The Role of Lactose in Milk
Lactose is the natural carbohydrate found in the milk of all mammals. It is a disaccharide, meaning it is made of two simpler sugar molecules: glucose and galactose. In milk, lactose provides a vital energy source for young offspring. A single cup of cow's milk contains about 12 grams of lactose, which gives it a slightly sweet taste. For human consumption, this lactose is normally broken down in the small intestine by the enzyme lactase. However, many people lose the ability to produce enough lactase after childhood, leading to lactose intolerance and the associated digestive issues.
The Journey from Milk to Cheese: How Sugar Disappears
The transformation of sugar-rich milk into low-sugar cheese is a complex process rooted in microbiology and chemistry. It involves several key steps that systematically remove and consume the lactose.
Step 1: Acidification with Starter Cultures
The cheesemaking process begins by adding special bacteria, known as starter cultures, to the milk. These friendly bacteria play a crucial role by feeding on the lactose present in the milk. As they consume the lactose, they convert it into lactic acid. This acidification process serves two purposes: it lowers the milk's pH, which is necessary for coagulation, and it begins to reduce the overall lactose content before the milk even becomes cheese.
Step 2: Separating Curds from Whey
After acidification, a coagulant like rennet is typically added to the milk. Rennet causes the milk proteins (casein) and fats to coagulate, forming solid clumps called curds. The remaining liquid, known as whey, is rich in water and, importantly, the majority of the lactose. The curds are then cut and drained, allowing most of the lactose-rich whey to be separated and discarded. This initial draining is the most significant step in reducing the sugar content.
Step 3: Aging Consumes Residual Lactose
While draining removes most of the lactose, some still remains trapped within the curds. This is where the aging, or ripening, process comes into play. As cheese ages, the starter bacteria continue their work, fermenting the last bits of residual lactose into lactic acid. This continued fermentation, which can last for months or even years depending on the cheese, further lowers the lactose concentration to negligible levels. The longer a cheese is aged, the lower its lactose content will be, resulting in aged, hard cheeses like Parmesan that are virtually lactose-free.
Comparison of Aged and Fresh Cheeses
The amount of sugar in cheese depends heavily on the production process and aging period. This is the main reason for the difference in lactose levels between fresh, soft cheeses and hard, aged varieties.
| Feature | Aged, Hard Cheeses | Fresh, Soft Cheeses |
|---|---|---|
| Lactose Content | Minimal to virtually none (<0.1 g per serving) | Higher (1 to 5+ grams per serving) |
| Whey Content | Very low, most is drained off and/or pressed | Higher, retains more moisture and liquid whey |
| Aging Period | Longer, allowing more time for bacteria to consume lactose | Minimal or no aging; consumed shortly after production |
| Texture | Firm and hard | Soft, moist, and creamy |
| Examples | Parmesan, Aged Cheddar, Swiss | Ricotta, Cottage Cheese, Cream Cheese |
Implications for Lactose Intolerance
For individuals with lactose intolerance, this distinction is crucial. Because aged cheeses have so little lactose, they are often well-tolerated and can be enjoyed without causing the digestive discomfort associated with milk. A person who cannot drink a glass of milk may be able to eat aged cheddar or Parmesan without any issues. However, soft cheeses like ricotta or cottage cheese, which retain more whey and are not aged, will have higher lactose levels and may cause symptoms. This is why people with lactose sensitivities should learn to distinguish between different types of cheese.
Nutritional Spotlight: Beyond the Sugar
While the sugar content is a key difference, milk and cheese offer different nutritional profiles overall. Cheese, being a concentrated milk product, is typically much higher in fat and protein per serving than milk. It is also an excellent source of calcium. However, cheese is also often higher in sodium and saturated fat. On the other hand, milk is a good source of hydration and contains other important nutrients like potassium, vitamin D, and B vitamins. Both can be part of a healthy diet, with the best choice depending on individual dietary needs and health goals. You can learn more about the scientific principles behind this process at the Science Learning Hub.
Conclusion
The short answer to why there is sugar in milk but not cheese is twofold: the physical separation of curds from lactose-rich whey, and the biochemical fermentation of remaining lactose by beneficial bacteria during the aging process. This transformative journey, which has been perfected over millennia, changes milk's nutritional makeup and explains why aged cheeses are a suitable dairy option even for many with lactose intolerance. By understanding this process, we can better appreciate the art of cheesemaking and make informed dietary choices.
