Environmental impacts

A range of physical, chemical and physicochemical parameters impact the growth of micro-organisms, of which we may consider temperature, pH, water activity, oxygen, radiation, pressure and 'static' agents.

Temperature

The rate of a chemical reaction was shown by Svante Arrhenius (1859-1927) to increase two- to three-fold for every 10°C rise in temperature. However, cellular macromolecules, especially the enzymes, are prone to denaturation by heat, and this accordingly limits the temperatures that can be tolerated. Although there are organisms that can thrive at relatively high temperatures (thermophiles), most of the organisms discussed in this book do not fall into that class. Neither do they tend to be psychrophiles, which are organisms capable of growth at very low temperatures. They have a minimum temperature at which growth can occur, below which the lipids in the membranes are insufficiently fluid. It should be noted that many organisms can survive (if not grow) at lower temperatures and advantage is taken of this in the storage of pure cultures of defined organisms (discussed later). Organisms which prefer the less-extreme temperature brackets, say 10-40°C, are referred to as mesophiles.

HH Guanine O Thymine O

hnAAJ An^

Thymine Adenine

Sugar O

Cytosine Guanine

Fig. 1.6 Nucleic acids. (a) Nucleic acids comprise three building blocks: bases, pentose (sugars with five carbon atoms) and phosphate. There are four bases in DNA: the purines adenine (A) and guanine (G) and the pyrimidines thymine (T) and cytosine (C). A and T or G and C can interact through hydrogen bonds (dotted lines) and this binding affords the linking between adjacent chains in DNA. The bases are linked to the sugar-phosphate backbone. (b) In the famous double-helix form of DNA, adjacent strands of deoxyribose (D)-phosphate (o) are linked through the bases. The sequence of bases represents the genetic code that determines the properties of any living organism. In ribonucleic acid (RNA), there is only one strand: thymine is replaced by another pyrimidine (uracil) and the sugar is ribose, whose C2 has an -OH group rather than two H atoms.

Fig. 1.6 Nucleic acids. (a) Nucleic acids comprise three building blocks: bases, pentose (sugars with five carbon atoms) and phosphate. There are four bases in DNA: the purines adenine (A) and guanine (G) and the pyrimidines thymine (T) and cytosine (C). A and T or G and C can interact through hydrogen bonds (dotted lines) and this binding affords the linking between adjacent chains in DNA. The bases are linked to the sugar-phosphate backbone. (b) In the famous double-helix form of DNA, adjacent strands of deoxyribose (D)-phosphate (o) are linked through the bases. The sequence of bases represents the genetic code that determines the properties of any living organism. In ribonucleic acid (RNA), there is only one strand: thymine is replaced by another pyrimidine (uracil) and the sugar is ribose, whose C2 has an -OH group rather than two H atoms.

Table 1.2 Role of vitamins in micro-organisms.

Vitamin

Coenzyme it forms part of

Thiamine (vitamin Bj ) Thiamine pyrophosphate

Riboflavin (B2) Niacin

Pyridoxine (Bg)

Pantothenate

Biotin

Folate

Cobalamin (B12)

Flavin adenine dinucleotide, flavin mononucleotide Nicotinamide adenine dinucleotide Pyridoxal phosphate Coenzyme A Prosthetic group in carboxylases Tetrahydrofolate Cobamides

Most organisms have a relatively narrow range of pH within which they grow best. This tends to be lower for fungi than it is for bacteria. The optimum pH of the medium reflects the best compromise position in respect of

(1) the impact on the surface charge of the cells (and the influence that this has on behaviours such as flocculation and adhesion);

Table 1.3 Exogenous enzymes.

Enzyme

Major sources

Application in foods

«-Amylase

Aspergillus, Bacillus

Syrup production, baking,

brewing

P-Amylase

Bacillus, Streptomyces,

Production of high maltose

Rhizopus

syrups, brewing

Glucoamylase

Aspergillus, Rhizopus

Production of glucose

syrups, baking, brewing,

wine making

Glucose isomerase

Arthrobacter, Streptomyces

Production of high fructose

syrups

Pullulanase

Klebsiella, Bacillus

Starch (amylopectin)

degradation

Invertase

Kluyveromyces,

Production of invert sugar,

Saccharomyces

production of soft-centred

chocolates

Glucose oxidase

Aspergillus, Penicillium

Removal of oxygen in

(coupled with

various foodstuffs

catalase)

Pectinase

Aspergillus, Penicillium

Fruit juice and wine

production, coffee bean

fermentation

P -Glucanases

Bacillus, Penicillium,

Brewing, fruit juices, olive

Trichoderma

processing

Pentosanases

Cryptococcus,

Baking, brewing

Trichosporon

Proteinases

Aspergillus, Bacillus,

Baking, brewing, meat

Rhizomucor, Lactococcus,

tenderisation, cheese

recombinant

Kluyveromyces, Papaya

Catalase

Micrococcus,

Cheese (see also glucose

Corynebacterium,

oxidase above)

Aspergillus

Lipases

Aspergillus, Bacillus,

Dairy and meat products

Rhizopus, Rhodotorula

Urease

Lactobacillus

Wine

Tannase

Aspergillus

Brewing

P -Galactosidase

Aspergillus, Bacillus,

Removal of lactose

Escherichia,

Kluyveromyces

Acetolactate

Thermoanaerobium

Accelerated maturation

decarboxylase

of beer

(2) on the ability of the cells to maintain a desirable intracellular pH and, in concert with this, the charge status of macromolecules (notably the enzymes) and the impact that this has on their ability to perform.

