Whenever I empty a carton of a drink such as milk or orange juice, I replace the cap on the container and leave it aside ready to throw out. Invariably, when I return the carton seems to be under some pressure as the sides are bulging and stiff. Why does this occur? This does not seem to happen with cartons that have held carbonated drinks.
Two possibilities exist for this effect 鈥� Ed
鈥� When you finish the last of the milk or orange juice and put the top back on, most of the air in the bottle is still chilled from being in the fridge. As the air reaches room temperature it expands, swelling the container. Try leaving the empty container with no top on until it reaches room temperature and then put the lid on you鈥檒l find the container won鈥檛 swell.
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You can produce the reverse effect by washing out the empty milk container in hot water then immediately putting the lid on. The container will now suck in its sides as the air inside it cools.
Carbonated drinks containers, however, are designed to hold their contents under pressure. If you have been sprayed by a fizzy drink you will be able to vouch for the pressure such containers can withstand. When the cool air expands to room temperature in an empty carbonated drink bottle the pressure on the more rigid structure is well within its design constraints and it does not visibly swell.
Mark Edwards, Chapel Hill, Queensland, Australia
鈥� Fruit juice and milk cartons bulge through accumulation of fermentation gases from a range of micro-organisms. These are thriving on the nutritional soup left behind in the cartons. Long-life drinks are usually pasteurised during processing by heating them sufficiently to kill most spoilage organisms. As soon as you open the container, however, bacteria, yeasts and microbial spores will contaminate your product.
If you keep the carton in the fridge, the growth of these organisms will be slow. However, once you leave an empty carton at room temperature the organisms multiply quickly on the film of juice or milk left inside. They will generate enough carbon dioxide to create a discernible positive pressure, especially in warm weather.
Fermentation is much less likely in carbonated drinks because microbial inhibitors such as sodium benzoate or potassium sorbate are often added. Furthermore, the concentration of nutrients is often low, especially in sugar-free drinks, as these products contain little or no real fruit juice.
The organisms you might find in your fruit juice cartons are typically types of yeast, such as species of Saccharomyces or Candida which thrive in the high acidity environment of fruit juices.
Milk is a great food for most micro-organisms and many types of bacteria can proliferate in it. Even if milk is not contaminated from the outside, fermentation may still occur thanks to the growth of bacilli whose spores are resistant to the temperatures used in milk pasteurisation.
Julia Aked, Bedford, UK