Last night I made these chocolate chip peanut butter bars from Chocolate Covered Katie for my stepson. Then I thought, why not also whip up some chocolate ganache to spread over the top, like frosting? Because, chocolate.

I used the ganache recipe I’ve always known and loved: equal parts heavy cream and chopped-up 70% dark chocolate (I used half a cup of each). Heat the cream until it starts to steam, pour over the chopped-up chocolate, DON’T STIR, cover and wait for 5 minutes. Then uncover, whisk up a storm, and voila — beautiful chocolate sauce that will set up to a super-thick frosting consistency after 30 minutes in the fridge.

Let’s just say my stepson was very, very psyched about dessert last night.

I love ganache. I’ve been making it a lot recently. It’s a great way to use up the leftover chocolate I bought for my tempering experiments. Mostly I end up stirring all this ganache into whipped cream (I have a half-pint whipped cream dispenser, which changed my life) to make lazy-woman’s chocolate mousse. But lately I’ve been thinking about using ganache as the filling for my first attempt at dipped chocolate truffles.

I know the ganache used to fill truffles is thicker than the stuff I make for frosting desserts (truffles are typically filled with a 2:1 chocolate-to-cream ratio ganache). But I’m really confused about one thing. Even a thick ganache still has a good amount of cream in it. Cream needs to be refrigerated. Why don’t truffles filled with ganache need to be refrigerated too?

I did a little digging and found some very helpful information on the Paul Bradford Sugarcraft School website, which I’ve summarized below. But please check out the school’s website for a more thorough explanation.

Why does ganache spoil?

Ganache goes bad because moisture in the cream promotes microbial growth. A typical ganache lasts about two weeks in the fridge, or two days on the counter. However, while all ganaches contain some water from the cream, most of that water is chemically unavailable because it’s bound up with other ingredients.  It’s the amount of unbound water in a ganache (also known as its water activity or available water) that has the biggest effect on the rate of microbial growth. Measuring water activity is how scientists predict the expected shelf life of food products.

As a side note, cocoa mass is actually ANTI-microbial, so pure chocolate (even without preservatives) has a long shelf life — most of the bars I’ve bought recently have an expiration date 1.5 years from their production date.

How can I make my ganache last longer?

First, for obvious reasons, your ganache will stay fresh longer if you scald the cream before pouring it over the chocolate, since the heat will kill a lot of microbes. Scalding cream repeatedly also helps condense the cream somewhat by evaporating a small amount of the water. Less water = lower water activity = less microbial growth.

I’ve never been lucky enough to make ganache in the UK, but they have something there called double cream, which contains 48% dairy fat — it must do wonders for the shelf life of British ganache. In the US, the heavy cream I find at most supermarkets only contains about 36% dairy fat, so evaporating some of that extra water by repeatedly scalding the cream would likely produce longer-lasting ganache. Though, I wonder if scalding the cream affects its taste? If anyone knows, please share.

All that aside, it’s actually the type of sugar that has the biggest impact on the shelf life of ganache-filled truffles. When professional chocolatiers make ganache, they typically add invert sugar (or even honey, a natural invert sugar) to hot cream at a ratio of 5-8 grams of invert sugar to 100 grams of cream. Invert sugar binds with water, dramatically reducing water activity.

Long story short: a ganache-filled chocolate truffle made with invert sugar should be shelf-stable at room temperature for 6-8 weeks — which is why, to go back to my original question, professionally made ganache-filled chocolate truffles can sit out on the counter for weeks without going bad.

What if I don’t like the idea of cooking with an ingredient my grandma wouldn’t recognize?

I hear you. I’m kind of bummed about the idea of putting chemically altered sugar in my truffles too. It just sounds so…. processed. But when all is said and done, invert sugar really isn’t so bad. You can actually make your own by adding cream of tartar or citric acid to sugar and water and reducing them into a simple syrup.

Plus, your grandma probably ate plenty of invert sugar in her time. It’s ubiquitous in candy.

In any event, I’m ordering some invert sugar on Amazon this week for use in my continuing ganache experiments. I’ll keep you posted.