Dino’s Daily Dose – Day 5 – Why does my milk go bad before the expiration date?

This is a great question and one of the biggest mysteries of the universe. In this issue i’m going to also answer the questions, “why does milk go bad?” and “what does the date on the carton mean?” I am going to answer all three of these questions and give you some information about milk you probably didn’t know. Most importantly, California has the best milk in America. This is not just boasting. California milk is more nutritious than milk in other states. Ill explain this in a bit, but first lets deal with the question at hand.

Ron Nodder


Thank you Ron “RB (Ron Baby)” Nodder, for the question. RB & I were in a band together in 1989 called Tongue-N-Groove. Ron is currently in a band on the central coast called Rhythm Method and is an award winning wine maker. His label Chateau Lettau is a small boutique winery in Paso Robles. Ron produces some of my favorite central coast blended varietals. If you liked my blog post on making wine, you will like Chateau Lettau even better.

Now back to the question at hand. Why does the milk go sour before the date on the carton? Lets first look at what is in milk.

Milk is a combination of:
Water
Lactose
Fat
Protein
Minerals
Vitamins
and…..BUGS!

Yes bugs. Really, really good bugs. The most important of these bugs is called Lactobacillus. Lactobacillus is one of my favorite of all bacteria. If it weren’t for the LB bug we wouldn’t have such amazing things as yogurt, cheese, sauerkraut, pickles, beer, wine, cider, kimchi, chocolate, and other fermented foods. Some LB bacteria are Probiotics and yoghurt is full of this stuff. Eat lots of yoghurt if you want a healthy digestive system.

Lactobacillus basically spends it’s day eating Lactose (milk sugar) and pooping out Lactic Acid. As the milk becomes more acidic the milk begins to curdle or coagulate. This is how many dairy products are made including cheese. Some cheese is coagulated with an enzyme called rennet and some are acid coagulated with edible acids like lemon juice and vinegar. So basically what is happening when your milk start to sour (the sour smell is from the lactic acid) is the milk is turning it’s self into cheese.

The reason the milk doesn’t turn to cheese on the first day you get it is because it was pasturized. This process kills bacteria by heating it up.

The date on the bottle is the “sell by” date. That date is usually 14 days after pasteurization. This is basically a date in which the processor of the milk guarantees it to be fresh. Depending on how the milk is handled the milk could last much longer or much shorter.

So how should you handle the milk? Milk needs to be cold. Real cold. We keep our milk on the dairy at 40 degrees F. If you let the milk get warm the bugs become active and start turning your milk to cheese. So here are some tips to help keep the milk cold.

1. Buy the milk last when you are at the store
2. Put it in an insulated container like an ice chest
3. Go strait home, don’t get your car washed or go to a soccer game and leave the milk in the car.
4. Keep the milk on a bottom shelf toward the back of the fridge. It is very convenient to keep it in the door but every time you open the door you are exposing it to warm air.
5. Take the milk out of the fridge, pour it in the bowl, then put it right back in the fridge. Don’t leave it on the table while you eat the cereal.
6. Your nose is the only one who really knows when the milk is bad. I never open a carton of milk without smelling it. If it stinks, it’s no good. If it doesn’t stink it’s good. It’s that simple. Milk is one of the few substances in the world that tells you exactly when it goes bad. If you don’t know what bad milk is supposed to smell like, take some milk, put it in the garage, come back in a couple of days and smell it.

One side note. Back in the old days, many people preferred milk to be soured a little bit. It gives it a more robust flavor. It would be the equivalent of drinking Bud Light (fresh milk) vs Guinness Stout (soured milk). It really is a matter of taste. Next time you smell sour milk take a sip, you might like it.

I promised I would tell you why California milk is better than all the other milk but I have run out of time. Ill tell that story on another day. 25 days still to go.

One last thing. There is a really cool website that will tell you where your milk actually comes from. Every bottle has a plant number on it. This website will show you where that plant is. And don’t forget to buy Real California Milk.

7 thoughts on “Dino’s Daily Dose – Day 5 – Why does my milk go bad before the expiration date?

  1. Thanks for the information. My husband and I recently became empty nesters. The BIG milk drinker left the nest and suddenly our milk purchases became smaller and smaller. I love milk but we weren’t drinking it as quickly. Any who, lately we noticed the milk is not lasting even a few days let alone to the expiration date. In fact I bought a half gallon in a paper carton on Sunday and by Wednesday it was close to curdling. Yuck. I was flabergasted. I have always kept it in the door and never had this problem before. the temp in the fridge is 33. I will take your advice and let you know!

  2. Whoah, I’m having the same problem as Joy down there from 2011… I just went to live at a boarding school and I occasionally buy half-quarter gallons of milk. At home it was always kept in the door and we never had problems. Now I keep it in the door and it curdles within days! Does it have something to do with the amount of milk or something? It’s really annoying, because it’s not like I’m in college so I can’t really just leave to go shopping for milk whenever I want, and I would rather not have to drink it all in a few days to prevent having to waste it…

  3. Carly,

    It could have to do with the temperature of your fridge, warmer than at home, how often the door is opened, and the temperature of the room. Could also be that the milk is older. Maybe the store at home has more turnover, therefore the milk is generally fresher. Here is a trick. The stores alway put the newer milk behind the older stuff. Primarily because they often load the refrigerators from the back, but mostly because they want the older stuff sold first. Next time you go to the store, dig all the way to the back and check the dates compared to the ones in the front.

    As for size. Yes, size could be a factor. For one, the smaller cartons may not be as popular, therefore, less turnover in the store. Second, there is a thing called thermal mass. A larger carton of milk, say a gallon, when exposed to heat will not heat up as fast as a smaller one because there is less milk exposed to the warm air as a percentage. For the same reason it would take longer to freeze a gallon of water than a quart of water.

    Whatever the case, thank you for buying milk. Me & all my dairy friends appreciate you and are glad you are making good nutrition choices. Good luck in school!

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