Does The Erlenmeyer Flask Belong in Bioprocessing?
Throughout the evolution of biological lab practices, we have made some great advancements and much of the equipment we use has changed to keep up with the times. But for some strange reason, the Erlenmeyer Flask, a flat bottom flask invented by German chemist Emil Erlenmeyer in 1861 has stayed with us far longer than it should have.
Described as an inverted conical flask with a long straight neck, the ubiquitous glassware that bears his name was perfectly designed for his needs. It just so happened that those needs were the mixing and heating of liquids. The long straight neck of the flask allowed for the precipitation of vapor, which in his chemical synthesis experiments proved quite useful. Dr. Erlenmeyer used his flasks in the chemical synthesis of tyrosine, guanidine, creatine, and creatinine, and the upper section of the flask was great at reducing solvent loss, making them especially appropriate for recrystallization. Microbiologists however have something important to consider that chemists don’t. Our science is living. The reactions we research and the proteins we produce happen in living systems. We must therefore consider parameters like metabolic activity, glucose consumption, pH, and oxygen uptake rate. The oxygen transfer rate in Erlenmeyer flasks, however, depends on an incredible number of variables from fill volume to shaker speed, and a variety of different flasks based on the Erlenmeyer design exist. With wide or narrow necks, with or without baffles, shorter, taller, or more squat, they all face many of the same problems. They are variations of a device designed for something else. If cells have a greater oxygen demand, there is a greater need for aeration. In Erlenmeyer flasks this means lower fill volumes. In a 500mL flask the fill volume will likely never exceed 100mL, and most often 1/10 of the fill volume is all that is used. This alone does not solve all of the problems with dissolved oxygen and for long fermentations with high oxygen demands the rotational shaking speed is increased as much as the shear stress allows. Baffles have been added to improve gas exchange and they work to a degree but they’re not adequate solutions when there is a high oxygen demand. Increased shaker speed in baffled flasks can lead to a phenomenon known as out-of-phase. This is where the media starts to shake out of sync with the rotation of the flask, and the culture quickly stops growing. Baffled flasks also have to be watched more closely because foam can build up in the culture and increased shear stress on the cells is often fatal.
Biologists have made do with the ubiquitous flask designed by a chemist for precipitation experiments, and despite its faults that flask has been at the heart of countless scientific discoveries for the last century including the fights against HIV, cancer, and even COVID-19. They have been used in research of polio, measles, whooping cough, cholera, and countless others not because they are ideal, but because they were already there. Imagine all the time and money that could have been saved if scientists were using flasks specifically designed for biological cell culture. When we look at the founding fathers of microbiology, Antoni van Leeuwenhoek, Robert Hooke, or Louis Pasteur, it’s important to remember that their glassware was handblown, mostly round bottomed, and specifically designed for their needs. A visit to the Pasteur Museum in France will not feature an Erlenmeyer flask and the reason is not availability. It’s because they weren’t the right tools for the job. Simply changing the size or shape of an Erlenmeyer flask in order to make it work for cell culture is akin to fixing a dump truck to race in Formula 1. It’s just not the right tool for the job, and it’s time we all admit it.