Clostridium heat resistance

The most frequently utilized to challenge moist heat sterilization cycles are Bacillus stearothermophilus and Clostridium sporogenes, spore-forming bacteria are selected because of their relatively high heat resistance. In addition to the selection of an appropriate organism for use as a biological indicator, the concentration and resistance of the indigenous microbial population is established. 

Reduction of microorganisms in milk before cheese production must be achieved in such a way that the functionality of the milk proteins is not affected. Heat-resistant spores, such as Clostridium tyrobutyricum and C. sporogenes can cause severe spoilage of the cheese by late fermentation that can result in the production of H2 and CO2 gases, and unpleasant smelling fermentation products [102]. Although heat sterilization reduces these spores, because of the heat-induced complexation between 3-Lg and K-casein, UHT milk normally does not form a rennet gel and consequently could not be used efficiently for cheesemaking [87]. 

Microbes are killed by heat. If food is heated to an internal temperature above I6O0F, or 78oC, for even a few seconds this sufficient to kill parasites, viruses or bacteria, except for the Clostridium bacteria, which produce a heat-resistant form called a spore. Clostridium spores are killed only at temperatures above boiling. This is why canned foods must be cooked to a high temperature under pressure as part of the canning process. 

Under certain circumstances, many bacteria produce capsules outside their ceU envelopes. Capsules are usually made of polysaccharide however, some Bacillus capsules are composed of D-glutamic acid polypeptide. Certain Gram positive bacteria, most notably strains of Bacillus and Clostridium, produce modified ceUs (endospores) capable of surviving in adverse environments. Spore PG is found between an inner and outer membrane and differs from that in the normal vegetative cell muramic acid is mostly in the lactam form the spore PG has fewer peptide cross-links and the ceU exterior is coated with keratin. The spore also contains large quantities of calcium dipicolinate, a substance involved in microbial heat resistance. 

A second group of wort bacteria include Bacillus and Clostridium species, spo-rogenic species with high heat-resistant spores that may enter into beer processes via raw materials (malt and hops). These are associated with the production of butyric acid and sulfur compounds (Back, 2005). A third group includes the genera Lactobacillus, Enterococcus, Lactococcus, and Pediococcus many producing lactic acid and diacetyl (Back, 2005). 

For foods it is desired to kill essentially all the spores of Clostridium botulinum, which produces a toxin that is a deadly poison. Complete sterility with respect to this spore is the purpose of thermal processing. Since Cl. botulinum is so dangerous and often difficult to use, other spores, such as Bacillus stearothermophilus, which is a non-pathogenic organism of similar heat resistance, are often used for testing the heat-treating processes (A2, Cl). 

Bacteria from the genus Clostridium are anaerobic, sporeforming gram-positive rods. The spores are heat-resistant and can survive in foods that are incorrectly or minimally processed. The genus contains a number of dangerous pathogens, including Clostridium botulinum, Clostridium difficile, and Clostridium tetani. 

The D-value is always defined for a certain temperamre and for a defined process. Table 30.1 presents a number of D-values for micro-organisms. Spores of Bacillus and Clostridium strains have the highest heat resistance. 

Heat treatment is used to kill non-sporing bacteria and most yeasts and moulds. Although most or all spore-forming bacteria will be killed, spores will remain, as they are much more heat-resistant. Spores of the pathogenic Clostridium botulinum and Clostridium perfringens will survive. C. perfringens is effectively controlled by refrigeration, but C. botulinum must be controlled as it causes the potentially fatal disease, botulism, if sufficient toxin is allowed to develop in the product and is then consumed (Table 6.1). Bacillus spp. are probably not a serious threat as they are aerobic (Leadbetter, 1989). 

The co-culture consisted of two gram-positive, strictly anaerobic rods with heat-resistant endospores, apparently a Clostridium species and another similar coccoid bacterium. The culture grew on plates as a mixture of white and clear colonies, but both colony types contained co-cultures of the two cell types, as shown by scanning electron micrographs (SEM) of a white colony, and transmission electron micrographs (TEM) of a clear colony. As described below, co-cultures obtained from stocks derived from both the white and clear... 

Through electron microscope examinations, LJP-1 originally appeared to be a co-culture of two heat-resistant bacteria, a rod and a coccus. It was purified to a single strain, the rod-shaped bacterium, by heat treatment at 82°C for 20 min. The bacterium was a gram-positive (negative in older cultures), non-motile, strictly anaerobic rod with heat-stable spores, identified as a Clostridium strain (8). 

Food may be contaminated with toxins produced by bacteria, such as botulinum toxin. This is produced by the bacterium Clostridium botulinum and is one of the two most potent toxins known to humans (the other being ricin). As little as one hundred-millionth of a gram (1 X 10-8 g) of the toxin would be lethal for a human. Fortunately, the toxin is destroyed by heat so that cooked food is unlikely to be contaminated (although the bacterial spores are quite resistant). The bacteria grow in the absence of air (they are anaerobic), and consequently, the foodstuffs most likely to be contaminated are those that are bottled or canned and eaten without cooking, for example, raw or lightly cooked fish. 

Sporulation. Bacteria of the genera Bacillus and Clostridium form metabolically inert spores when deprived of adequate nutrients (Fig. 32-1L83,203,20 1 Bacterial spores are remarkably resistant to heat and can survive boiling water for prolonged periods. 

The stationary phase classically represents the stage at which the rate of cell division equals the rate of cell death, so the number of viable cells remains constant. This phase usually occurs when the cell concentration reaches a sufficient size that some property of the environment restricts growth rate, often a nutrient where phytoplankton are concerned. Physiological changes can occur, including adaptations that promote cell survival through periods of limited growth. Some bacteria (e.g. species of Bacillus and Clostridium) form endospores, which represent a dormant state that is resistant towards levels of desiccation, heat, chemical or radiation exposure that would prove lethal to the non-endospore forms. 

In a study of the resistance of the spores of Clostridium botulinum to moist heat, Tanner gives the following summary and conclusions ... 

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