There are several different ways to infuse flavor, but we're going to stick with the basics. The first is heated maceration, the second is maceration, the third is decoction, then percolation and, finally, instant infusion. Each process will highlight different qualities in your botanical ingredients. Classically, to infuse flavor into a spirit drink, the crucial ingredient is time.
Combine botanical ingredients and liquor in a container, close it, label it, and wait for days or weeks while the soluble flavoring compounds filter out of the plant and enter the liquor. An infusion is both the process and the result of extracting flavors from food to convert them into a liquid. This is usually done by soaking food in the liquid for a long period of time. The liquid is usually water, alcohol, oil, or vinegar and can be cold or hot.
The infusion process also works both ways, and foods take on some of the flavors of the liquid. Now they are becoming more and more frequent, as many talented chefs and waiters are creating personalized infusions for use in both food and cocktails. However, there is a fine line between infusing and macerating, as many people say that maceration involves finely grinding botanical ingredients to increase their surface area and extract more flavor. The higher the temperature, the faster the infusion will be made, although bitter components are usually extracted faster as well.
This technique was popularized by Dave Arnold, from the French Culinary Institute, and is often referred to as rapid nitrous infusion. When the pressure is released, the liquid is removed from the solids, which draws out more flavor and leaves some of its flavor. The second method of hot infusion involves combining the liquid and the flavoring agents in a pot and bringing them to a simmer. If you are going to make this reverse infusion, make sure that the liquid you use to flavor the solid is strong enough in terms of flavor.
In a basil infusion, you want it to be the freshest basil experience possible, and the pressure process is the way to achieve that. It is a technique taken from perfumery and is used to accelerate the maturation of a perfume or essence. And prolonged infusions are indiscriminate in terms of what they extract; therefore, care and experience are necessary to avoid flavors that are over-extracted or unpleasantly unbalanced. The longer the liquid is infused and the hotter it is, the stronger and more bitter the resulting infusion will be.
We said that heat can speed up infusion time (significantly), and normally the technique used to do so is Sous Vide. Honestly, it's very unlikely that he let the infusion work for months before filtering it. Infusions are probably one of the most important practices used in bars in the last ten years, and are increasingly being explored as waiters learn the use of techniques and equipment from modernist cuisine and other disciplines. The amount of time the infusion is heated affects both the intensity of the infusion and the flavor profile.