"But the Jar Looks Clean..." - How Biofilms can spoil your raw milk (and what to do about it)

Dried milk residue in the bottom of a returned Herdshare jar.

If you’re part of our raw milk community, you already know how passionate we are about providing the freshest, cleanest, most delicious milk straight from our cows to your family. But today, we want to talk about something you can’t see—biofilms—and how they can quietly affect your milk if we’re not all careful with our jars.

Let’s Start with Something Relatable

If you’re a parent, you’ve probably lived this moment:

You find a long-lost sippy cup under the car seat. You know the one—your stomach turns the moment you pick it up. That cup used to have milk in it, but now it’s a science experiment. You open it, gag a little (or a lot), and debate whether it’s worth even trying to clean it. Because the truth is: no matter how many times you wash it, that smell sticks around.

Now imagine that happening, invisibly, inside one of your returned milk jars. This is where biofilms come into play.

What Is a Biofilm, Anyway?

Biofilms are microscopic communities of bacteria that stick to surfaces—especially where there’s moisture and organic material, like leftover milk. They form a thin, slimy layer that can’t be seen by the naked eye, but they’re tough. Once a biofilm starts to form, it can be incredibly resistant to standard cleaning—even more so if the jar wasn’t rinsed right after use.

Here’s the kicker:

Even if you rinse your jar and it looks clean, if it hasn’t been scrubbed properly with hot water and soap, those biofilms can still be present.

And in a raw milk setting, that’s a big deal.

Why Biofilms Matter So Much for Raw Milk

Unlike pasteurized milk, raw milk is a living food. It’s full of beneficial bacteria and enzymes that are great for your gut and immune system—but it also means it’s more sensitive to contamination from outside sources.

When milk is poured into a jar that hasn’t been fully cleaned or has hidden odors or bacteria, it can:

    •    Spoil more quickly

    •    Develop off-flavors or odd smells

    •    Have its beneficial bacteria outcompeted by unwanted microbes

Even with our rigorous cleaning and sanitizing process, if the starting jar wasn’t truly clean, it can interfere with the milk’s quality.

Why the State Requires Single-Use Bottles for Licensed Raw Dairies

You might be surprised to learn that biofilms are exactly why the Department of Agriculture requires all licensed raw milk to be bottled in single-use containers. It’s not just red tape—it’s because reused containers, no matter how carefully cleaned, can harbor these persistent bacteria.

So as we move toward full licensing (which we’re excited about!), we’ll be retiring our current returnable glass jar system and switching to state-approved, tamper-evident, single-use bottles.

We know many of you love the reusable jars—and we do too! That’s why we’ll be offering a variety of beautiful, practical reusable containers for sale in our farm store. These can be used for milk storage at home, kefir-making, cream separation, and all your homestead-style milk needs.

And most importantly: we’re putting every step in place to ensure the milk quality you’re receiving now remains exactly the same—fresh, rich, and handled with care from cow to bottle.

The Smell Factor: Why “Milk Only” Matters

Glass is wonderful for milk. It’s non-toxic, non-reactive, and easy to sanitize. But here’s the downside: glass holds on to strong smells and residues. If you’ve ever stored spaghetti sauce, kombucha, or soup in a glass jar, you know the lid and jar can smell like it forever.

When a jar that once held onion soup is reused for raw milk—even after a wash—those smells and oils can leach into the milk. It won’t make the milk harmful, but it will absolutely affect the taste and cause it to “go off” sooner than expected.

That’s why we kindly ask: please only use our milk jars for milk.

So What Can You Do? (It’s Simple!)

Here’s how you can help us maintain the highest standards of safety and freshness during this final stretch before we’re licensed:

    1.    Rinse your jar immediately after you pour the last of the milk.

    2.    Wash it thoroughly with hot, soapy water and a brush—especially around the rim and shoulder of the jar.

    3.    Let it air dry completely before putting the lid back on.

    4.    Never use the jar for anything else—milk only, please!

Milk build up under the lid of a returned herdshare jar.

At Der Dairy, we’re committed to providing raw milk that’s not only nourishing and delicious but also handled with the highest care from cow to customer. Thank you for being part of this journey and for working with us through the changes as we prepare to go fully licensed.

Every clean jar returned, every thoughtful question, and every bit of support helps us continue doing what we love—and doing it better.

With gratitude,

Brittany Kanagy

Der Dairy at Boundary Creek Farm

“From our farm to your fridge—pure, raw, real.”