Mold floating in juice?

If you’re tempted to strain out that little raft and drink the rest, well, we don’t recommend it. That’s because as the mold is doing its thing, it’s dropping who knows what into the juice. Some of the compounds the mold releases might make the juice taste bad; others might not be good for your body, either in the short or long term. For example, some types of mold that grow on fruit, particularly apples, make a toxin called patulin, which can be present even if there’s no obvious sign of mold. Patulin is enough of a concern that the U.S. Food and Drug Administration sets limits on patulin levels in products like apple sauce and apple juice.

Here’s another reason to dispose of the juice: mold also releases spores into the air, which can cause allergic reactions if inhaled by sensitive folks. 

Fungi tolerate acidic conditions better than bacteria

While the acid in juice can help deter most bacteria, fungi don’t mind it much. According to Fungi and Food Spoilage by John I. Pitt and Ailsa D. Hocking, most fungi can grow comfortably between a pH of 3 and 8. Orange juice and apple juices have pHs in the low 3s or very low 4s, according to this table from Kansas State University. 
Meanwhile, yeasts can handle a pH as low as 1.5. For reference, our stomachs usually have a pH of 1.5 to 3.5, according to Harvard Health Publishing. 

Moisture, on the other hand, is essential for mold growth. According to Fungi and Food Spoilage, mold cannot grow in juice concentrates because there’s not enough water available. Mold also needs oxygen, though it can grow in relatively low oxygen conditions. 

Preventing mold growth in juice

Manufacturers use a number of tricks to prevent microbial growth in juice. Heat treatments, like pasteurization, kill off an array of microbes, including those that could cause rapid spoilage or foodborne illnesses.  Manufacturers may also “hot fill” bottles, which means they fill bottles with pasteurization-temperature juice, then seal and invert the container so all inner surfaces have been heat-treated by the juice itself. 
They might also ensure their packaging doesn’t contain any mold-fueling oxygen.  Ingredients like benzoic acid, sorbic acid, sulphur dioxide and acetic acid might be added to prevent microbial growth. Alcohol preserves products like wine. Cold temperatures won’t prevent growth, but can slow it. 

If juice isn’t pasteurized, it will probably succumb to acid-tolerant bacteria before fungi can settle in. If you see mold or anything problematic inside a sealed container or just-opened container of juice, that signals contamination on the manufacturer’s side (Let them know! They may not realize there was an issue.). 

At home, keeping the cap sealed, not drinking straight from the bottle and keeping the juice cold will reduce contamination risks or stall growth of anything problematic. This is generally true for all bottled food products that require refrigeration after opening. Ultimately, you probably won’t see this kind of growth very often. 

Yeast flourishes more easily in juice

As long as we’re talking about fungi in juice, it’s worth mentioning that yeast has an easier time messing with your juice than a fuzzy mold. As a single-celled organism, yeast can disperse in the liquid and can grow without oxygen. Yeasts are also even more tolerant of acidic conditions and especially love sugar. While fuzzy molds (also known as filamentous fungi) can grow in juice, it’s easier for them to operate on a firm surface, in which their filaments can spread out and invade, than to float on a liquid. Yeast, by contrast, can’t penetrate solids so will only operate on a solid food’s surface.

If yeast has taken over your juice, you’ll know something’s up. It might be oddly fizzy or taste off. If the yeast’s fermentation shenanigans go on long enough, they could even cause a miniature explosion and break through the seal of the container. Given how successful yeasts can be in acidic, low-oxygen environments, it’s perhaps not a surprise that they’ve been estimated to be responsible for 75 percent of spoilage in acidified foods.