While there is a body of knowledge, both written and oral, on essential-oil-distillation, there is much less knowledge about the co-production hydrosols. My own conversation with distillers reveal that making hydrosols differ quite a lot, depending on both the size of the still and the quantity and type of material being distilled.
It also varies from year to year, just as the parameters for oil production vary depending on the amount of rainfall, length of season, hours of sun,mean annual temperature, geographical location,and so forth. In a wet year, such as 1998, the plants contain more water, and this greatly affects both the oils and the hydrosols. In fact, the 1998 crop of hydrosols was very prone to bloom, had significant variations in pH, and had the shortest life spans of any I have dealt with thus far.
There are some basic rules for collecting hydrosols. Generally the still is allowed to run for a period before distillate waters begin to be collected. This is a bit complex to explain. First, the steam may actually run for some time before distillate begins to flow. In a still that holds five hundred kilograms of plant material, it may take from as little as thirty minutes to as long as two hours before the steam travels through the charge (plant material packed into the still) and makes it's way to the condenser vessel known as a Florentine flask. As was explained earlier. The Florintine allows the hydrosol to flow out, once the flask is full, while the precious essential oils remain within the flask. A neat bit of design.
When the distillate water begins to flow from the condenser, it is rare for there to be any oil in the few minutes. A small still with a high-oil-volume charge may produce oil within one to two minutes after flow commences; a larger still or a lower-oil volume plant material may not produce oil for quite some time. Some distillers believe that the hydrosol should be collected only from the moment the oil droplets start appearing. Others believe you should take it from the beginning of the run.
Then there is the question of when to stop collecting the distillate. It is widely agreed that you do not collect all the hydrosol from a run. The chemicals that come over in the oil at the late stages of distillation are primarily non-water-soluble, large, and heavy molecules, as these would not have an appreciable water-soluble component, the hydrosol at this point is becoming more and more water-like and could in fact dilute the hydrosol collected from the early stages.
But what is the cut off point? Here the alchemy and knowledge of the distiller are trump cards.
However, there is a scientific component based on the changing chemistry of the distillates as the process happens and the pH changes. As a rule of thumb, no more than two-thirds of the distillate waters, frequently as little as 20 percent, is generally kept for use as hydrolate; the rest is allowed to flow away or is returned to the fields and the plants that were its source.
Reference: Hydrosols: The Next Aromatherapy: Suzanne Katty