I’ll start this post with some numbers you might find suprising.
- A shrink wrapped cucumber will last about four times as long as a loose one.
- Bananas in a perforated plastic bag will last twice as long as a loose bunch.
- Pears will keep 14 days longer in a plastic bag in the fridge as they will do loose at ambient temperature.
- Shrink wrapped broccoli lasts twice as long as the unwrapped product.
Yet despite this, we hear people complaining that fruit and veg is over packaged, and we have environmentalists urging us to leave the plastic at the checkout. You can now buy reusable netting bags to take to the supermarket instead of using plastic ones. But this idea that packaging is a bad thing is worth some exploration.
The two most important functions of packaging are to prevent physical damage to the product, and to slow the rate of biological decay. Physical damage can occur at any point in the grower-supplier-retailer-consumer chain, and results in unsaleable produce. Most of this is hidden from view, but we see some evidence of it on the supermarket shelves containing loose fruit and veg. Almost inevitably, there’s a battered looking product at the bottom of the box. Nobody is going to buy this, and it ends up as food waste. The role of packaging in slowing the rate of biological decay is a bit more complicated. Fruit and veg are living plants, and biological activity continues after the product has been harvested. The main process that continues post-harvest is respiration; the fruit/veg will take up oxygen from the surrounding environment, break down some of the sugars they’re made up of, and emit heat, water and carbon dioxide.
Respiration and transpiration – it’s why packaging is necessary
Harvested fruit and veg respire at very different rates (see the table below). Broadly speaking, fruit and veg with low rates of respiration tend to store well. Temperature also has an influence, and since respiration itself generates heat, products with high respiration rate are best stored at cooler temperatures, and/or in environments with low oxygen levels. Suitable packaging and storage will often result in double or triple the shelf-life, and therefore reduce food wastage.
Closely associated with respiration is water loss. Fruit and veg are between 65% and 95% water. Whilst the plant is growing, the water lost by transpiration is replaced by water entering the plant from its roots. Once water has transpired through the plant skin, it evaporates and is then carried away into the wider surrounding air. After the plant is picked, it has no way of replacing this water, hence the wilting/shrivelling. Fruit and veg packaging is designed to limit this process. But there is a balance to strike; if we kept the plant in a completely sealed container, the transpired water would condense, and form ideal conditions for mould growth. In contrast, if we unwrap the fruit/veg and leave it in an airy environment, it will lose water very quickly and shrivel. Where to strike the balance depends on the characteristics of the product; the rate of transpiration varies a lot between different fruits and vegetables (see table).
Transpiration and respiration are often closely linked and are considered in combination when packaging is designed. For many products, the sweet spot is packaging with breather holes in it, which allow some moisture to dissipate, but not so much that the product wilts, and also changes the air composition in the vicinity of the product. This type of packaging is used for the majority of soft fruit (e.g. strawberries, grapes, plums), it’s also good for other fruits (e.g. bananas, apples), and a lot of vegetables (e.g. courgettes, carrots, potatoes). Leafy veg is more challenging to package correctly; it has a high surface area from which it can lose water, but this surface area is also liable to condensation and therefore mould growth if the humidity is too high. Packages for these often allow more air circulation, in combination with storage in the fridge in order to limit respiration rate.
Keeping it cool
A report commissioned by WRAP looking at how best to help consumers reduce food waste is an interesting read if you want to marvel at our collective ignorance on fresh furit and veg storage. Retailers recommended instructions for 63 different fruit and veg were that 56 of them were best stored in the fridge. Exceptions were bulky items like potatoes and onions, and products that don’t ripen at cool temperatures or suffer from cold induced injury, such as bananas, mangoes and pineapples. However because the air in refrigerators is very dry, most products will last at least twice as long if left in their original packaging as well as putting them in the fridge.
Plant hormones and ripening
Just like us, plants have hormones. The main one affecting plant ripening is ethylene. It’s produced by all plants to a greater or lesser extent, but in certain fruits, ripening is associated with a big increase in ethylene production. These fruits include apples, bananas, melons, nectarines, pears, plums and tomatoes. Food producers use this to their advantage; these fruits can be picked before they’re ripe, stored in controlled environments, and then ripened using ethylene. We can take advantage of this effect ourselves; a ripe fruit from one type placed in a paper bag with unripe fruit of any other type above will assist ripening. Similarly, removing any ripe fruit from a bag will slow down the ripening of the others (this is really useful if you are a gardener with an annual tomato glut, followed by a long and frustrating harvest of green tomatoes).
Some fruit and veg packaging is sealed and doesn’t contain normal air; its been modified in some way in order to slow decay. This is particularly common for ready to eat bagged salads. The air inside is low oxygen, slightly higher carbon dioxide with the bulk being nitrogen. This slows respiration right down, so stops the leaves going brown, and also prevents bacterial activity.
The bags are slightly inflated to cushion the contents and prevent damage. Despite these measures, the Waste Resource Action Programme (WRAP) estimate that around 40% of bagged salad is thrown away by consumers without being eaten. It’s as
If in doubt, read the label
It’s ok to be confused by all of this; the fruit and veg we eat differ so much in biological terms that there is no single best way of packaging and storing them. But the producer/processor has worked out the best packaging method for you; all you have to do in most cases is to leave it in the packaging and put it in the fridge. Storage information is also usually on the label and there is also a comprehensive and easy to navigate source of information here. Key points to remember:
- If the product comes in packaging with breather holes, leave it in the package; the package is limiting the rate of transpiration and it should be at a happy equilibrium.
- If the product comes shrink wrapped in film (e.g. cucumber, broccoli), leave it in there. This type of packaging is used on products that lose water very quickly.
- If the packaging was sealed to start with and it’s possible to reseal it once you’ve taken some of the product out, do it.
- For root vegetables bought with the leaves still on (e.g. carrots, beetroot), remove the leaves. These provide huge surface areas for transpiration and will suck moisture out of the vegetable really quickly.
- Most fruit and veg will do better in the fridge, particularly those with high respiration rates.
- If there’s condensation on the inside of fruit/veg packaging, unwrap it to prevent mould forming. Don’t wash stuff before you store it; adding moisture will increase the likelihood of decay.
- Remove ripe and over-ripe fruit from the package if you want to slow the ripening of the remainder.
Fast and loose?
Some people prefer buying fruit and veg loose, which makes sense if you want a small quantity. If you want to minimise spoilage, when you get it home then you can mimic the package it would have come in (e.g. perforated bag, shrink wrapped film). However, buying loose because of concerns about packaging waste is misunderstanding the point of packaging and will increase the rate at which your food goes off. When I took a walk round the Co-op last week, the only packaging I found in the fruit and veg aisle that didn’t look like it was peforming a useful physical/biological protective function was the netting bags around citrus fruits. Ironically, it is precisely this type of bag that you can now buy to reduce your packaging waste.
To be continued…
This post is getting on the long side, so I’ve decided to split it. Coming next time:
- The relative importance of food waste and packaging waste
- Where and why food is wasted.
- The future of packaging design.