There are so many reasons we love shopping at Costco—the incredible prices! The free samples! The butcher-quality meats! But one that always draws us back to the dairy aisle, in particular, is that the milk seems to stay fresh for days past its sell-by date. And it turns out we aren’t the only ones who have noticed this.
“I open milk pretty much the same day I buy it. And Costco milk lasts significantly longer,” writes one user on the r/Costco subreddit. “I prefer Costco milk because it does last longer than anything else,” writes another.
So what gives? What kind of sorcery is Costco engaging in that’s making its milk last longer than milk you’d get literally anywhere else? Well, it might not be actual sorcery, but close enough! Read on to find out what’s behind the magic of Costco’s milk.
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Why does Costco milk tend to stay fresh past its sell-by date?
There are multiple factors that may contribute to this sell-by date phenomenon, starting with what’s going on at the dairy supplier and ending with the milk’s journey to your local Costco store. Here are the biggest ones.
Costco conducts surprise annual audits
This keeps those dairy suppliers on their toes! All Costco dairy suppliers are audited annually (while those located in China are audited twice a year)—and the audits must be unannounced, according to the Food Safety & Quality Audit Expectations for Costco Suppliers. “Surprise audits are a standard practice in the dairy industry, albeit the frequency can vary by company,” says Bryan Quoc Le, PhD, a food scientist and food-industry consultant. “Costco’s experience of annual surprise audits helps ensure compliance with quality and safety standards, which can contribute to longer-lasting freshness by reducing contamination risks and spoilage.”
To pass an audit, Costco suppliers must score over 85% on food-safety standards without a single critical finding. (These are things like visible contamination, falsification of records and non-potable water—the really bad stuff.) A failing grade or one or more critical findings would trigger a re-audit, and that re-audit would have to happen within 60 days and also be unannounced. All this makes for a vigorous audit program that helps ensure freshness.
Costco rigorously tests every batch of milk
Costco’s milk is a Kirkland Signature item, and it’s one of the categories of products that Costco places on their “high-risk” list. While this doesn’t mean that it’s more susceptible to contamination and other biological hazards, it does mean that the brand tests this milk more thoroughly than some other products to ensure optimal quality. After all, it is their private-label brand. As a result, it’s required to undergo extensive microbiological testing, with a reported 60 tests run on every batch of milk. If you’re thinking that sounds like a lot, that’s because it is!
“Running extensive microbial tests is common among high-standard dairy producers, [though] 60 tests per batch is above average. Typical dairy producers usually run a few tests, such as total bacterial count, coliforms, somatic cell count and pathogen detection,” Le explains. “The rigorous testing ensures the milk is free from harmful bacteria and contaminants, which helps the milk stay fresh longer.”
Costco also tests the water used in milk processing
“Annual water testing for E. coli is standard in dairy processing,” says Le. However, Costco requires all water used in its facilities—whether it’s being used for processing or sanitation purposes—to be tested for E. coli by a certified third-party laboratory.
“Testing ensures that the water used doesn’t introduce contaminants,” Le explains. “The absence of the pathogenic bacteria helps maintain the milk’s purity, which preserves its freshness closer to its original state.” In other words, Costco isn’t just testing the milk itself—it’s testing all the water that may come into contact with the milk too.
Costco buys milk in bulk
Even the biggest supermarket chains are likely not buying as much milk at a time as Costco, which could contribute to it being fresher when it arrives on store shelves. “Because Costco buys milk in bulk, these products may be shipped directly and more frequently from producers, which reduces time spent in distribution,” explains Le. “The faster logistics can help preserve freshness of the product by shortening the time between production and sale.”
According to the American Dairy Association, milk typically travels from the farm to store shelves in about 48 hours. But if Costco is buying an entire truckload at a time, it may be even faster than this, meaning the milk is that much fresher when you bring it home.
How else can you make milk last longer?
Even if you occasionally need to pick up a gallon of 2% at your local grocery store, the following tips can help keep your milk fresh until you’ve had the last drop:
- Keep it covered and sealed as much as possible to minimize contaminants.
- Avoid leaving it out at room temperature to reduce the chances of spoiling.
- Don’t drink directly from the container or touch the container with spoons or cups. This will prevent bacterial contamination.
- Store it in the back of the fridge to keep it at a consistently cold temperature.
Got milk? If you’re shopping at Costco, we’ll assume that’s a yes.
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Sources:
- r/Costco subreddit: “Is it just me or does Costco milk last longer past the expiration date than other milk brands?”
- Bryan Quoc Le, PhD, food scientist and food-industry consultant; email interview, July 2025
- Costco Wholesale: “Food Safety & Quality Audit Expectations for Costco Suppliers”
- American Dairy Association: “How Does Milk Get to Stores? Exploring Milk’s Farm to Table Journey”
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