With the exceptions noted above, adverse effects on the product are
slight; nutritional quality, flavour, appearance and texture resemble the
fresh material very closely. To the consumer it is a ‘natural’ process with
none of the negative associations of processes such as irradiation or
chemical preservatives.
Initially, commercial application of high-pressure technology was
limited mainly to acidic products. The yeasts and moulds normally
responsible for spoilage in these products are pressure sensitive and the
bacterial spores that survive processing are unable to grow at the low pH.
In 1990, the Meidi-Ya company in Japan launched a range of jams
treated at 400–500 MPa in pack. These have a chill shelf-life of 60 days
and have sensory characteristics quite different from conventional heat-
processed jams since more fresh fruit flavour and texture are retained.
Refrigeration is necessary to limit residual enzyme activities which give
rise to browning and flavour changes. Other products introduced include
salad dressings, fruit sauces, and fruit flavoured yoghurts. More recently
a number of pressure-treated foods have been introduced in Europe, the
United States and elsewhere. These include fruit purees and juices and
some more novel products such as guacamole, cooked ham and oysters.
Pressure-treated guacamole has been a success in the USA where pres-
sures of around 500 MPa for 2 minutes extend its chill shelf life from 7 to
30 days. Similar treatments are applied to packs of sliced cooked ham
and other delicatessen meat products in Italy, Spain, Germany, the USA
and Japan to reduce the risk posed by any post-cooking contamination
withListeria monocytogenes. In the United States and South Korea,
pressure-treated oysters are also available. The process used releases the
adductor muscle which holds the oyster shell closed, so it has the dual
safety benefits of eliminating any Vibrio species present as well as
reducing the number of stab injuries incurred during abortive attempts
to open the shell.
In the future, the range of products may be increased by coupling
moderate pressure with a heat treatment equivalent to pasteurization. In
one trial, shelf stable, low acid foods were produced by combining a
pressure of just 0.14 MPa with heating at temperatures of 82–103 1 C. Other
developments such as equipment capable of semi- or fully-continuous
operation will also considerably improve commercial feasibility, so that we
may see and hear a lot more about pascalization.
4.4 Low-Temperature Storage–Chilling and Freezing
The rates of most chemical reactions are temperature dependent; as the
temperature is lowered so the rate decreases. Since food spoilage is
usually a result of chemical reactions mediated by microbial and
92 The Microbiology of Food Preservation