Controlling or inhibiting growth of microorganisms

It is important to regulate those organisms that are present during the making of fermentation products and also those that are able to grow and survive in the finished product. On the one hand, we have nowadays the deliberate seeding of the desired organism(s), which therefore gain a selective advantage in outgrowing other organisms. Conversely, there are physical or chemical '-cidal' treatments or sterilisation procedures that are employed to achieve the depletion or total kill of organisms. Relevant factors are

(1) how many organisms are present;

(2) the types of organism that are present;

(3) the concentration of antimicrobial agents that are present or the intensity of the physical treatment;

(4) the prevailing conditions of temperature, pH and viscosity;

(5) the period of exposure; and

(6) the concentration of organic matter.

Fermentation by itself comprises a procedure that originally emerged as a means for preserving the nutritive value of foodstuffs. Through fermentation there was either the lowering of levels of substances that contaminating organisms would need to support their growth or the development of materials or conditions that would prevent organisms from developing, for example, a lowering of pH. In the case of a product like beer, there is the deliberate introduction of antiseptic agents, in this case, the bitter acids from hops.

Heating

Moist heat is used for sterilising a greater diversity of materials than dry heat. Moist heat employs steam under pressure and is very effective for the sterilisation of production vessels and pipe work. Dry heat is less efficient and requires a higher temperature (e.g. 160°C as opposed to 120°C); it is used in systems like glassware and for moisture-sensitive materials.

The microbial content of finished food products is frequently lowered by heat treatment. Ultra-high temperature (UHT) treatments are used where especially high kills are necessary. Pasteurisation is a milder process, one in which the temperature and the time of exposure are regulated to achieve a sufficient kill of spoilage organisms without deleteriously impacting the other properties of the foodstuff. In batch pasteurisation, filled containers (e.g. bottles of beer) are held at, say, 62°C for 10min in chambers through which the product slowly passes on a conveyor (tunnel pasteurisation). In flash pasteurization, the liquid is heated as it flows through heat exchangers en route to the packaging operation. Residence times are much shorter so temperatures are higher (e.g. 72°Cfor 15s). In the specific example of beer, this might be the way in which beer destined for kegs is processed. One pasteurisation unit (PU)

is defined as exposure to 60°C for 1 min. As the temperature is increased, the shorter exposure time equates to 1 PU. The more organisms, the more extensive is the heat treatment, so the onus is on the operator to minimise the populations by good hygienic practice.

Cooling

The ability of organisms to grow is curtailed as the temperature is lowered (refrigeration, freezing).

Drying

As organisms usually require significant amounts of water (discussed earlier), drying affords preservation. Thus, for example, starting materials for fermentation (such as grains and fruits) may be subjected to some degree of drying if they are to be stored successfully prior to use. The other way in which water activity can be lowered is by adding solutes such as salt or sugar. In this book, we encounter several instances where there is deliberate salting during processing to achieve food preservation, for example, in fermented fish production.

Irradiation

The use of irradiation to eliminate spoilage organisms is charged with emotion. Critics hit on the tendency of the technique to reduce the food value, for example, by damaging vitamins. However, the procedure really should be considered on a case-by-case basis, and only if there is some definite negative impact on the quality of a product should it necessarily be avoided. Thus, to take beer as our example again, there is evidence for the increased production of hydrogen sulphide when beer is irradiated.

Filtration

Undesirable organisms can be removed by physically filtering them from the product. Depth filters operate by trapping and adsorbing the cells in a fibrous or granular matrix. Membrane filters possess defined pore sizes through which organisms of greater dimensions cannot pass. Typically these pore sizes may be 0.45 ^m or, for especially rigorous 'clean-up', 0.2 ^m. Practical systems may employ successive filters - for example, a depth filter followed by membranes of different sizes. The approach may be most valuable for heat-sensitive products.

Chemical agents

Modern food production facilities are designed so that they are readily clean-able between production runs by chemical treatment regimes, often called

'cleaning in place' or CIP. This demands fabrication with resilient material, for example, stainless steel, as well as design that ensures that the agent reaches all nooks and crannies. CIP protocols generally involve an initial water rinse to remove loose soil, followed by a 'detergent' wash. This is not so much a detergent proper as sodium hydroxide or nitric acid and it is targeted at tougher adhering materials. Next is another water rinse to eliminate the detergent, followed by a sterilant. Various chemical sterilants are available, the most commonly used being chlorine, chlorine dioxide and peracetic acid.

Some foodstuffs are formulated so that they contain preservatives (Table 1.4). In other foodstuffs there are natural antimicrobial compounds present, for example, polyphenols and the hop iso-a-acids in beer. And, of course, the end products of some fermentations are historically the basis of protection for fermented foodstuffs, for example, low pH, organic acids, alcohol, carbon dioxide. Of especial interest here is nisin (Fig. 1.9) that is a natural product from lactic acid bacteria, capable of countering the invasion of other bacteria.

An essential aspect of the long-term success of lactic acid bacteria as a protective agent within the fermentation industries is the multiplicity of ways in which it counters the growth of competing organisms. Apart from nisin and other bacteriocins, we might draw attention to the production of

(1) organic acids, such as lactic, acetic and propionic acids, with acetic acid being especially valuable in countering bacteria, yeasts and moulds;

(2) hydrogen peroxide, which, as we have seen is an activated (and therefore potentially damaging) derivative of oxygen;

(3) diacetyl and acetaldehyde, although some argue that the levels developed are not of practical significance as antimicrobial agents.

Table 1.4 Food grade antimicrobial agents.

Preservative

Acetic acid and its sodium, potassium and calcium salts Benzoic acid and its sodium, potassium and calcium salts Biphenyl

Formic acid and its sodium and calcium salts Hydrogen peroxide p-Hydroxybenzoate, ethyl-, methyl- and propyl variants and their sodium salts Lactic acid Nisin

Nitrate and nitrite, and its sodium and potassium salts o-Phenylphenol

Propionic acid and its sodium, potassium and calcium salts Sorbic acid and its sodium, potassium and calcium salts Sulphur dioxide, sodium and potassium sulphites, sodium and potassium bisulphites, sodium and potassium metabisulphites (disulphites) Thiabendazole

Fig. 1.9 Nisin. This antimicrobial destroys Gram-positive organisms by making pores in their membranes. It includes some unusual amino acids, including dehydrated serine (Dha), dehydrated threonine (Dhb), lanthionine (Ala-S—Ala) and ^-methyllanthionine (Abu-S—Ala). The last two originate from the coupling of cysteine with dehydrated serine or threonine, respectively. See also http://131.211.152.52/research_page/nisin.html.

Fig. 1.9 Nisin. This antimicrobial destroys Gram-positive organisms by making pores in their membranes. It includes some unusual amino acids, including dehydrated serine (Dha), dehydrated threonine (Dhb), lanthionine (Ala-S—Ala) and ^-methyllanthionine (Abu-S—Ala). The last two originate from the coupling of cysteine with dehydrated serine or threonine, respectively. See also http://131.211.152.52/research_page/nisin.html.

Energy source Cell components

Heat

NAD(P)H

Degradation products ► Building 'blocks'

Catabolism Anabolism

Fig. 1.10 Energy sources (e.g. sugars) are successively broken down in catabolic reactions, resulting in the capture of energy in the form of ATP and reducing power (as reduced NADH). Building blocks are transformed into the polymers from which cells are comprised (see Figs 1.3-1.6) in anabolic reactions that draw on energy (ATP) and reducing power (many of the anabolic processes use the phosphorylated form of NADH, i.e. NADPH).

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