At breakfast one morning, I read the label of my container of plain yogurt and wondered how partly skimmed milk, milk protein and bacterial cultures could result in what I was eating. Here's what I found out about these ingredients.
To be called yogurt, a product is supposed to contain Lactobacillus bulgaricus and Streptococcus thermophilus bacteria. These are nice, probiotic bacteria that are supposed to be good for your intestines and, in some cases, already live there. In general, these bacteria produce lactase, an enzyme that breaks down lactose, the milk sugar. The fermentation of lactose results in lactic acid. Lactic acid, as the name suggests, lowers pH and gives yogurt that tangy taste. The lower pH causes the protein molecules to denature or unravel, which thickens the yogurt. Some of what makes yogurt good is like what happens when milk goes bad. Different brands use different bacteria, which can affect flavour and other properties of the yogurt. Mine listed Bifidobacterium lactis, Lactobacillus acidophilus and Lactobacillus casei.
If the yogurt has live bacteria in it, they can still survive being eaten by humans and continue to help break down the lactose. This is why people who might be lactose intolerant can eat yogurt, although there's still some lactose in it. My yogurt container claimed that it contained "more than 1 billion probiotic bacteria per serving that contribute to a healthy intestinal flora." That sounds good, but the effectiveness of yogurt to improve your intestines is tricky when you consider that you could have tens of trillions of microbes in your gut.
Partly skimmed milk
Milk is the main ingredient of yogurt, and by eating yogurt you get most of its nutrients, like calcium. Many sites have instructions for making yogurt at home and experimenting with how you make it. Commercial yogurt comes with different amounts of fat. Mine said 2%. Fat free yogurt might not be as thick, but the fat does not seem to be integral to the key processes in making yogurt. Mostly I choose by what's on sale.
Adding milk protein is not necessary to make yogurt but is included in the commercial process to make it thicker. The milk gets heated to kill off unwanted bacteria. Keeping the milk above 90°C (195°F) causes lactoglobulin, one of the main proteins in milk, to denature. This adds to the thickness of the yogurt. As the culture becomes acidic because of bacterial action, the proteins form a gel. Greek yogurt strains out the liquidy whey and so even if fat free, will be thicker.
This is just a taste of all that goes into yogurt. It's a lot to digest. If you have any tips on making yogurt please leave them in the comments and maybe one day I'll try it myself.
Still feeling curious? Keep reading! Have you ever wondered about expiry dates on food or why we put fluoride on our teeth?