Why does fresh food go off?

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.

owlpp
Reusable netting bags. Why not just reuse plastic ones?

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.

respiration rate
Frut and veg respire at very different rates. In this table, the examples in the ‘medium’ category respire twice as fast as those in the low category. those in the ‘high’ category respire about four times as fast. Data from Handbook of Food Preservation (Rahman, 2007).

 

 

 

 

 

 

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
Rates at which different fruit and veg lose water. A brussel sprout loses water around 240 times as fast as the potato.  Data from Handbook of Food Preservation (Rahman, 2007).

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.

useful packaging
The shelf life of all of these products is signficantly improved by packaging. Bananas will keep twice as long in a perforated plastic bag as they will as a loose bunch. Pears will last around 14 days longer in a bag in the fridge than loose at ambient temperature. Broccoli respires very quickly and it’s shelf life is doubled by shrink wrapping. Mushrooms are best in sealed punnets for the same reason. Data sources: bananas, pears, broccoli, mushrooms.

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).

Modified atmospheres

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.

LIDL - ITALIAN STYLE SALAD -Supermarket salad bags for Amanda Cable.
Ready to eat salad, packaged in a protective atmosphere to keep it fresh. As soon as the bag is opened, it goes off very quickly.

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.

cucumber
A shrink wrapped cucumber will last around 4 times as long as a loose one. Data source.

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.

 

9 thoughts on “Why does fresh food go off?”

  1. Thank you for this informative article. It’s interesting to read something from a different perspective.

    Have you considered the following:
    1. The damage to the environment caused by the disposal of this packaging and how that compares to the damage caused by food waste?
    2. That the effect of most of the packaging is negated by the opening of the packaging to use the product so the benefits of longer term storage to the householder are negated
    3. I’m confused about the potatoe facts. From my own experiments at home potatoes (main crop not new) last longest stored dry in a dark sack oreferably still with soil on not in the fridge, they can last a couple of months this way, was this included in the analysis?

    I do think the need for this packaging is symptomatic of the long food chains we have and shortening these by changing the way people shop should reduce carbon in many ways.

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    1. Hiya. Thanks for your comments.
      (1) is the subject of the next blog post.
      (2) you’re right, it depends on the function of the packaging. So for the nitrogen puffed ready to eat salad, the benefit is negated as soon as you open it. For a lot of the perforated bags, I suspect most of the benefit is maintained if you don’t make an enormous hole in the bag. For the shrink wrapped stuff, I think rewrapping it in the same packaging is probably better than leaving it open. You could do a nice experiment at home on this.
      (3) You’re right. Dry, dark, cool is good. Presumably the soil on the potatoes drys out fairly quickly? The link to the study that included stuff on how to store potatoes is: http://www.wrap.org.uk/sites/files/wrap/WRAP%20RTL044-001%20Final%20report.pdf
      Potatoes are one of the few fruit/veg that aren’t likely to be better in the fridge. Which is lucky given how bulky they are.

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  2. Hi Judith, thanks for the information, but as a food producer it does puzzle me as to why people think it’s a good thing to eat fruit and veg that are weeks old? No amount of packaging can prevent the loss of taste and crispness that truly fresh produce has. Like the person commenting above already mentioned; plastic packaging mainly serves the needs of our CO2 heavy food systems, to allow for it to be transported, stored and handled. What we need is a re-localisation of our food production, with the amount of time between harvesting and eating reduced to a couple of days if not mere hours and both our health and that of the planet will vastly benefit from that.

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    1. Completely agree. Short food supply chains would be ideal. Until we have them, we need to do what we can to stop food we grow from going to waste. Packaging and correct storage is part of the solution to food waste, but I would hope that people understand that not buying food you’re not going to eat is much more important. I agree that packaging and storage enables a longer supply chain. This is done in the name of ‘consumer choice’ which is apparently a good thing?!

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  3. Thank you for this post, which is so clear and useful. I don’t know much, so I wondered what you thought of this as a defense of the “anti-packaging” position: Most food waste happens in the home [1], so perhaps it is possible to reconfigure people’s food habits so they both use less packaging and waste less food? Could suppliers change how they preserve food, so that it used less plastic (ie not individually wrapped), getting food unspoilt to retailers, and then consumers take it home without the wrapping. Yes, more risk of spoilage at home (as you so convincingly show in this post), but less plastic used overall, but the increased risk of spoilage is countered by changes in other aspects of people’s behaviour, not just their choice to not have plastic packaging?

    [1] I learned this in your other post! https://lowcarbonbuildings.wordpress.com/2018/03/15/in-praise-of-plastic/

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    1. Apologies Tom, I completely missed this comment but have now approved it.
      Suppliers definitely have a role in all of this; it is in their interests to sell you more food, so the enticing special offers that make you buy more food than you need are problematic. A lot of suppliers are doing what you suggest already; many loose foods (bananas for example) are in protective atmospheres whilst in storage or in transit, and are only actually loose when they get to the store.
      To me the critical change in behaviour would be for us to get away from this idea that we should be able to eat whatever we like, whatever the season. This is a major driver of the need for long supply chains that involve packaging/protective atmospheres.

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