Microorganisms can multiply very quickly under optimum conditions. The time it takes for a microbial cell to reproduce is called the generation, or doubling, time. The type of microorganism, type of food, and the storage conditions determine the generation time. At optimum conditions, most microbial generation times range from eight to 45 minutes. At refrigeration temperatures, generation times can be slowed to one to 10 days, if the cells reproduce at all.
For spoilage organisms, a slime or odor may begin to appear at one to 10 million microbial cells per gram of food. For pathogens, the hazardous dose varies. Clostridium perfringens requires 10 to 100 million cells per gram to cause disease, whereas 10 Listeria monocytogenes cells can cause disease. Because the identity of the initial contaminants is not known, it is important to restrict all microbial growth as much as possible.
Example:
The generation time of a bacterium is 20 minutes at 98°F. One cell will generate more than one million cells in less than seven hours at this temperature (Table 1, page 58).
Table 1 illustrates the importance of initial bacterial numbers. The initial contamination level affects how rapidly the food will spoil or become hazardous to humans. The table shows two initial contamination levels one bacterial cell and 1,000 bacterial cells. The more microbial cells that are initially present, the less time it takes to reach spoilage or hazardous levels. Proper sanitation of food contact surfaces is essential to keep contamination levels low.
| Table 1. Effect of Time and Contamination Level on Cell Numbers of a Bacterium With a 20-Minute Generation Time at 98°F. | ||
|
Time (hours:minutes) |
Number of Microbial Cells (low contamination level) |
Number of Microbial Cells (high contamination level) |
| 0 | 1 | 1,000 |
| 0:20 | 2 | 2,000 |
| 0:40 | 4 | 4,000 |
| 1:00 | 8 | 8,000 |
| 1:20 | 16 | 16,000 |
| 1:40 | 32 | 32,000 |
| 2:00 | 64 | 64,000 |
| 2:20 | 128 | 128,000 |
| 2:40 | 256 | 254,000 |
| 3:00 | 1,024 | 1,016,000 |
| 3:40 | 2,048 | |
| 4:00 | 4,096 | |
| 4:20 | 8,192 | |
| 4:40 | 16,384 | |
| 5:00 | 32,768 | |
| 5:20 | 65,536 | |
| 5:40 | 131,072 | |
| 6:00 | 262,144 | |
| 6:20 | 524,288 | |
| 6:40 | 1,048,576 | |
| Food storage conditions, such as temperature, can dramatically impact microbial generation time. Microorganisms can grow from 40 to 140°F, though most grow best at 65 to 100°F. At temperatures below each microorganism's optimum, the generation time is longer, so it takes more time to reach spoilage or hazardous levels. | ||
| Table 2 shows examples of bacterial cell numbers over time at three temperatures. Refrigerated storage of sensitive foods is essential to keep microbial levels low. | ||
| Table 2: Effect of Temperature on Cell Numbers Over Time for One Bacterium. | |||
|
Time (hours) |
Number of cells (at 98°F, generation time = 20 minutes) |
Number of cells (at 70°F, generation time = 1 hour) |
Number of cells (at 36°F, generation time = 24 hours) |
| 0 | 1 | 1 | 1 |
| 1:00 | 8 | 2 | |
| 2:00 | 64 | 4 | |
| 3:00 | 512 | 8 | |
| 4:00 | 4,096 | 16 | |
| 5:00 | 32,768 | 32 | |
| 6:00 | 262,144 | 64 | |
| 24:00 | 4,722,366,483,000,000,000,000 | 16,777,216 | 2 |
| (in theory only; practically, numbers will not exceed 10,000,000,000) | |||