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| | PRESERVATION OF FOOD BY HIGH HYDROSTATIC PRESSURE (HPP)
By Dr. D. F. Farkas, Vice-President, Food Technology Branch Office
INTRODUCTION
Food processors have long desired to preserve foods without
heat. Excessive heat reduces perceived freshness. Shelf stable and refrigerated
full moisture foods preserved with minimum heat treatments are designated as
minimally processed foods. Combined with refrigeration, minimally processed
foods processed by high pressure can provide an extended shelf life and a high
degree of microbiological safety. High pressure has been known, for over 100
years, to inactivate vegetative forms of yeasts, molds, and bacteria. The
application of high pressure to the processing and preservation of foods has
been under intensive investigation since the early 1980s. The availability of
commercial isostatic pressing equipment, used for forming metal and ceramic
parts, sparked this interest. High pressure appears to inactivate microbes by
denaturing the proteins in the microbe's cell membranes. This denaturation, if
not repairable by the cell, prevents it from transferring water, ions, and
nutrients across its membranes and the cell dies.
High pressure also denatures proteins in foods, but does not
alter covalent bonds. Thus the chemistry of the food is not changed. Colors,
flavors, and nutritional values are not affected. The appearance of some foods
may be affected due to protein denaturation and possible cell leakage similar to
the liquid loss in some frozen foods upon thawing.
The effects of high pressure are instantaneous throughout a
food product and are independent of product composition, size, mass, or
geometry. This absence of transport limitations gives high pressure processing a
unique advantage over all other processing methods. A 55 gallon metal drum of
product receives the same treatment effect as a five gram sample in a plastic
pouch sharing the same pressure vessel.
The following presentation describes the technology of food preservation and
processing using high pressure. Equipment, packaging, and product formulation
requirements are presented so that the technology can be evaluated for
application to existing and new products. Regulatory issues will be discussed
along with good manufacturing practices and HACCP needs. Factors affecting the
profitability of high pressure processing will be presented.
PRESERVATION OF FOODS BY HIGH PRESSURE PROCESSING
Microbiological requirements
Work since the turn of the century has demonstrated that high hydrostatic
pressures can be used to preserve foods in commercial containers. However, to
date, successful commercial preservation by high pressure has depended upon the
use of refrigeration or the hydrogen ion, to insure a pH below 4.5, to block the
germination of Clostridium botulinum and other spoilage causing spore forming
bacteria. Attempts to produce commercially sterile low acid foods such as meat,
milk, and vegetables have fail due to the extreme pressure resistance of spores.
Vegetative microbes, parasites, insects, and insect eggs are susceptible to
inactivation by pressures in the range of 150 to 600 Mpa (25,000 to 90,000 psi).
This indicates that high pressure can be used to: Prepare
shelf stable, commercially sterile, naturally acid or directly acidified
foods. Prepare pressure pasteurized, extended shelf life, low acid
refrigerated foods. Prepare high water activity ingredients free of
pathogenic vegetative microbes.
A water activity near 1.0 is essential for effective high pressure inactivation
of vegetative microbial cells. Thus high pressure has limited application in the
pasteurization of low water activity ingredients such as dry spices, sugar,
starch or gums.
Parasites in meat and fish can be inactivated at pressures in the range of 200
MPa. These pressures have little effect on quality (such as protein denaturation).
Insect and insect egg inactivation is another possible application. Pressure has
been used to inactivate insects and insect eggs in dry ingredients.
Acid foods such as fruit preserves, yogurt, fruit juices, and pourable dressings
have been preserved by high pressure (400 Mpa for 20 minutes) and are available
commercially in Japan. An extended shelf life, low acid, refrigerated product,
guacamole, is on the market in the United States. Vegetative microbial pathogen
free ingredients produced by high pressure treatment, such as fruits for yogurt
or ice cream and milk for non-heat pasteurized cheese production, are not yet
available.
Because bacterial spores are so pressure resistant, the development of shelf
stable or commercially sterile pressure treated low acid foods requires
additives such as nitrate and nitrite or possibly bacteriosims to block the
germination of spores. Work on the combination of heat and pressure indicates
that temperatures in the range of 90 to 110 0C in conjunction with pressures of
600 MPa can inactivate spores in a matter of minutes (Rovere, 1996). A series of
short or pulsed pressure treatments appears to be more effective in inactivating
spores than an equivalent single pressure holding time. That is, five one minute
cycles may result in a higher inactivation of a spore population than a single
five minute exposure.
The compression of foods, during high pressure treatment, causes a temporary,
uniform, temperature increase of about three degrees centigrade per 100 MPa
(15,000 psi). The temperature of the food returns to the starting value upon
decompression.
Formulation and Processing Requirements
Work sponsored by the United States Department of Defense has shown that it is
technically feasible to produce a wide range of commercially sterile acid foods
using high pressure. Table 1. lists examples of products preserved by high
pressure and held up to 120 days at 34 and 80 0F for sensory analysis.
Formulated products were filled, vacuum sealed, and pressure treated at room
temperature in the packages and for the times and pressures shown. The resulting
products were found to be microbiologically stable at 0, 10, 30, 60, 90, and 120
days at 80 0F using plate count and acidified potato dextrose agar. Descriptor
analysis panels were used to follow sensory changes in the products in
comparison to heat treated controls (packaged products brought to 90 0C internal
temperature and cooled) stored under the same conditions. Yogurt could not be
preserved by heat since it was found that product quality was lost on heating.
