1. What is the significance of the Lag Phase Duration (LPD)?
The Lag Phase is the time required for the cell population to adjust to the food environment and begin growth.
The Lag Phase is the most unpredictable aspect for a growth model, compared to the Generation Time. This is because the Lag Phase will vary depending on the previous "history" of the organism. For example, the Lag Phase Duration (LPD) of bacteria grown at 98°F in culture medium and then transferred to ground beef at 50°F will be different than the LPD of bacteria grown at 70°F. This is because the previous environment of the bacteria will result in different cellular changes that are necessary before it can again grow in a different environment.
The LPD represents a distribution of lag times for individual cells in the matrix. As you notice on most growth curves, this produces a concave-shaped curve between the lag phase and the growth phase. Consequently, a portion of this curvature is included in the calculated lag phase and a portion is included in the growth phase.
2. How do I use the Lag and No Lag options in the Growth Models?
Selecting the "Lag" option will result in a prediction of the Lag Phase Duration based on the experimental data for the model.
Selecting "No Lag" will remove the period of time for the calculated Lag Phase, and will begin predictions with growth of the bacteria. Note - a portion of the curvature between the LPD and the Growth Phase is included in the Growth Phase. Therefore, the starting level that appears in the chart and table will not be the same as the value that you set as the Initial Level. The time calculated to reach the Level of Concern will use the Initial Level, although it will not appear in the table or chart.