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Method taken from: FDA, Center for Food Safety and Applied Nutrition January 2001, Chapter 3, Aerobic Plate Count
The spiral plate count (SPLC) method for microorganisms in milk, foods, and cosmetics is an official method of the APHA (2) and the AOAC (3). In this method, a mechanical plater inoculates a rotating agar plate with liquid sample. The sample volume dispensed decreases as the dispensing stylus moves from the center to the edge of the rotating plate. The microbial concentration is determined by counting the colonies on a part of the petri dish where they are easily countable and dividing this count by the appropriate volume.
Pre-freeze step (Not necessary in Vacuum concentration)
The Pre-freeze step occurs when the material temperature is taken from normal room temperature and pressures to a point below the freezing point of the sample. Any free non “bound” liquid and aqueous material contained within the sample moves from its liquid phase to a solid phase below the freezing point of the product. This is done by removal of energy from the material - freezing. Typical aqueous solutions will be Eutectic forming, which means that there will be an increase in the concentration of the solution as the product temperature falls and water is frozen.
Water Activity Definitions
Wikipedia describes Water activity as an element amount used to symbolize the energy status of the water in a system. It is determent as the vapor pressure of water above a sample divided by that of pure water at the same temperature, consequently, pure distilled water has a water activity of exactly one. It is extensively used in food science as a easy, clear-cut measure of the dryness of food, foods characteristically have an optimum water activity at which they have the greatest shelf life. Water activity is also used in the pharmaceutical production and chemicals for moisture sensitive products.