A cut apple browns within minutes. Unless that reaction is stopped, it continues in dried product and ruins the colour.
How does browning happen?
Two components sit in separate compartments within an apple's cells.
The first is polyphenols: compounds such as chlorogenic acid and catechin.
The second is an enzyme called polyphenol oxidase.
While the apple is whole those two are kept apart.
Cutting, bruising or decay tears the cell walls and they meet.
In the presence of oxygen the enzyme converts the polyphenols into quinones.
The quinones link together and form brown, melanin-like pigments.
That is the fruit's response to a wound; a defence mechanism in the plant.
It is not spoilage, but it is visually unwanted and the flavour shifts slightly too.
Which conditions accelerate it?
Four factors speed the reaction.
The first is oxygen. The enzyme cannot work without it; that is the most critical variable.
The second is temperature. The enzyme is most active between 20 and 40 degrees.
It slows in the cold but does not stop.
The third is pH. The enzyme is most efficient between 6 and 7; below pH 3 it stops.
The fourth is the apple's own composition. Varieties high in polyphenols brown faster.
Some varieties naturally contain less polyphenol oxidase and brown late.
Ripeness matters too; enzyme activity is higher in overripe fruit.
| Factor | Accelerates browning | Slows browning |
|---|---|---|
| Oxygen | In open air | Vacuum, under water |
| Temperature | 20-40 °C | 0-4 °C or above 80 °C |
| pH | 6-7 | Below 3 |
| Cut surface | Wide, bruised | Minimal, clean cuts |
| Variety | High in polyphenols | Low in polyphenols |
| Ripeness | Overripe | Fully ripe |
Which pre-treatments are used?
There are four ways to stop the enzyme.
The first is acidification, using lemon juice or an ascorbic acid solution.
The pH falls and enzyme activity diminishes.
Ascorbic acid also reduces the quinones back, reversing the colour compounds that have formed.
That is the commonest method.
The second is blanching. The fruit is held briefly in hot water or steam.
Above 80 degrees the enzyme denatures and stops entirely.
But that method softens the texture of the apple.
The third is removing the oxygen. The slices are held under water or under vacuum.
The fourth is using sulphites. Effective, but an allergen that must be declared.
Industrial production generally uses a blend of ascorbic and citric acid.
How is colour retained in freeze-drying?
The freeze-drying process offers two advantages against browning.
The first is low temperature. The product is processed frozen; enzyme activity is close to nil below freezing.
The second is vacuum. Oxygen is removed and the enzyme cannot work.
Together those two conditions stop browning throughout the process.
But there is a problem: the enzyme is not killed, only halted.
Once packed and exposed to oxygen and moisture, it can start working again.
Pre-treatment is therefore still necessary.
Freeze-dried apple without pre-treatment browns in the pack over time.
In pre-treated product the colour holds for months.
Properly produced freeze-dried apple comes very close to the colour of a freshly cut one.
How does it differ from conventional drying of apple?
In conventional hot-air drying the temperature is 60 to 70 degrees.
That partly stops the enzyme, but it works throughout the early stage while the product heats.
Non-enzymatic browning also begins at that temperature.
The Maillard reaction and caramelisation come into play.
The result is the characteristic brown colour of conventionally dried apple.
That colour is not counted as a fault; it is what the product is expected to look like.
In freeze-dried apple the pale cream colour survives.
That is the most visible difference between the two products.
There is a difference in flavour too; conventional drying develops caramel notes, while freeze-drying preserves the aroma of fresh apple.
Does browning change the nutritional value?
Polyphenols are consumed during browning.
Polyphenols are antioxidant compounds; some are lost in a browned apple.
Vitamin C is affected too; ascorbic acid reacts and diminishes.
But the loss should not be overstated; the change in total nutritional value is limited.
Fibre, minerals and sugar are unaffected.
Vitamin C loss in freeze-dried apple runs 10 to 20 percent; in conventional drying, 50 to 80.
Polyphenol loss is markedly lower in freeze-drying too.
That shows the method is advantageous nutritionally as well.
A hundred grams of freeze-dried apple carries around 350 calories, 8 to 10 grams of fibre and 60 to 70 grams of sugar.
How should freeze-dried apple be stored?
Moisture is the greatest enemy of freeze-dried apple.
Two things happen in a product that takes up moisture.
The first is texture; the crispness goes and the product softens.
The second is colour; the enzyme resumes working and browning begins.
Airtight packaging is essential.
Once opened the product should go into a sealed glass jar with a desiccant sachet.
It should be kept in the dark; light degrades the remaining pigments.
An unopened vacuum pack keeps twelve to twenty-four months.
Once opened, two to four weeks.
In a humid kitchen that falls to a week.
Whether the product has taken up moisture is detectable by hand; if the crisp structure has softened, it has.
What to look for when buying dried apple?
There are four indicators.
The first is colour. Pale cream or faint yellow is normal; brown patches indicate missing pre-treatment or long storage.
The second is texture. The product should snap easily in the hand; if it flexes it has taken up moisture.
The third is the ingredients list. Apple alone is ideal.
Ascorbic and citric acid appearing is normal and harmless; they come from the pre-treatment.
Where sulphites appear, anyone with an allergy should take care.
The fourth is the packaging. Vacuum and opaque packaging gives a longer shelf life.
Transparent packaging makes the colour easy to check but offers no protection from light.
In choosing between the two, how quickly the product will be used is what decides it.
