Methods of food preservation

Meat Preserving And Curing Guide

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The five basic historical methods of food preservation still in use today comprise desiccation or drying, heating, freezing, fermentation, and chemical preservation (Grierson, 1997).

1. Desiccation. Desiccation (dehydration, or drying) prevents the rotting of meat, the germination of stored grains, and the sprouting of certain vegetables. It also inhibits the growth of micro-organisms, but some of these dormant microorganisms can become dangerous with rehydration of the food. The Chinese and the Italians optimally started to use their noodles when, independently, they invented starchy dried foods with a very long shelf life.

2. Heating. Heat can increase shelf life by pasteurising or sterilising food. Meats can be spit-roasted or held over a fire on a pointed rod but our ancient ancestors could not adequately cook many plant foods until they developed pottery cooking vessels.

3. Freezing. Ancient peoples living in areas with cold winters would observe that frozen foods remained in good condition (at least to unsophisticated taste buds) almost indefinitely, whereupon humans developed rudimentary cold storage by cooling the recesses of caves and other shelters with ice and snow.

4. Fermentation. Fermentation is a gradual chemical change caused by the enzymes of some bacteria, moulds, and yeasts. Fermented beverages were ubiquitous in the earliest civilisations of Mesopotamia and Egypt. Not only did wine facilitate conviviality, it was usually more potable than the available water. Winemaking also served as a means of storing nutrients from grapes almost indefinitely. Similarly, Asian steppe dwellers turned mare's milk into koumiss, a fermented beverage that keeps much longer than unprocessed milk. Many cheeses with a long shelf life are produced by lactic-acid fermentation. One means of pickling, a very early form of food preservation, is to treat foods with vinegar, a liquid obtained by further fermenting alcoholic beverages.

5. Chemical preservation. Many people consider food additives a modern innovation, but humans have used preservatives for millennia. Today it is hard to understand how precious salt was in ancient times, when it was valued partly as an effective preservative. Salted herring were exported in large quantities from North Sea fishing communities and consumed throughout most parts of Middle Europe. Meat from slaughtered livestock was salted for consumption over the winter. Smoking is another ancient and common means of chemical food preservation. Smoked foods include bacon, kippered herring, and salmon. Classical smoking introduced antioxidants, butylated hydroxyanisole (BHA) and butyl gallate, for example, in large amounts. However, currently permitted levels of such antioxidants as additives are far below such levels. Spices are rich in antioxidants and even bactericides (substances that kill bacteria). The hot curries and chilli dishes popular in the tropics, where food safety is most difficult to achieve, tend to be high in such natural preservatives.

As with chemical preservation, many people consider freeze-drying, a combination of two of the time-tested methods described above, a modern innovation. But native South Americans living on the Altoplano, in the Andes, have subsisted on freeze-dried potatoes of a sort for thousands of years. The plateau dwellers carry the potatoes up into areas where the atmospheric pressure is very low, the sky is usually almost cloudless, and the night-time temperatures fall well below the freezing point of water. The natives slice and crush the potatoes and 'freeze-dry' the result by spreading it on rocks. In modern food-processing facilities, freeze-drying involves freezing foods hard and then drying the result in a vacuum.

In the course of time food preservation, especially using heat, has undergone many developments. The most important ones include controlling the risk of survival of pathogenic organisms by the use of mathematical models (McMeekin et al., 1993 and Ross, 1999) and the introduction of flash-pasteurisation and heat processes also known as the high temperature-short time method (HTST) (Enright et al., 1956, 1957). They are now widely used in the food industry, not only to preserve liquids from microbial spoilage but also to inhibit unwanted enzymatic activity in beverages such as orange juice. Nowadays the food processing industry is faced with an ever increasing demand for safe and minimally processed food with a high degree of wholesomeness and a fresh appearance. This has resulted in a number of nonthermal processing technologies (Gould 2000, 2001).

High hydrostatic pressure, which is applied for commercial food products at pressure levels up to 700 Mpa, has been proven to inactivate vegetative organisms (Smelt et al., 2002b) as well as spores (Heinz and Knorr, 2002, Meyer et al., 2000). Farkas and Hoover (2000) have identified critical process factors and future research needs. Key advantages of high pressure processing are the quality advantages achieved for various foods as evidenced by the commercial products which are mainly fruit juices, fruit preparations and selected meat products.

Ohmic heating (Joule heating, electrical resistance heating, electroconductive heating) is achieved by passing electric currents through food which is placed between electrodes. For inductive heating electric currents are induced within food material, due to oscillating electro-magnetic fields generated by electric coils. With both processes rapid - and in many cases uniform - heating of liquids and particulates can be achieved. Limited information is currently available regarding industrial applications of the processes. Sastry and Barach

(2000) have presented critical process parameters and data on microbial inactivation. Microwave and radio frequency heating uses electromagnetic waves of given frequencies to generate heat. Due to difficulties in achieving uniformity of heating, industrial preservation processes have not yet been consistently successful. Datta and Davidson (2000) have summarised microbial aspects of these rapid heating processes.

