application of alternative food-preservation - Bentham Science

High Pressure Processing Application of Alternative Food-Preservation Technologies 115
without increasing the temperature, thus obtaining a “safe” and “fresh” egg, without off-odours. This is the HHP
processing.
This simple imagine offers a friendly description of how HHP works: hydrostatic pressure is usually applied to food
products through a water bath that surrounds the product; it can be applied both to liquid and packed solid foods.
Riva [4] reported that HHP is based on 5 basic principles:
1. Le Chatelier’s principle: pressure enhances reactions leading to a volume decrease (e.g. starch
gelatinization, protein denaturation and phase transitions). Le Chatelier’s principle can explain the
antimicrobial effectiveness of HHP, as high pressures denatures proteins, solidifies lipids and
destabilizes biomembranes [2].
2. Adiabatic heating: the increase of pressure results in a uniform increase of the temperature. This
phenomenon can be described through the following equation:
dT
dP
T
C
where T is the temperature (K); P, the pressure (Pa); α, the thermal expansion (1/K); ρ, the density
(kg/m 3 ); Cp, the heat capacity (J/kg*K). This equation indicates that temperature increase depends
on the characteristics of the system and the initial temperature: for example it has been evaluated
that water temperature increases of 2.8-4.4°C/100 MPa (2.8, 3.8 and 4.4 at 20, 60 and 80°C
respectively); otherwise the temperature of oil increases of 6-8°C/100 MPa.
3. Isostatic rule: HHP processing is not affected neither by the volume nor by the shape of the foods;
the pression is uniformly distributed around and throughout the product.
4. Squeezing: pressure enhances ionization phenomena inside the system, thus resulting in little
changes of the pH.
5. Energy of compression: energy input required by HHP is lower than that used in the traditional
thermal treatments; therefore, at room temperature pressure can affect only hydrogen and ionic
bonds. In contrast, covalent bonds remain unchanged.
Nowadays, HHP has been proposed and applied for the preservation of different product, as a suitable alternative
to the traditional heat processing.
In the following paragraphs, the reader will find some details on the antimicrobial effectiveness of HHP, a brief
description of the physico-chemical modifications caused by pressure in foods, some examples of the application
of this approach for some foods and a safety evaluation of the method.
EFFECT OF HHP ON THE MICROORGANISMS OF FOODSTUFFS
It is well known that HHP can be used successfully for the inactivation of the pathogens and spoiling microflora
of foodstuffs. As regards the kind of resistance, different reports suggest the following hierarchy of resistance:
Bacteria (cells)>fungi>protozoa-parasites and amongst the bacteria, the Gram positive are more resistant than
Gram negative ones, thus highlighting that pressure resistance could be inversely related to cell dimension,
although there are some exceptions to this general statement [2]. The viruses cannot be included in this scale, as
they are characterized by a broad range of sensitivity/resistance [2].
Pressure treatments at 400-600 MPa for 5-20 min at various temperatures are able to inactivate the vegetative
forms of foodborne pathogens (see Table 1); however, it is important to underline that pressure effectiveness is
influenced strongly by the temperature, the kind of treatment (single or multi-step) and food components. A final
consideration on the bacteria is the following: amongst Gram positive bacteria, lactic acid bacteria appeared as
the most resistant ones.
Despite bacteria sensitivity, HHP cannot be used to inactivate spores. In fact, bacterial spores are the most
difficult life-forms to eliminate with hydrostatic pressure [2]; for example, Hoover et al. [2] reported that it was
possible to detect viable spores of Bacillus spp. after a treatment at 1700 MPa for 45 min at room temperature.
This report, along with other data available in the literature [5], suggests that HHP alone cannot be used to
inactivate spore-formers; in contrast the use of the hurdle approach (i.e. the combination of two or more
preserving elements) is a reliable way [2, 6]. In the case of bacterial spores, it has been suggested the
p