Do you have any good links/posts on getting the correct PPM per gallon of wort? Micron size vs. rate vs. volume?
— Robert French
As homebrewer’s, it’s nice that we can measure many of the important variables in our process. We can measure the density of our wort with either a hydrometer or a refractometer. We can measure temperature with a thermometer. We can measure the pH of wort or beer with a pH meter. Unfortunately, there are no inexpensive ways to measure a few key variables. Most homebrewers, for example, do not measure the amount of alcohol in their beer. They estimate the percentage by volume via a calculation. Likewise, the level of carbonation in our beer may be estimated either via a temperature and pressure table for keggers, or by adding the appropriate amount of priming sugar for those who bottle condition their beer. Arguably more important, however, is the amount of oxygen in our worts before we pitch the yeast. For most fermentations, an oxygen level around 8 ppm is desired. However, some strains of yeast may respond better to slightly higher levels of oxygen.
The amount of oxygen in a liquid can be measured with the dissolved oxygen (DO) meter. These devices are relatively expensive. However, the prices have come down substantially in the past few years. (Most are made for measuring oxygen content in aquariums. The cheapest are under 200 bucks.) Still, it is not common for homebrewers to possess these. Instead, we try to estimate the amount of oxygen introduced into our worts by simpler means. Frequently, you’ll hear advice like “use an aquarium pump for five minutes” or “oxygenate for one minute.” Instructions such as these leave out the flow rate of oxygen, the pore size of the aeration stone, the volume of wort being aerated, and other variables such as temperature. They rely on the fact that the airflow rates, wort volumes, aeration stone pore sizes, and wort temperatures are — in most cases — going to be within the same ballpark. (For example, 2 micron and 0.5 micron sintered stainless steel aeration stones are commonly used in homebrewing for aerating wort. Incidentally, sintering is the process of forming a solid mass from a powder. This is usually done under pressure and at high temperatures. By changing the pressure and temperature a metal is sintered at, the average pore size can be manipulated.) Even though these instructions are fairly vague, they work remarkably well in practice in a wide range of homebrewing situations.
But, what if we want to get more specific? Is there a way of calculating oxygen levels (in ppm) from a specific flow rate, through a specific pore size, into wort of a given volume and temperature? Simply knowing the flow rate of the gas (either air or oxygen), you could calculate the amount of oxygen added to a volume of wort. [Air is mostly nitrogen (78%) and oxygen (21%), with the remaining being argon (0.93%), carbon dioxide (0.035%, and rising), and a variety of less abundant gases.] However, you would not know how much of that gas simply bubbled out of solution seconds after it was added. If someone were to do a series of experiments that measured all the important variables — gas flow rate, aeration stone pore size, work volume, and work temperature — and then measured dissolved oxygen levels, they could set up a table (or derive an equation) that estimated oxygen concentration in wort from these variables. To the best of my knowledge, however, that work hasn’t been done yet.
So, for the time being, homebrewers will either need to invest in a dissolved oxygen meter, or rely on secondary ways of determining if their work is oxygenated adequately. First and foremost, with properly pitched worts (held at the proper temperature), the amount of time it takes for the fermentation to start is a major clue to the effectiveness of your aeration. If fermentation starts within a reasonable amount of time, then you can infer that your work received a sufficient amount of aeration. What constitutes a reasonable time depends mostly on your pitching rate, wort temperature, and yeast strain. For most ales, you should see the start of fermentation in 8 to 12 hours, although you generally do not need to worry as long as fermentation starts within 24 hours. (And, with some yeast strains, fermentation may start even sooner.) If your start times are frequently long, and you’ve pitched an adequate amount of yeast, increasing the amount of time you aerate will likely help. Keeping track of all the relevant variables in your brewing notebook will help you figure out and an aeration time — for a given flow rate, pore size, wort temperature, and yeast strain — that works for you.