Shan Jordan, of Wellington, asks :-

Recently we had an abundance of lemons and decided to juice them. We stored the juice and pulp in the fridge in a plastic milk bottle and a glass bottle. After a few days you could see the plastic bottle, which was only half full, starting to swell and when it was opened it gave a loud fizzing sound and juice spilled out. The lemon in the glass bottle, which was filled to the top, also fizzed upon opening but wasn't as reactive.

We’d be interested to know what makes lemon juice fizz? And why more so when stored in plastic?

Steve Flint, a microbiologist at Massey University's Institute of Food Science and Technology, responded.

There is yeast on the skins of most fruit and they grow very well in juice and produce gas. It will be most noticeable in plastic bottles as they will swell. Plastic bottles are more difficult to clean than glass bottles so this could also be an explanation for the observation that the problem was more obvious in the plastic bottle.

Another consideration is that with the large headspace in the plastic bottle, there is more air to allow the yeast to grow compared with a full glass bottle.

The process by which yeast produce gas is known as fermentation. This results from the use of sugar to provide the energy that the yeast need to grow and reproduce. Fermentation makes use of a process called glycolysis that eventually results in the production of carbon dioxide as a by-product. This is what makes the “fiz” that we see in a naturally fermented product like ginger beer, or in this case, lemon juice.

Another product that makes use of yeast and produces gas as an important component of the product, is bread. The gas in bread is produced by the activity of yeast that provides us with the familiar texture of bread.

Yeast is responsible for the production of alcohol, although not all yeast can tolerate large amounts of alcohol. Specific yeast are used for the production of wine and beer – two of the oldest beverages known to mankind.

Traditionally, yeast on the skins of the grapes provided the source of yeast to ferment the grape juice and produce wine. Wine can have a fiz if it is allowed to carry out some of the fermentation in a bottle, as in the case of Champaign. For other wines, the gas is released from the fermenting juice and the wind is bottled at the end of the fermentation process when no more gas is produced.