D and z values of microorganisms of  importance in foods

The D and z values of several microorganisms in foods are described in Table 1.

Table 1  D and z values of several microorganisms of importance in foods.

Vegetative cells

The vegetative cells of microorganisms are sensitive to relatively low temperatures. Keeping them at 65.5ºC is already sufficient to destroy molds, yeasts and bacteria with D65.5ºC not greater than 2–3 min, in most cases.

 Heat-resistant mold spores

Some filamentous fungi produce heat-resistant spores. These molds are called heat-resistant, including the species Byssochlamys fulva, Byssochlamys nivea, Neosartoria fischeri, Talaromyces flavus, Talaromyces bacillisporus and Eupenicillium brefeldianum. The spores of B. fulva  have a D90ºC value varying from 1 to 12 minutes (Bayne & Michener, 1979) and a z value between 6 and 7ºC (King  et al., 1969). The heat resistance of  B nivea is slightly lower, with a D88ºC of 0.75 to 0.8 min and a z value between 6 and 7ºC (Casella et al., 1990). The heat resistance of  N. fischeri is similar to that of  B. fulva (Splittstoesser & Splittstoesser, 1977).

 Bacterial spores

Some bacteria are also capable of producing spores,. Bacterial spores are different from heat-resistant mold spores in that they are not structures of reproduction, but rather structures of resistance. They are not metabolically active, as are vegetative cells. They remain in a state of dormancy and, under favorable conditions, germinate and originate new vegetative cells. The heat resistance of bacterial spores varies with the species. Some bacterial species produce spores with heat resistance comparable to the resistance of mold spores, but others produce much more resistant spores, requiring temperatures above 100ºC to be destroyed.

 Strictly thermophilic aerobic spore forming bacteria: These are aerobic bacteria that grow well at high temperatures (optimal in the 55ºC range or higher) and do not grow at temperatures below 37ºC. The typical species of this group in foods is  Geobacillus stearothermophilus, which produces spores with D121.1ºC = 4–5 min.  G. stearothermophilus does not grow under acid conditions (minimum pH of 5.5).  Alicyclobacillus acidocaldarius is also a strictly thermophile, but does not grow at neutral pH or above 6.0 (strictly acidophilic) and its presence in foods is less common. The  A. acidocaldarius D120ºC value in water, citrate-phosphate buffer and orange juice is 0.1 min.

 Strictly thermophilic anaerobic spore forming bacteria: These are anaerobic bacteria that grow well at high temperatures (optimal in the 55ºC range or above) and do not grow at temperatures below 37ºC. The typical species of this group in foods are Thermoanaerobacterium thermosaccharolyticum (D121.1ºC  = 3 a 4 min, minimum pH of 4.7) and  Desulfotomaculum nigrificans (D121.1ºC  = 2 to 3 min, minimum pH 6.2). Both species produce spores that are among the most heat-resistant and do not grow under acidic conditions.

 Facultative thermophilic aerobic spore forming bacteria: These are aerobic bacteria which grow well at 55ºC but, contrary to strictly thermophiles, they also grow at temperatures below 37ºC. The typical species of this group in foods are  Bacillus coagulans and  Alicyclobacillus acidoterrestris, which produce spores considerably less resistant than those of strictly thermophiles. B. coagulans is a facultative anaerobe and has a D121.1ºC value between 0.01 and 0.07 min. Aciduric, grows well at neutral or slightly acidic pH (4.0 or higher).  A. acidoterrestris has a D95ºC value between 2.5 and 8.7 min and a z value between 7.2 and 11.3ºC. Aerobic, but some strains may be facultative anaerobic. Acidophilic, the optimal pH for growth is between 3.5 and 5.0 and generally does not grow at pH values above 6.0.

Mesophilic aerobic spore forming bacteria: The Compendium (Stevenson & Segner, 2001) defines this group as aerobic bacteria that grow better at 35°C than at 55°C, because some strains are capable of growing at temperatures above 50°C. The typical species belong to the genera  Bacillus (B. licheniformis,  B. cereus) and Paenibacillus (P. macerans, P. polymyxa), which produce spores less resistant than those of the thermophilic bacteria.  Sporolactobacillus,  Brevibacillus and  Virgibacillus also fit this definition, but their association with sterilized foods is little documented.

 Mesophilic anaerobic spore forming bacteria: The typical species of these groups are of the genus  Clostridium.  Proteolytic (putrefactive) clostridia ( C. sporogenes, C. bifermentans,  C. putrefasciens,  C. hystolyticum and  C. botulinum types A and B) produce spores of higher heat-resistance levels.  Non-proteolytic clostridia ( saccharolytic) ( C. butyricum,  C. pasteurianum,  C. tyrobutyricum,  C. beijerinckii and  C. acetobutylicum), which are capable of growing at pH 4.2–4.4, produce spores that are less heat-resistant than the spores of the putrefactive clostridia.

References

Bayne, H.G. & Michener, H.D. (1979) Heat resistance of Byssochlamys  ascospores.  Applied and Environmental Microbiology, 37, pp. 449–453.

Casella, M.L.A., Matasci, F & Schmidt-Lorenz, W. (1990) Influence of age, growth medium, and temperature on heat resistance of  Byssochlamys nivea ascospores.  Lebensmittel-Wissenschaft & Technologie, 23, pp. 404–411.

King, A.D., Michener, H.D. & Ito, K.A. (1969) Control of Byssochlamys  and related heat-resistant fungi in grape products. Applied Microbiology, 18, pp. 166–173.

Silva, N.D .; Taniwaki, M.H. ; Junqueira, V.C.A.;  Silveira, N.F.A. , Nasdcimento , M.D.D. and Gomes ,R.A.R .(2013) . Microbiological examination methods of food and water a laboratory Manual. Institute of Food Technology – ITAL, Campinas, SP, Brazil .

Splittstoesser, D.F. & Splittstoesser, C.M. (1977) Ascospores of  Byssochlamys fulva compared with those of heat resistant Aspergillus. Journal of Food Science, 42, pp. 685–688.