Agar, a polysaccharide extracted from red seaweed, provides the foundation for growing microorganisms in a laboratory setting. This gelling agent is mixed with specific nutrients to create a solid culture medium, which is then poured into a shallow dish to create an agar plate. The solidified surface allows for the isolation and growth of bacteria, fungi, or other microbes, making it a fundamental tool in microbiology. Agar remains firm at typical incubation temperatures, which is an advantage for cultivating organisms that thrive in warmer conditions.
The Standard Setting Process and Timeline
The process of solidification begins once the molten agar medium, typically held around 50°C for pouring, is transferred into the petri dish. Agar begins to solidify when its temperature drops below approximately 40 to 42°C, a range known as the gelling point. The time required to cool from the pouring temperature to this gelling point dictates the overall setting time.
Under standard laboratory conditions (pouring 15 to 25 milliliters into a 100-millimeter dish at 20 to 22°C), complete solidification usually takes about 20 to 40 minutes. During this period, the plates must remain undisturbed on a level surface to ensure an even, flat layer of medium. Although the agar appears solid within minutes of reaching the gelling temperature, allowing the full 20 to 40 minutes ensures the medium achieves maximum firmness before handling.
Variables That Alter the Setting Rate
The standard 20 to 40-minute timeline depends on several physical factors. One major influence is the volume of media poured, which affects the surface area to volume ratio. Pouring a thicker layer of agar substantially increases the time required for cooling because heat must dissipate from a greater mass through a constant surface area.
The ambient temperature of the environment also plays a large role. Setting the plates in a cold room (typically 4°C) accelerates the cooling process, potentially reducing the setting time to 10 to 15 minutes. Conversely, attempting to set plates in a warm climate or near a heat source will dramatically slow cooling, potentially causing the agar to take over an hour or fail to solidify properly.
Another factor is the concentration of the agar itself, though most standard microbiological media use about 1.5% by weight. While a higher concentration (such as 2.0%) does not change the gelling temperature, it can lead to a slightly faster initial onset of gelling and produces a firmer final gel. The concentration primarily affects the texture and strength of the final medium.
Ensuring Plate Readiness After Solidification
Achieving a solid gel is only the first step; the newly set plates require specific handling before use. Immediately after solidification, the petri dishes must be inverted (turned upside down). This practice prevents water vapor, which condenses on the cooler lid, from dripping onto the agar surface and creating wet spots.
The plates then need a period of drying, often called “curing,” to reduce excess surface moisture. If too much moisture remains, it can promote the spreading of colonies across the surface after inoculation, making isolation difficult. A typical curing process involves leaving the inverted plates slightly ajar in a sterile environment for 10 to 20 minutes at room temperature, or up to several hours in a laminar flow hood.
For long-term preservation, the cured plates should be sealed (often by stacking them inverted inside a sealed bag) and stored in a refrigerator or cold room at approximately 4°C. This cold storage slows the growth of potential contaminants and minimizes water loss through evaporation. Most prepared agar plates maintain their quality for several weeks to a few months when stored properly, though shelf life varies depending on the media type.