Spoilage of orange juice – a lesson in microbiology

In our home we don’t drink a lot of juices or soft drinks, it’s mainly water, tea, coffee and maybe some lemonade. Nevertheless, when we have visitors we tend to stack up on some juice, often orange juice as well. Of course, the drinks never completely finish and they end up standing in our fridge for quite some time, before either being drunk or being poured down the sink because we’ve forgotten about them.

When throwing out the juice I’ve had the experience of seeing some blue/white flakes when pouring the juice down the drain. It had spoiled and micro-organisms had taken their chance and had thrived on the juice.

Today we’re discussing this phenomenon, why do moulds and yeasts grow on orange juice and what else is there in the world of micro organisms?

Fresh juice is full of micro-organisms

Fresh fruits will of course contain micro-organisms on the surface of the peel. Micro-organisms are everywhere in this world and grow wherever they can. They sit on fruits and vegetables as well. However, the inside of fruits and vegetables is generally pretty sterile. Micro-organisms will only come in when a peel or skin has been damaged or when the fruit or vegetable is cut. Bacteria, moulds and yeasts are everywhere and will set on the new surfaces.

When peeling or squeezing an orange we break up the cellular structures, releasing a lot of delicious juices, rich of sugars. These sugars are the ideal food for a lot of bacteria, yeasts and moulds. Fortunately though, the pH of orange juice is only 3-4. A lot of bacteria cannot grow anymore at this low pH, despite the presence of sufficient food. However, most of them will not die and may linger in the juice.

Moulds & yeasts love orange juice

Moulds and yeasts are another story, a lot of these micro-organisms have absolutely no problem with a pH of 3-4 and will happily continue growing in orange juice. Happily ‘eating’ (converting) all the sugars into energy.

Of course, researchers have looked into this and have found a wide range of moulds and yeasts inside freshly made orange juice. It’s an interesting composition.

Fighting microbial spoilage of orange juice

Luckily for juice producers, microbial spoilage of orange juice can be prevented pretty well, thanks to the use of several techniques, one of them being pasteurization. It’s a relatively simple technique, heating the orange juice to a pre-specified temperature for a minimum amount of time. This will kill off unwanted micro-organisms. Of course, pasteurization of a juice is only useful if it is then packed in a way that it cannot be infected anymore. I’ll come back to that in another post, focussing all on the packaging of orange juice.

Pasteurization, unlike sterilization, doesn’t kill off all the micro-organisms in orange juice. Especially the so called spores will survive. These spores are very resistant to heat treatment. However, these generally do not grow at the pH of orange juice, they don’t appreciate the acidic environment. Therefore, pasteurization is good enough for stabilization of orange juice for a couple of months.

Fighting micro-organisms is nevertheless often a matter of setting up several barriers to prevent growth and infection of micro-organisms. Therefore, another commonly used trick is that the orange juice is de-aerated, which will take out the oxygen of the juice. A lot of the moulds and yeasts that could grow in the juice if they would still be there need oxygen to grow. So by taking out all the air (and thus oxygen), it’s even harder for micro-organisms to thrive!

Other hurdle that can be pulled up, is the addition of preservaties. However, this use of preservatives isn’t always accepted by consumers and we will not focus on this any further in this post.

Opening a pack of orange juice

A closed pack of orange juice will prevent micro-organisms from re-infecting the juice, however, as soon as you open it moulds, yeasts and bacteria can come in again. This is why orange juice has to be stored in the fridge again once you’ve opened it. You’d want the micro-organisms that have come in to grow as slowly as possible.

Once the bacteria, moulds and yeasts which don’t mind the pH of orange juice have come in they will start growing again, possibly forming this layer of moulds on top of your orange juice, once you’ve forgotten it in your fridge…

Sources

Want to know some more in-depth details? Have a lot at these sites: Indian evaluation of fresh orange juices, which yeasts sit in orange juice?, microbiology of juices and soft drinks.

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