Meal prepping is a great way to have meals and snacks readily available, and smoothies make for a great healthy snack alternative to many other options. But if you’ve ever tried make-ahead smoothies before, you might’ve noticed they go bad pretty quickly.

Luckily there’s a fairly simple solution to this problem.

Oxidation is what causes fruit and fruit juices to turn brown, go bad, and lose some of their nutrients. It’s caused by exposure to air (oxygen specifically).

This means that keeping your smoothie out of the open air is key. The less contact it has with air and oxygen, the longer it’ll last.

A glass food or drink storage container (or two, or three, or however many you need depending on the size of your batch) with an airtight lid is key. But the container by itself isn’t enough even if it’s airtight.

You’ll have to ensure that the smoothie fills up the entire jar, bottle, or container, right to the brim. Any space between the lid and smoothie means there’s still some air trapped in the container– and even a little bit is enough to cause some oxidation.

Adding a bit of lemon juice to your smoothie will help it to last longer, too, if you don’t mind the taste. Its vitamin C will help to ward off oxidation as well.

If you own a juicer to make your own juices at home, these tips will also help your juice to last longer since fruit juice can suffer from oxidation as well.