Water activity

The majority of microbes comprise between 70% and 80% water. Maintaining this level is a challenge when an organism is exposed variously to environments that contain too little water (dehydrating or hypertonic locales) or excess water (hypotonic).

The water that is available to an organism is quantifiable by the concept of water activity (Aw). Water activity is defined as the ratio of the vapour pressure of water in the solution surrounding the micro-organism to the vapour pressure of pure water. Thus, pure water itself has an Aw of 1 while an absolutely dry, water-free entity would have an Aw of 0. Micro-organisms differ greatly in the extent to which they will tolerate changes in Aw. Most bacteria will not grow below Aw of 0.9, so drying is a valuable means for protecting against spoilage by these organisms. By contrast, many of the fungi that can spoil grain (Aw = 0.7) can grow at relatively low moisture levels and are said to be xerotolerant. Truly osmotolerant organisms will grow at an Aw of 0.6.

Oxygen

Microbes differ substantially in their requirements for oxygen. Obligate aerobes must have oxygen as the terminal electron acceptor for aerobic growth (Fig. 1.7). Facultative anaerobes can use oxygen as terminal electron acceptor, but they can function in its absence. Microaerophiles need relatively small proportions of oxygen in order to perform certain cellular activities, but the oxygen exposure should not exceed 2-10% v/v (cf. the atmospheric level of 21% v/v). Aerotolerant anaerobes do not use molecular oxygen in their metabolism but are tolerant of it. Obligate anaerobes are killed by oxygen.

Clearly these differences have an impact on the susceptibility of foodstuffs to spoilage. Most foods when sealed are (or rapidly become) relatively anaerobic, thus obviating the risk from the first three categories of organism.

Irrespective of which class an organism falls into, oxygen is still a potentially damaging molecule when it becomes partially reduced and converted into

FADH2 1

NADH -> Coenzyme Q ■*• Cytochrome b->Cytochromes c ■*• Cytochromes a ■ Oxygen

Fumarate Dimethyl Trimethylamine Nitrate sulphoxide N-oxide

Fig. 1.7 Electron transport chains. Reducing power captured as NADH or FADH2 is transferred successively through a range of carriers until ultimately reducing a terminal electron acceptor. In aerobic organisms, this acceptor is oxygen, but other acceptors found in many microbial systems are illustrated. This can impact parameters such as food flavour - for example, reduction of trimethylamine N-oxide affords trimethylamine (fishy flavour) while reduction of dimethyl sulphoxide (DMSO) yields dimethyl sulphide (DMS), which is important in the flavour of many foodstuffs.

Hydroxyl Ol-T

Perhydroxyl

Singlet *O2 oxygen

Peroxide

' peroxide

Superoxide

Ground-state oxygen

Fig. 1.8 Activation of oxygen. Ground-state oxygen is relatively unreactive. By acquiring electrons, it become successively more reactive - superoxide, peroxide, hydroxyl. Superoxide exists in charged and protonated forms, the latter (perhydroxyl) being the more reactive. Exposure to light converts oxygen to another reactive form, singlet oxygen.

radical forms (Fig. 1.8). Organisms that can tolerate oxygen have developed a range of enzymes that scavenge radicals, amongst them superoxide dismutase, catalase and glutathione peroxidase.

Radiation

One of the radical forms of oxygen, singlet oxygen, is produced by exposure to visible light. An even more damaging segment of the radiation spectrum is the ultraviolet light, exposure to which can lead to damage of DNA. Ionising radiation, such as gamma rays, causes the production of an especially reactive oxygen derived radical, hydroxyl (OH^), and one of the numerous impacts of this is the breakage of DNA. Thus, radiation is a very powerful technique for removing unwanted microbes, for example, in food treatment operations.

Hydrostatic pressure

In nature, many microbes do not encounter forces exceeding atmospheric pressure (1 atm = 101.3 kPa = 1.013 bar). Increasing the pressure tends to at least inhibit if not destroy an organism. Pressure is of increasing relevance in food fermentation systems because modern fermenters hold such large volumes that pressure may exceed 1.5 atm in some instances. Although they do not necessarily kill organisms, high pressures do impact how organisms behave, including their tendency to aggregate and certain elements of their metabolism. The latter is at least in part due to the accumulation of carbon dioxide that occurs when pressure is increased.

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