Thus a heat treated control was not possible.
TABLE 1. Examples of Commercially Sterile Acid Foods Preserved by High Pressure.
|
Product
|
Pressure (MPa)*
|
Time (minutes)
|
Package
|
|
Spanish rice
|
340
|
30
|
Saran coated nylon
|
|
Spaghetti with meat sauce
|
340
|
30
|
Saran coated nylon
|
|
Yogurt with peaches
|
340
|
30
|
Saran coated nylon
|
|
Grapefruit, Orange, Pineapple
|
340
|
30
|
Saran coated nylon
|
|
Spanish rice
|
580
|
15
|
Omni bowls
|
|
Lemon Pudding
|
580
|
15
|
Omni bowls
|
|
Yogurt Drink
|
580
|
15
|
Omni bowls
|
|
Oriental Chicken (with rice)
|
580
|
15
|
Omni bowls
|
|
Seafood Creole (with rice)
|
580
|
15
|
Omni bowls
|
|
Vegetarian Pasta
|
580
|
15
|
Omni bowls
|
|
Salsa
|
540
|
3
|
Scholle bags**
|
|
Apple Juice
|
540
|
3
|
Scholle bags**
|
*
100 MPa is approximately equal to 15,000 psi ** Aseptically filled
TECHNIQUES
TO POTENTIATE HIGH PRESSURE INACTIVATION
The effect of multiple pressure cycles, for example several cycles of five
minutes each given between 0 and 400 MPa, on the survival of spores in foods has
been studied. Results indicate improved spore inactivation rates over a thirty
minute single pressure treatment. Batch pulsed pressure treatments may allow
fairly simple pressure vessels with very long cycle lives since the vessel is
not held at the treatment pressure for long time periods.
Pressure is known to cause the germination of spores. Combinations of pressure
with mild heating, bacteriosins, and other antimicrobial agents are being
explored to increase the effectiveness of high pressure treatment in the
inactivation of spores in low acid foods.
PACKAGING
A major consideration for the packaging of foods for pressure treatment is the
effect of pressure on the structure and barrier properties of the package.
Additionally, pressure treatment of some foods may result in off-flavors
(taints) possibly due to chemical changes in the plastic packaging, or from off
flavors transferring from the plastic to the food. All packaging materials and
incoming lots of packages should be tested for their potential contribution to
off flavor development. While most foods, with the exception of some soft
fruits, withstand compression to pressures in the range of 600 MPa, rigid,
semi-rigid, and some packages made of flexible materials may deform, crack,
delaminate, or suffer hydration of their inner barrier layers under pressure
treatment. Microwave able plastic bowls with double seamed, easy open, metal
ends (Omni bowls) were found to withstand pressures to 600 MPa if completely
filled with product. Plastic cups, bowls, and even steel cans can be treated at
pressures of 550 MPa provided the geometry is such that compression forces are
distributed so as to prevent permanent distortion. All packages used in the high
pressure preservation of foods must be capable of filling and sealing with no
remaining head space volume. Any head space will be compressed to zero volume
due to the compressibility of gasses in the head space. Packaging for high
pressure preserved foods must accommodate a possible 15% volume compression
while retaining extremely low oxygen transmission rates. Initially, the
packaging system must facilitate the removal of oxygen to extremely low levels.
Acceptable residual oxygen levels must be determined as part of preservation
process development.
Ultimately
the product-package system must be treated, stored, and evaluated for product
quality after storage for the desired time at appropriate storage temperatures.
Double seamed containers may provide certain economies since they can be filled,
sealed, and handled at relatively high line speeds, however these structures
must be evaluated in relation to the volumetric efficiency of the pressure
treatment system.
PRODUCT SAFETY AND REGULATORY ISSUES
Product safety and regulatory requirements for shelf stable, low acid foods,
preserved by high pressure, and packed in hermetically sealed containers have
not been developed in the United States. Good manufacturing practices (GMP) can
be expected to be similar to heat processed foods. The requirement that the
process provide the equivalent of a twelve fold decimal reduction of a pressure
resistant strain of Clostridium botulinum has not been determined.
Because spores are pressure resistant, all commercially sterile high pressure
preserved foods to date have been acid products with a water activity very close
to one. Examples are yogurt, fruit preserves, fruit juices, and pourable salad
dressings. Viable counts can be obtained when pressure treated acid foods are
plated on neutral pH media. Pressure treatment is presumed to inactivate
microbes by disrupting their membrane structures so that they cannot reproduce.
Injured microbes have been shown to repair themselves if they have access to
appropriately enriched media. For this reason all pressure preserved products
should be held, for possible regeneration of microbes, for a time in excess of
their projected shelf life.
STORAGE, AND SHELF-LIFE ISSUES
Ultimately,
sterile, enzymatically inactive foods, such as those preserved by high pressure
in combination with a mild heat treatment, when held in hermetically sealed
containers at room temperature lose flavor, color, and some nutritional value
through temperature induced chemical reactions in the food. These reactions may
be between carbohydrates and proteins as in non-enzymatic browning, may be due
to oxidation from residual active oxygen in the package, or may be due to
internal oxidation-reduction reactions among individual compounds. Room
temperature storage can result in a loss of the fresh quality of food in as
short a time as ten to 30 days. Refrigerated storage can prolong fresh quality
characteristics to 60 or more days by slowing down chemical quality degradation.
Thus the development of preservation methods which minimize the use of heat to
deliver fresh tasting products benefit most from refrigerated storage. |
