Lactose and Lactic Acid

Who knew lactose in cheese could be so complicated?

What's Lactose?

Lactose is a type of sugar. More precisely, lactose is a disaccharide. This means lactose is a sugar made up of two simple sugars (or monosaccharides as scientists would say). We’re pretty familiar with disaccharides; lactose is probably the second most popular disaccharide after sucrose (table sugar). Lactose is made of a molecule of glucose and a molecule of galactose. Those two monosaccharides are linked together by a particular type of bond, a β-1→4 glycosidic bond to be exact. That’s important because our intestines need to produce lactase enzyme to break that bond. If you don’t produce that enzyme in high enough amounts, you’ll suffer from lactose intolerance. If the enzyme doesn’t get a chance to break that bond, the microbes in your gut will take a whack at it and will ferment the lactose. Just like other fermentation reactions, common by-products include acid, gas, etc. None of which our digestive system likes very much.


The scientific name for lactose is β-D-galactopyranosyl-(1→4)-D-glucose

So why does any of this matter? For many cheesemakers the chief concern is not breaking lactose into its two parts, but instead having bacteria convert it to lactic acid. Understanding why/how much/when lactose is converted into lactic acid can have dramatic effects on cheese. But before we get too far, let’s take a look at how much lactose is actually in the milk starting off.


Lactose in Milk

Most folks realize that the milk they drink is mostly water. What most folks don’t understand is that the next most prominent ingredient is lactose. Most breeds of cow, goat, and sheep have around 4-6% lactose on average. You heard me right, cows, goats, and sheep. One of the biggest misconceptions surrounding lactose is that goat and sheep milk don’t have any. That’s not the case. As we’ll discuss later on, the type of cheese and how it’s made are the main determining factors of lactose content in the final cheese. The chart below outlines lactose content for some the major milk producing breeds.

Lactose content

Check out this interactive graph for more information and citation!

The main response I get after I tell people this information is “But I feel better when I drink/eat goat/sheep milk products. If it’s not the lactose, then what gives?” My response to that is “Check at a later date for a post about that.”


Lactic Acid Formation

For many cheeses, lactose ends up as lactic acid. This is accomplished by the metabolic processes of the bacterial starter culture the cheesemaker uses. Here’s a reminder what those are. It’s a little more complicated than that since some cultures utilize lactose in different ways. The image below shows a breakdown of how most mesophilic cultures (think cheddar) convert lactose and how some thermophilic cultures (think mozz/parm) convert lactose. You’ll see it gets complicated with some cultures leaving some residual sugar (galactose). This can cause problems down the road, like burnt cheese on pizza. In fact, some starter cultures might metabolize lactose into things other than lactic acid.

Lactose formation

Basic lactose metabolization in cheese

What cheeses are naturally low in lactose? Simply put: aged, cultured, non-“fresh” cheeses. The ones that come to mind right away are aged cheddars, aged gouda, aged parms, etc. What cheeses will have more lactose? Those that aren’t cultured, like many fresh Latin American cheeses. Those that are very young, like fresh cheese curds. Those that have lactose added back, like gjetost. Lactic acid has a tremendous impact on flavor, texture, and functionality of the final cheese.

Let’s talk about how lactose content changes in a model cheddar cheese. I’ve gathered some data from a few scientific papers and made a graph below. Feel free to hover over the blinking dots to get a sense of what’s going on. I’ll also walk through it here. We’ll start with milk and end with an aged cheddar cheese. Buckle up. It’s gonna be a wild ride.

  • A - As already discussed, lactose content starts off around 5%. Once starter is added they go to work converting lactose to lactic acid. Once rennet is added, whey begins to pool. We now have two different areas for lactose to partition (until the whey is drained).
  • B - As the curds are heated the starter keeps at it. If high temperatures are reached (~100°F), starter could be slowed down.
  • C - After the curds are cut, large amounts of whey is coming off the curd. Lactose will migrate with it. Given enough time though, some culture in the whey can start to metabolize the lactose into lactic acid like it's doing in the curd.
  • D - By the time the curds are ready to be hooped a lot of the lactose is gone. The addition of salt at this stage can also slow the culture.
  • E - After a few months practically no lactose remains.

Hover over the blinking dots to learn more! Data adapted from Fox et. al. 1990.


Lactic Acid Metabolization

At this point you might be saying “Pat, what else is there to talk about? Once lactose is converted to lactic acid, that’s the end of the story!” Like an M. Night Shyamalan movie, the story still goes on. Lactic acid itself can be further metabolized into all sorts of things. We’ve already touched on this when we talked about eyes in Swiss cheese. The image below will take you through a few other things lactic acid can be converted into.

Lactate metabolization

Basic lactic acid metabolization in cheese



Where does this leave us? Fully entrenched in the wonderful world of cheese chemistry. The take home point is that lactose is more important than just being the causative factor of tummy aches, but goes on to be transformed into many different compounds.

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