pasteurization.
- Examples: Feta, Brie, Camembert, Queso Fresco, Queso Blanco, Requesón, and blue-veined cheeses.
- Safety Precaution: Avoid if you are in a high-risk group unless the cheese is cooked thoroughly to an internal temperature of 165°F. Always check the packaging to confirm it's made from pasteurized milk, but be aware of the cross-contamination risk.
Hard Cheeses (Lower Risk)
- Characteristics: Low moisture, higher acidity, and longer aging time.
- Examples: Cheddar, Parmesan, Romano, and Colby.
- Safety Precaution: These are generally considered safe for all populations, including pregnant women, when made with pasteurized milk. The environment is inhospitable for Listeria to survive or grow.
Cheese Risk Comparison Table
| Feature | Soft Cheeses | Hard Cheeses |
|---|---|---|
| Moisture Content | High | Low |
| Acidity (pH) | Can be lower (less acidic) | Generally higher (more acidic) |
| Listeria Growth Potential | High | Low |
| Associated Outbreaks | Yes, frequently | No, rare |
| Recommended for Vulnerable Groups | Avoid, unless cooked | Generally safe |
| Example Cheeses | Brie, Feta, Queso Fresco | Cheddar, Parmesan, Romano |
The Raw Milk Factor: A Significant Hazard
While cheese texture is a major indicator of risk, the pasteurization status of the milk used is a critical, separate concern. The FDA and Health Canada have quantified the risk, noting that soft-ripened cheeses made from raw milk are significantly riskier than those made from pasteurized milk.
- Raw Milk Contamination: Raw milk can become contaminated with Listeria and other harmful bacteria directly from the animal or during milking. Since no heat treatment is applied, this contamination can persist in the final cheese product. For instance, the FDA emphasizes that unpasteurized milk and products made from it should be avoided entirely by high-risk individuals.
- Legal Ageing is Not Enough: A common misconception is that aged raw milk cheese is safe. However, the FDA's research indicates that the 60-day aging period for raw milk cheeses is not sufficient to eliminate Listeria, particularly in soft-ripened varieties.
Conclusion: Making Informed and Safe Cheese Choices
The question of which cheese poses a higher risk of listeriosis is answered by focusing on a combination of factors: moisture content, acidity, and pasteurization. For individuals who are not in a vulnerable group, the risk is relatively low. However, for pregnant women, the elderly, and immunocompromised individuals, exercising caution is essential. By understanding the science behind Listeria's survival, you can confidently navigate the cheese aisle. Always check labels to ensure milk is pasteurized, and for soft cheeses, be aware of the post-pasteurization contamination risk and cook them thoroughly to be extra safe. To learn more about food safety guidelines, visit the CDC's page on Listeria.
What to Look for When Buying Cheese
- Check the Label: Always look for the word "pasteurized" on the label of soft cheeses.
- Avoid Unpackaged Cheeses: Be cautious of soft, unpackaged deli cheeses or items from a salad bar, where cross-contamination is more likely.
- Know Your Textures: Remember that soft cheeses carry a higher risk. This includes soft-ripened, blue-veined, and fresh soft varieties.
- Heed Recalls: Pay attention to food recalls from official sources like the FDA and CDC. Recent recalls have been linked to contaminated soft cheeses.
- Consider Cooking: If you are in a vulnerable group and craving a soft cheese, ensure you cook it until it's steaming hot to destroy any potential bacteria.
