Ask Mr. Wizard

Handling With Frozen Fruit


Brad Carpenter — Raleigh, North Carolina asks,

Whenever I’ve sought online advice for adding fresh fruit into a secondary, people consistently recommend using frozen fruit. I’ve used pasteurized purees, vodka-infused tinctures, and artificial flavoring, but those methods alter the flavors. I understand lowering the temperature will slow yeast growth. But does it actually kill off the baddies? Hollywood has taught me that freezing an organism, even for thousands of years, and thawing them out will result in disastrous monsters. I’d like to know the scientific answer, is there a chance anything would survive freezing?


Hollywood movies generously apply dramatic license in all matters of the world. But when it comes to portraying the resurrection of frozen life forms, there is not much exaggeration when the organism is something like a bacterial or yeast cell. And most of us kind of know, perhaps without really knowing it, that freezing is not a form of sterilization. Let me give you an example. A pound of ground chicken is popped in the deep freeze and stored for 30 days at -40 °F/-40 °C; are you feeling eager to try some chicken tartare when this package is thawed?

OK, this is not an answer to your question, but you get my point. Freezing food may reduce microbial populations, but it is certainly not a substitute for pasteurization. In fact, freezing is a really great way to preserve cell viability as long as the cells or tissue being frozen are protected from the damaging effects of ice crystals with a cryoprotective compound, like glycerol, that interferes with the formation of ice crystals. So freezing yeast cells can actually be used to prevent cell death.

When it comes to brewing with fruit, today’s homebrewer really has quite the array of options. Many brewers find it hard to resist going au naturale and simply adding clean, damage-free fruit to beer. Fruit additions are usually made after primary fermentation is complete; not sure why this is, but the fact that the concentration of fermentable sugars is low is certainly a help when it comes to minimizing the growth of wild yeast. Spoilage bacteria are still able to grow in beer, but if the fruit is added to a sour beer, the bacteria and yeast on the surface may not be a problem. Clean beers, however, may be contaminated by this process and brewers wanting to use fresh fruit can use sulfite to knock back wild populations.

While going au naturale is attractive to brewers who like the romance of using fresh fruit or who simply want to use locally-grown fruits, many brewers either don’t have access to the right types of locally-grown fruits or don’t want to risk ruining beer with a potential source of spoilage organisms. This is where products like aseptically-processed purees, fruit extracts, and even crystallized fruit juices come in handy. While thermal processing can certainly change the flavor and color of fruit juices, it is also possible, and quite common, to thermally process fruit juices without negatively altering flavor. Single-strength citrus juice is a great example of how gentle pasteurization can be on juice flavor; seriously, few consumers are even aware that these products are pasteurized. Another option is to simply add the fruit at the time of serving. Lots of options, indeed.

There was a time when many US craft brewers were living life on the lunatic fringe and throwing caution to the wind. After several brewers had very costly, barrel-aged beer recalls due to spoilage in the market, a good chunk of craft brewers adopted a more conservative and reasoned approach to packaging beers that potentially could cause problems months after leaving the brewery. Flash-pasteurization prior to bottling is no longer an unusual method, nor is it frowned upon by “cool brewers,” and the polymerase chain reaction (PCR) method of DNA amplification is now routinely used in brewing labs of all sizes as a rapid method to detect the presence of a wide range of beer spoilers before aged beers get anywhere close to being packaged.

Brewing with fruit opens up another chapter of flavor opportunities to an already impossible number of ingredient permutations. The risk of contamination is real but also realistically managed through fruit selection, time of addition, and brewing technique. Thank you for the great question and viva la fruta!

Response by Ashton Lewis.