I didn’t know it was a ‘thing’ until then! Luckily the food was fine, but the process of freezing and defrosting must have weakened the seal, and the jar broke. I found this out when the base of a jar I’d just used to freeze something fell out as I was washing it up. (Breadcrumbs yes, frozen chickpeas yes, sauces or stock, no.) If you can’t, choose to use only with low water content foods. This join is another point of stress weakness, and it’s better to avoid these jars if you can. Sometimes you’ll see jars that have a distinctive round circle at the bottom, which is a separate piece of glass fused to the jar. Jars from the reject shop won’t, and are often thinner glass. Jars that previously contained jam, pickles or sauce will have been heat-treated, and are my preference. I always choose jars that would have been through some kind of heating process in a factory. For beans, lentils or chopped vegetables, it’s less important, but still preferable. I can also stack them in my freezer, which maximises the space.įor liquids, a wide neck is very important. They are also free (I rescue from a local cafe, and neighbours who eat jam.) Each jar fits a one-portion serve, and the tapered sides mean the frozen contents almost slip straight out when I’m ready to heat them up. My absolute favourite is the Bonne Maman jam jar. I prefer wide-necked jars for freezing, and ideally those that taper outwards. How to freeze food in glass jars: choosing suitable jars Leaving a lid ajar until the contents are frozen will prevent this.įoods like chickpeas or frozen sweetcorn have spaces between them, so there is less pressure applied to the sides of the glass (the air gaps can be filled) meaning narrower jars can be used for these types of food. This is why wide-necked glass jars, or those with sides that taper outwards, are better for freezing, especially when freezing liquids.įilling a wide-necked jar to the brim and then screwing a lid on tightly may also cause a jar to crack, because now there is no space at the top for the food to expand. If a jar is narrow, or has sides that taper inwards, the extra pressure placed on the glass sides as the contents freeze will make it crack (which relieves the stress). The sides of a glass jar are rigid and so the only way to expand is up. Stock, soup and sauces are all liquid, and so will expand more when frozen than foods like bread or wraps, that have a low water content.Īs liquids freeze and expand, they need a place to go. (You might have noticed when you make ice cubes that the level of the cubes rises as they freeze.) Food contains varying amounts of water and so different foods expand to different amounts when frozen. When the outside and the inside of the glass have different temperatures (such as putting a glass jar of frozen food into a bowl of hot water, or putting hot liquid into a cold glass) the glass is shrinking on one side and expanding on the other, which creates stress. Nothing that you can see, but it happens. It can’t absorb the impact, so it breaks.)Īlthough it’s rigid, glass actually shrinks a little when it’s cooled and expands a little when heated. (Like, for example, when you drop glass on a hard surface. It’s rigid and solid, so when pressure is applied it tends to crack. Understanding the science of freezing in glassįirst, a science lesson! It’s helpful to understand what’s going on.
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