High pressure processing of fruitand vegetables its methods and techniques and dos and dont's of food processing

1. FST-502 -
Fruits and
Vegetable
Processing
HIGH PRESSURE
PROCESSING OF FRUIT
AND VEGETABLES

2.  By subjecting foods to high pressure (HP), microorganisms
and enzymes can be inactivated without the degradation of
flavour and nutrients associated with traditional thermal
processing.
 The technology was first commercialised in Japan in the early
1990s for the pasteurisation of acid foods for chilled storage.
 In spite of massive research efforts, particularly in Europe
and the USA, commercial development outside Japan has
been slow so far, mainly because of the very high investment
and processing costs of HP processing as well as regulatory
problems in regions such as Europe
INTRODUCTION

3.  The HP process is non-thermal in principle, even if the
pressure increase itself causes a small rise in temperature.
 HP affects all reactions and structural changes where a
change in volume is involved, as in the gelation of proteins or
starch.
 The mechanism behind the killing of microorganisms is a
combination of such reactions, the breakdown of non-covalent
bonds and the puncturing or permeabilisation of the cell
membrane.

4.  Vegetative cells are inactivated at about 300MPa at ambient
temperature, while spore inactivation requires much higher
pressures (600 MPa or more) in combination with a
temperature rise to 60–70°C.
 Certain enzymes are inactivated at 300 MPa, while others are
very difficult to inactivate at all within the pressure range
which is available today.
 Moisture level is extremely important in this context, little
effect being noticeable below 40% moisture content.

5.  The main components of a high pressure system are:
 • a pressure vessel and its closure
 • a pressure generation system
 • a temperature control device
 • a materials handling system
 Most pressure vessels are made from a high tensile steel alloy
‘monoblocs’ (forged from a single piece of material), which can
withstand pressures of 400–600MPa.
 For higher pressures, prestressed multilayer or wire-wound
vessels are used.
 Vessels are sealed by a threaded steel closure, a closure having
an interrupted thread (which can be removed more quickly), or by
a sealed frame that is positioned over the vessel.
 Temperature control in commercial operations can be achieved
by pumping a heating/cooling medium through a jacket that
surrounds the pressure vessel.
HIGH PRESSURE (HP) TECHNOLOGY

6.  The formation of ‘spores’ is a survival strategy of some bacterial
genera in extreme stress conditions, especially Bacillus and
Clostridium.
 Although bacterial spore counts can be lowered by exposure to
high pressure, combination with other preservation methods,
such as mild temperature elevation, is required for substantial
reduction of viable spore counts.
 Sojka and Ludwig (1994, 1997) have suggested the use of a two-
step process to overcome the problems associated with the
extreme pressure resistance of bacterial spores: an initial mild
pressure treatment to induce spore germination followed by a
treatment at higher pressure and temperature to kill the
germinated spores.
 A combination of pressure with temperatures of 60°C and higher
is required for extensive inactivation of spores: the lower the
pressure applied, the higher the required temperature to induce
a preset extent of inactivation.
IMPACT OF HP ON SPORE-FORMING
BACTERIA

7.  At temperatures below 60°C in combination with a pressure
of about 400 MPa, maximal three log-cycle reductions were
obtained for Clostridium sporogenes and Bacillus coagulans
spores.
 To achieve sterility with minimal impact on nutritional value,
flavour, texture and colour, high pressure processing using
multiple high pressure pulses and achieving an end
temperature above 105°C under pressure for short time has
also been proposed.

8.  At moderate pressure, growth and reproduction rate of
vegetative bacteria are retarded while at higher pressures
inactivation occurs.
 Although pressure stability is largely dependent on the type of
microorganism, the species and the medium conditions,
pressures between 200 and 600 MPa at room temperature are
usually sufficient to cause a substantial reduction in viable
vegetative cells.
 Vegetative forms of prokaryotes such as yeasts and moulds
are most pressure sensitive and inactivated by pressures
between 200 and 300 Mpa.
 In general, the protective effect of real food products has
been attributed to the presence of proteins and sugars.
IMPACT OF HP ON VEGETATIVE BACTERIA