In the last step, you learnt why food processing occurs. Now you’ll examine some of the challenges facing crops post-harvest which food processing aims to overcome.
Once harvested, fruit and vegetables need to stay at a certain level of quality before they can be sold and eaten. However, these products are highly perishable because of biological processes that happen after harvesting. These processes lead to cell ageing and ultimately, cell death (scientifically referred to as senescence). We can’t prevent senescence, but we can postpone it through post-harvest processing.
Some characteristics of plant-based food, such as colour, texture and flavour, influence whether we choose to buy the product or not. Changes in these qualities happen naturally after harvesting, but external factors such as temperature, light and humidity can speed up the ripening process.
Some of the changes plant-based products go through include:
- Respiration. This is the breakdown of sugars and other cell components of plant-based food into simpler molecules with the release of energy. Fruits with higher respiration rates like bananas and avocados ripen rapidly, reducing their shelf life.
- Ripening. This is caused by several hormones and other compounds. Ethylene is a hormone that plays a key role in ripening and senescence in fruits and vegetables and controls many changes in colour, texture and flavour. Post-harvest control of ethylene is important to extend shelf life and maintain quality.
- Colour. Would you buy green strawberries or a brownish lettuce? The colour of fruits and vegetables is mainly determined by three families of pigments: carotenoids, anthocyanins and chlorophylls. The amount of these pigments is affected by factors like light and temperature. Browning is caused by enzymatic reactions that mainly occur after cutting or slicing. Loss of green colour, yellowing or discolouration are mainly due to chlorophyll degradation.
- Texture. Hardness, firmness, juiciness and crispness are determined by the structure of cell walls and membranes, the amount of fibrous components, and the amount of water in the cells. Most fruits soften during ripening due to degradation of cell membranes and starch breakdown. Vegetables are less susceptible to softening, but water loss leads to wilting, which makes vegetables like spinach or lettuce look much less appetising.
- Flavour. For many of us, the smell and taste of food is the most important decider of quality. Flavour depends on the degree of ripeness and its effect on the components responsible for the food’s smell and taste, like organic acids, sugars, phenolic compounds or esters. In fruits and vegetables, ripening results in the conversion of starch into sugars, which increases sweetness. Organic acids usually decrease with ripening, which also affects flavour.
Loss of nutrients
These are changes that affect the physical appearance and taste of crops, but other less obvious changes occur. As soon as crops are picked, their nutrient content starts to decrease through respiration and enzyme activity. This continues to decrease the longer food is stored, and even more so when it’s cooked.
Vitamin C is often used to measure how much nutrient degradation has occurred in food because it decreases rapidly after harvesting – green beans stored for 7 days lose 77% of their vitamin C content. Vitamin C is also water soluble and sensitive to heat, light and oxygen, making it easily lost during cooking.
For hundreds of years humans have preserved crops through processes such as pickling, making jams and chutneys, and freezing or canning. These processes change the shape, form and texture of the product. You might assume they also decrease the nutritional content, but studies suggest frozen food contains at least an equal amount of nutrients to fresh food. In fact, some frozen foods were found to contain more vitamin C than their fresh equivalents. For more on this, see Are frozen fruit and vegetables as good for you as fresh? in the See Also section.
The changes above can lead to food being rejected, and much of this rejected food goes to waste. Food waste is a huge concern around the world. It’s estimated that of all the food produced for human consumption, around one third of it gets wasted - this would be enough to feed all the hungry people in the world! Take a look at 10 Food Waste Facts in the See Also section for some eye-opening facts on food waste.
While waste is often caused by sellers and consumers throwing away food, it can also be generated through processing. For example, making fruit juice results in a large quantity of unused rind and pulp. The good news is that some of these by-products can be used for other purposes – as you’ll discover in the next step.
© University of Cambridge and the Spanish National Research Council