Preservation of fruits and vegetables
Principles of preserving produce, drying, freezing, canning, jams and jellies, pickling and fermentation, preservatives, and a jam-making sum.
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Fruits and vegetables are seasonal and perishable: a glut of mangoes or tomatoes in one month can be followed by scarcity and high prices a few months later. Preservation turns surplus produce into products that keep for months or years, such as dried fruit, jams, pickles, juices, frozen vegetables and canned goods. It reduces post-harvest losses, adds value, creates rural jobs, and evens out supply through the year.
Methods and principles of fruit and vegetable preservation are standard exam topics in horticulture and food technology.
Principles
Fresh produce spoils because of:
- Microorganisms: moulds, yeasts and bacteria.
- Enzymes in the produce, which cause browning, softening and off-flavours.
- Chemical reactions: oxidation of fats and vitamins, non-enzymic browning.
- Physical damage, insects and rodents.
Preservation works by killing, removing or inhibiting microbes and enzymes, and by protecting the product from recontamination. Most methods combine several hurdles.
Methods
Drying and dehydration
Removing water lowers water activity below the level at which microbes can grow. Methods range from sun drying (raisins, apricots, chillies, tomatoes) and solar dryers, which are faster and cleaner, to mechanical cabinet, tunnel, spray and freeze dryers. Many fruits are treated with sulphur dioxide or dipped in acids before drying to prevent browning. Dried fruits are typically dried to about 15 to 25 per cent moisture, vegetables to under about 5 to 10 per cent.
Freezing
Freezing to about −18 °C stops microbial growth and slows enzymes and chemical change. Vegetables are usually blanched first, briefly heated in water or steam, to inactivate enzymes such as peroxidase and catalase that would otherwise cause off-flavours in storage. Individual quick freezing (IQF) freezes small pieces rapidly in cold air, forming small ice crystals that damage texture less.
Heat: canning, bottling and pasteurisation
Food is sealed in containers and heated enough to kill spoilage and disease-causing organisms. Acid products (pH 4.6 or below), such as most fruits and tomatoes with added acid, need only boiling-water processing; low-acid vegetables need pressure processing to destroy Clostridium botulinum spores. Juices and pulps are pasteurised and often hot-filled or aseptically packed.
Sugar
High sugar concentrations bind water and create osmotic conditions in which few microbes grow.
- Jam: fruit pulp cooked with sugar to about 68 per cent total soluble solids (TSS).
- Jelly: clear fruit juice, rich in pectin, set with sugar and acid. A good set needs about 0.5 to 1 per cent pectin, a pH of roughly 3.0 to 3.3, and about 65 to 70 per cent sugar.
- Marmalade: citrus jelly with suspended peel shreds.
- Squashes and cordials: juice with sugar, acid and usually a preservative, diluted before drinking; ready-to-serve (RTS) drinks contain less juice and sugar.
- Candied and glazed fruits: fruit impregnated with syrup of increasing strength.
Salt, acid and fermentation
- Pickles in vinegar (acetic acid), oil or brine.
- Fermented vegetables, such as sauerkraut (cabbage with about 2 to 2.5 per cent salt), kimchi and fermented cucumbers, in which salt favours lactic acid bacteria that lower the pH and inhibit spoilage organisms.
Chemical preservatives
Permitted preservatives include sulphur dioxide (often added as potassium metabisulphite), widely used in light-coloured squashes and dried fruits; sodium benzoate, effective in acid products and used in coloured juices and sauces; and sorbic acid and sorbates. Each country sets maximum levels, and labels must declare them; sulphites can trigger reactions in some asthmatic people.
Other methods
- Irradiation at controlled doses to control insects, sprouting and microbes, permitted for certain products in many countries.
- High-pressure processing of juices and guacamole, which kills microbes with little heat.
- Modified-atmosphere packaging for fresh-cut produce.
Quality and safety
- Use sound, ripe produce and clean water; hygiene throughout.
- Control critical points: blanching time, sugar content, pH, processing temperature and time, and seal integrity, using HACCP principles.
- Prevent browning with blanching, acids (ascorbic and citric), sulphiting or excluding air.
- Meet national food standards and labelling rules.
How much water must be boiled off to make jam?
A small processor makes mango jam from 10 kg of pulp containing 12 per cent soluble solids, adding 10 kg of sugar, and aims for a jam with 68 per cent TSS.
- Soluble solids from pulp = 10 × 0.12 = 1.2 kg; from sugar = 10 kg; total = 11.2 kg.
- Final mass of jam = 11.2 ÷ 0.68 ≈ 16.5 kg.
- Starting mass = 10 + 10 = 20 kg, so water to evaporate = 20 − 16.5 = 3.5 kg.
Cooking stops when a refractometer reads about 68 °Brix, or when the boiling point reaches about 105 to 106 °C at sea level. The processor would also check pH and add citric acid if needed for a good set. Ignoring the small amount of added acid and pectin, 10 kg of pulp gives about 33 jars of 500 g.
Key points
- Spoilage comes from microbes, enzymes, chemical reactions and physical damage; preservation kills, removes or inhibits them.
- Main methods: drying, freezing (with blanching), canning and pasteurisation, sugar (jam, jelly, squash), salt, acid and fermentation, and permitted preservatives.
- Jam is cooked to about 68 per cent TSS; jelly needs pectin, acid (pH about 3.0–3.3) and sugar.
- Acid foods (pH 4.6 or below) can be processed in boiling water; low-acid foods need pressure processing.
- Sulphur dioxide suits light-coloured products; benzoate works in acidic products; levels are legally limited.
- Final mass = total soluble solids ÷ target TSS fraction.
What the exams ask
- School-leaving and entrance papers: principles and methods of preservation, and products such as jam, jelly, squash and pickles.
- Degree and research-fellowship entrances: pectin–sugar–acid gel formation, blanching, dehydration, freezing, preservatives and their limits, and fermentation.
- Professional and food technology examinations: product formulations and calculations, HACCP in fruit processing units, and food standards.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why doesn't my jelly set?
Usually too little pectin, too little acid, or the wrong sugar concentration. Underripe fruit has more pectin; adding lemon juice raises acidity; and boiling to the right TSS completes the set.
Is sun drying safe?
It can be, if produce is clean, protected from dust, insects, birds and rain, and dried quickly to a safe moisture. Solar dryers give cleaner, faster and more uniform results.
Why are vegetables blanched before freezing but fruits often not?
Vegetables contain enzymes that cause off-flavours and colour changes even in the freezer, and blanching inactivates them. Fruits are usually protected instead with sugar, syrup or ascorbic acid, because blanching would soften them.
Sources
- Fellows, P. J. Food Processing Technology: Principles and Practice, 4th edition. Woodhead Publishing, 2017.
- Lal, G., Siddappa, G. S. and Tandon, G. L. Preservation of Fruits and Vegetables. Indian Council of Agricultural Research, revised edition.
- Barrett, D. M., Somogyi, L. and Ramaswamy, H. (eds) Processing Fruits: Science and Technology, 2nd edition. CRC Press, 2005.
- Codex Alimentarius Commission. Standard for Jams, Jellies and Marmalades (CXS 296-2009).
- FAO. Fruit and Vegetable Processing. FAO Agricultural Services Bulletin 119, 1995.
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