Processing of Fruits and Vegetables
Scope of the domain
This domain tests the sequence of fruit and vegetable processing, browning control, blanching, canning, freezing, drying and product making such as juice, jam and pickles.
Core rules and concepts
- Typical operations are harvesting, sorting, grading, washing, peeling, cutting, blanching, filling, processing and packaging.
- Enzymatic browning occurs when polyphenol oxidase acts on cut tissue in the presence of oxygen. Control by blanching, acidification, ascorbic acid, sulphiting or excluding oxygen.
- Blanching inactivates enzymes, removes air and fixes colour before freezing or drying.
- Jam and jelly set requires pectin, sugar and acid at the right pH. TSS (Brix) and acidity are critical control measures.
- Canning of low-acid vegetables needs pressure processing; acidic fruits need less severe heat.
- Freezing requires proper blanching and rapid freezing to maintain texture.
Worked example
A jam batch has 68% TSS and pH 3.2 but does not set. Reasoning: sugar and acid are adequate for a standard jam, so the likely missing factor is pectin - either too little or degraded by prolonged heating. Adding acid alone will not form a gel without pectin. The correction is to use fruit with adequate pectin, add pectin, or adjust the boil time. The key is to check all three gel components: pectin, sugar and acid.
Traps that cost marks
- Thinking blanching is the same as cooking; its purpose is enzyme inactivation and preparation.
- Ignoring oxygen in enzymatic browning; acid alone may not prevent it.
- Assuming more sugar always improves jam; excessive sugar can crystallise or mask fruit flavour.
- Forgetting that low-acid vegetables need pressure canning, not open-boiling.