Strawberry
Except in regions where strawberries are grown for processing, the cultivars
available and supply of strawberries for puree and juice is usually dependent upon the vagaries
of the fresh market. When the fresh market price drops due to competitive pressure from
cheaper growing regions, the crop is harvested and either frozen whole (four parts fruit, one
part sugar) for bakery, jam, or jelly use or pureed for pulp/juice manufacture.
Furthermore, when the demand for processing raw material drops too far, or the
processing/frozen storage pipeline is full (no available capacity), the crop is abandoned in the
field. This unfortunate situation often faces juice processors a seasonal peak in fruit that
exceeds fresh market demand and factory capacity or processing plant operational
difficulties that reduce juice throughput. In either case sound fruit is wasted.
The extremely soft and delicate strawberry is one of the easiest to juice. A rigorous
inspection is necessary to remove rotted and mouldy fruit. A gentle chlorinated water spray
then eliminates soil. The whole fruit can be fed directly to a paddle or screw pulper fitted with
a 0.5 to 10 mm screen to pulp and separate the puree from the cap, leaves and larger seeds; a
few small seeds are acceptable in many puree uses. A second pulper with a less than 0.5mm
screen effectively removes remaining seeds and produces a smooth, is then filled into 20 to
200 litre containers and frozen at –18ºC. Actually, the puree should be deaerated prior to
freezing and frozen in less than 25 litre portion at less than 25ºC for optimum quality retention
but this is rarely done. It takes many hours for the centre of a 200 litre barrel of puree or frozen
fruit to freeze, ample time for quality deteriorations to commence.
Although frozen and thawed fruit are quite mushy, almost a puree, juice does not
separate easily from either fresh or frozen pureed strawberries. The thick seed-containing
puree requires a macerating enzyme treatment. About 50 to 100 ppm of enzyme followed by
holding at 10 to 20ºC for two to three hours (cold press) or 30 minutes at 60ºC (hot press)
produces juice at a 70 to 80 percent yield (puree basis). The cold press provides a better
strawberry flavour, while from the hot press yield and colour are higher. Centrifugation
followed by filtration produces a clear juice with typical strawberry flavour and colour.
Unfortunately, strawberry anthocyanin pigments are less stable than those in many
fruits are. Thus rapid handling and deaeration are recommended. An additional problem is the
formation and subsequent precipitation of ellagic acid in the juice. While ellagic acid is a
desirable phytochemical in strawberries, in juice it forms unsightly, powdery sediment. This
precipitate forms slowly in a brilliantly clear juice, even after microfiltration or sterile
filtration and is accelerated by pasteurization (Musingo, et al., 2001). The “brute force”
remedy is heating the juice rapidly to ~80ºC, immediately cooling, holding until sedimentation
is complete and then polish filtering. If the juice is later subjected to elevated temperature
during concentration or pasteurization, quality suffers.
Steam extraction is well suited to strawberry (Figure 6.8). This procedure yields a
juice with diminished fresh flavour, but excellent colour and overall stability. Strawberry juice
with a ~5 to 8ºBrix and acid level of 0.5 to 1.0 percent is rarely consumed as is, but contributes
flavour, ascorbic acid, moderate red colour, (and a good image) to blends (Figure 14.2a. and
b.). Strawberry juice can be concentrated, but the colour is somewhat diminished by even the
gentlest process.
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