Heat can be applied as either dry heat or wet heat. Wet heat is a more effective means of killing microbiological organisms than dry regarding temperatures and time applied. According to Jay et al (2005), heat resistance is decreased with increased humidity, moisture or water activity. Additionally, there are many other factors that affect the heat resistance or sensitivity of microorganisms. Salts have a variable effect, both positive and negative, on heat sensitivity. Carbohydrates can increase heat resistance by decreasing the water activity of cells. In descending order of impact, glucose, fructose and glycerol are impact carbohydrates. pH values above or below optimum pH of growth will lower microorganism heat resistance. Proteins and colloidal particles act as protective agents and increase heat resistance. Large populations or biofilms can increase heat resistance, possibly due to the excretion of protective compounds. Microorganisms are more heat sensitive during the log phase then during the stationary phase of the life cycle. Growth temperature can also have an impact through natural selection of more heat resistant strains when grown under higher temperatures of the opposite when grown under cooler temperatures. Combination therapies with compounds such as SO2 can also have a compounding on heat sensitivity.
The general hierarchy of heat resistance is essentially a function of optimum growth temperatures. Thermophiles are the most heat resistant, followed by mesophils and psychrophils. Sporeforming bacteria are more heat resistant than non-sporeforming bacteria. Gram positive bacteria are more heat resistant than gram negative bacteria and cocci more so than rods. Yeast ascospores are more heat resistant than vegetative yeast and asexual spores more so than mold mycelia. Sclerotia are the most heat resistant of these fungal types.
There are numerous concepts important to the thermal destruction of microorganisms. Thermal Death Time (TDT) is the time necessary to kill a given number of organisms and is established by maintaining constant temperature while time is determined. D values represent the time required to kill 90% of microorganisms at a given temperature. z values incorporate TDT and offer relative resistances to various temperatures. z values can then be used to determine varying temperature and times to achieve the same killing capacity. For instance, with a z = 8, either 0.35 minutes at 148oC or 35 minutes at 132oC can be used. F values represent the time needed to kill a microorganism population at 121.1oC (Jay et al (2005).
Application in Wine Microbiology:
There is little worthwhile use of heat to deal with microorganisms in wine, as there is an adverse effect on the flavor profile and color (Boulton et al 1996). However, historically heat has been used in pasteurization processes to neutralize spoilage organisms (Splittstoesser et al 1975; Pilone 1953). Splittstoesser et al (1975) did show that yeast were generally more heat resistant than bacteria in wine media. More recently, Couto et al (2005) showed that wine heated to 32.5oC experienced significant inactivation of Dekkera/Brettanomyces. There was no sensory data given. Heat can be, and is, used on juices, however this is primarily done to denature proteins and not necessarily used to control microorganisms.
The bottling line must be sterilized using heat. Hot water or steam is sent through filters and their holders, all lines leading to filler, filler bowl and all filler spouts. It is recommended that hot water (entering at 82oC and exiting at 72oC) or live steam (with 2 cm of invisible vapor) flows out of the filler spouts for at least 20 minutes (Boulton et al 1996). Neradt (1982) showed that the incidence of bottle contamination was significantly reduced with the use of increased sterilizing practices, including heat sterilization. In addition to bottling line sterilization, heat can be used to clean tanks, barrels, tools, and any other surfaces that may harbor microorganisms.
- Boulton, R., V. Singleton, L. Bisson, and R. Kunkee. 1996. Principles and Practices of Winemaking. Chapman & Hall, New York.
- Couto, J.A., F. Neves, F. Campos, T. Hogg, 2005. Thermal inactivation of the wine spoilage yeastsDekkera/Brettanomyces. International Journal of Food Microbiology 104: 337– 344.
- Jay, J.M., M.J. Loessner, D.A. Golden, 2005. Modern Food Microbiology 7th edition. Springer Scientific and Business Media, Inc., New York.
- Neradt,. Fritz, 1982. Sources of reinfections during cold-sterile bottling of wine. American Journal of Enology and Viticulture, 33: 140-144.
- Pilone, Frank J.,1953. The role of Pasteurizatin in the stabilization and clarification of wines. American Journal of Enology and Viticulture, 4: 77 – 83.
- Splittstoesser, D.F., L.L. Lienk, M. Wilkison and J.R. Stamer, 1975. Influence of Wine Composition on the Heat Resistance of Potential Spoilage Organisms. Applied Microbiology, 369-373.