and freezing can prevent discoloration - blanching actually destroys the enzymes, whereas freezing
slows down their activity. (However, care must be taken to not under-blanch the fruit or vegetable - this
actually stimulates the activity of enzymes and produces more pronounced browning effects than does
Alternatively remove the substrates: This is most easily done by removing the oxygen from the
surrounding area (the enzymes or substrates themselves are very hard to remove), by replacing the
air with either pure nitrogen or CO2 - this is often how pre-prepared cut vegetables are packaged.
Citrus fruits: Citrus fruits do not brown. Often, lemon juice is added to freshly cut fruits to prevent their
browning. This is not due to the acidity of the lemon juice slowing down enzyme activity, since the
addition of vinegar does not slow browning. Lemon juice contains high amounts of ascorbic acid (also
known as vitamin C). This molecule functions as an antioxidant (i.e. reverses the oxidation process),
and therefore reconverts the quinines formed back into their original uncoloured polyphenol molecule,
before they have the chance to form melanoidins.
Quinines + ascorbic acid _ polyphenols
To see this effect for yourself, cut three slices from an apple and leave them on a plate exposed to air.
To one add lemon juice, to the second vinegar, and the third a small amount of ascorbic acid powder.
The browning is...
The browning is prevented most by the ascorbic acid (a pure form of vitamin C), to a lesser by the
lemon juice (a less pure source of vitamin C), yet the vinegar provides little anti-browning effect.
III/IV - 3 (of 11)
· The smell of garlic:
Raw garlic contains a sulfur based compound called alliin as well as an enzyme that acts on the alliin
called alliinase. In raw garlic, these two compounds are separated by different compartments - a whole
garlic clove therefore has little aroma. On cutting garlic, these special sub-compartments are broken,
allowing the enzyme to react with its substrate the alliin and convert it to allicin, which has a very