Few sampling systems supplied with laboratory fermenters are easily modified for unattended aseptic sampling. Most commercial sampling systems are manually operated and are either hooded samplers or bottom-harvesting valves. The former operate on a vacuum principle and are widely accepted as being safe and simple for culture sampling. They can be sterilized either separately or with the culture vessel. During operation, aseptic conditions are maintained by a break in the liquid stream, along with the hood on the exit line. The latter-bottom harvesting valves -are usually found on larger (2-25 L) fermentation vessels. They use in situ steam sterilizable diaphragm or piston valves. These valves are easily sterilized between sampling times and are very reliable.During studies of batch fermentation kinetics of yeast cultures, neither type of standard sampling system was effective. Hooded samplers were ineffective when used with an airlift fermenter using high air flow rates. Bottom harvesting valves regularly resulted in excessive sample volumes. Moreover, manual operation of these sampling systems was inconvenient for short fermentation runs (less than 24 h). Therefore, several systems were tested to develop an automatic aseptic sampler capable of removing small (5-10 mL) culture samples at regular intervals during a fermentation. The system described here has proven to be a valuable tool in our overall research program. MATERIALSThe centerpiece of the automatic sampling system is a Sartorius three-way valve (SM 16639). It is autoclavable, and sold as a rapid syringe refill unit. As shown in a crosssectional view in Figure 1, this valve is fitted with luer-lok connections on both the syringe and effluent ports and a barbed tubing connector on the sampling port. The unit operates using two rubber check-valves, with the exit valve held in place by a stainless-steel spring. For the present application, the valve was modified in two ways: the rubber check-valves were replaced with machined teflon valves, resulting in freer operation of the valves, even after repeated usage and sterilization cycles; the hose barb was machined off to give a 3.175 mm 0.d. tube which was connected to 2) A Gilson model 201 programmable fraction collector with a 27-position multipurpose rack. Glass funnels (13 rnm O.D.) with silicon tubing attached were placed in the collection rack. The liquid samples could then be directed, by gravity, to sampler tubes in a refrigerated tube holder. Using the fraction collector in the manual mode permitted the serial
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