High intensity pulsed electric fields involving the subjection of foods placed between electrodes to pulses of high voltage have been demonstrated to effectively and controllably permeabilise (reversible or irreversible) biological membranes (Anon, 2001). Pilot scale equipment is available in Europe as well as in the USA. The impact of pulsed electric fields on vegetative micro-organisms has recently been demonstrated by Wouters et al. (2001a, b), Heinz and Knorr

(2001) and Barbosa-Carnovas et al. (2000). Ultra sound energy is generated by sound waves of 20 KHz and above. Ultrasound has a wide range of applications in medicine and biotechnology with limited applications regarding the inactivation of micro-organisms (Hoover, 2002). Selective inactivation of various micro-organisms has been accomplished recently and quality retention achieved due to removal of air from food systems by ultrasound treatment

(Zenker et al., 2002). Finally, in many applications food manufacturers also make use of ultraviolet (UV) light to inactivate micro-organisms.

Examples of naturally occurring antimicrobial compounds include for instance the use of antimicrobial small organic biomolecules (flavours/fragrances), peptides from plant (crop) and microbial origin and microbial wall lytic enzymes (Brul and Coote, 1999; Cleveland et al., 2001; Lewis, 2001; Lopez-Malo et al., 2002). Some of the discussed non-thermal (or rather sometimes low thermal) techniques are already used commercially or are very close to commercial application. All the techniques discussed have their own specific applications. For example, the pulsed electric fields technique is suited only for pumpable liquid products. Ultraviolet light treatment is suited only for surface decontamination and for treatment of fluids with a high transparency. Natural antimicrobials often, but clearly not always (!), have a strong flavour or taste when applied in concentrations needed to be antimicrobial and thus have a limited scope in terms of product formulation. Alternatively, some natural antimicrobials constitute novel ingredients or have to be produced using modern genetic modification (gmo)-based biotechnology and as such have to pass the ever more severe legislation criteria. With respect to gmo technology it is furthermore needless to say that consumer attitudes towards this technology is, right or not, extremely suspicious currently discouraging the viability of this type of approach for the food industry.

Finally, combination preservation has often been proposed as the way forward, minimising adverse organopeltic effects of individual treatments while maximising their (combined) antimicrobial effect through synergistic action on the microbes. In order to apply combination preservation systems and to optimise the use of the currently available systems, a better understanding of the physiology and molecular cell biology of food spoilage micro-organisms is needed for a real breakthrough (Abee and Wouters, 1999; Brul et al. 2002a). Moreover, new techniques for a rapid and thorough assessment of the distribution of the various strain variations of harmful micro-organisms throughout the food chain need to be developed (see below; de Boer and Beumer, 1999; Ferretti et al., 2001; Weimer and Mills, 2002; see also http://www.biochip-technologies.com/ of the Germany based company GeneScan). The following discussion provides a practical example of how ingredients containing herbs and spices might aid in the construction of a preservation system.

Byrne et al. (2002) tested the effects of commercial beefburger production and product formulation on the heat resistance of Escherichia coli 0157:H7. In their experiments beef trimmings were inoculated with 6-7logcfu of E. coli 0157:H7, following which the trimmings were frozen and stored at —18 0C for one month. After that time the trimmings were tempered using a microwave in a temperature range from —3 0C to 20C. After mincing, 10-gram beefburger samples were formed either without addition of ingredients (Economy burgers) or with addition of ingredients (Quality burgers) including among others seasonings, spices, salt and soya concentrate. Thereafter the burgers were frozen to —18 0C and stored at —18 0C. After a storage period of one month thermal inactivation studies were conducted at 55, 60 and 65 0C and decimal reduction

Table 23.3 Comparison of fl-values for Escherichia coli 0157:H7 in Quality and Economy beefburger formulations (Byrne et al, 2002)

Temperature

0C

Quality formulation

Economy formulation

Control

Frozen

Control

Frozen

55

20.8(1.0)*

9.3(1.0)

41.1(3.4)

11.7(1.7)

60

2.7(1.2)

1.9(0.3)

4.2 (0.9)

2.4(0.5)

65

0.6(0.2)

0.5(0.1)

0.7 (0.3)

0.6(0.1)

* D-value in min. (with standard deviation)

* D-value in min. (with standard deviation)

times in minutes were assessed. Freshly inoculated beefburgers not subjected to freezing and tempering steps were used as controls. The results are presented in Table 23.3. The results clearly demonstrate that the stresses caused by freezing (sublethal injury) and tempering did not evoke a protective effect against heat treatment. The stress caused by adding ingredients containing herbs and spices (natural antimicrobials) to the Economy formulated beefburgers clearly demonstrate an enhanced preservation effect. Both effects were very clear at the lowest heating temperature of 55 0C used and were not any more significant at a heating temperature of 65 0C. A protective effect was observed when the control samples of both formulation were compared with the heat resistance observed in brain heart infusion broth. In broth the D-value at 55 0C was 13.1 min. Finally, a relationship between aw and heat resistance has been reported in liquid systems by Kaur et al. (1998), who noted a decrease in the heat resistance of E. coli 0157:H7 in salt and sucrose solutions.

It is here important to note that in the UK (ACMSF, 1995) and Ireland (FSAI,1999) it is recommended that minced beef and minced beef products, including beefburgers, should be heated to an internal temperature of 70 0C for 2 minutes or its equivalents (i.e., 75 0C for 30 seconds, 65 0C for 10 minutes and 60 0C for 45 minutes). These treatments are considered to be sufficient to give a 6 decimal reduction in numbers of E. coli 0157:H7. At these temperatures the extra antimicrobial effects of the addition of ingredients to the product formulation on the survival of E. coli 0157:H7 upon heat treatment is no longer significant.

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