When infusing oils there are two main considerations, the taste of the oil and the taste of the flavoring. If you only want the flavoring to stand out, you should use a neutral oil like canola or grapeseed oil which do not bring their own flavors to the infusion. If you want a specific flavor, you can turn to other oils that have distinct characteristics, such as olive oil, walnut oil, or sesame oil.
For this recipe I want to focus on the subtle flavors of the rosemary and sage so I call for a neutral oil. The infusion process is very easy, simply combine the oil and herbs, heat in a sous vide machine for several hours, then cool and store. The time and temperature used will depend on the flavoring agents used but typically ranges from 131°F to 176°F (55°C to 80°C) for 1 to 5 hours. The more herbs and other flavoring agents used the stronger the end infusion will be.
I often use this infused oil to finish fish dishes or to add extra herb notes to vinaigrettes and other sauces.
Preheat a water bath to 131°F (55°C).
Combine the oil, rosemary, and sage in a sous vide bag or mason jar then seal. Infuse in the water bath for 1 to 2 hours.
Prepare an ice bath with 1/2 ice and 1/2 water. Remove the bag or mason jar from the water bath and place in the ice bath for 15 to 20 minutes. Strain the oil and store in a sealed container. It will last for a week or two in the refrigerator.
With a whipping siphon you can easily turn this infused oil into a foam. Place the strained oil in a pot and place on medium heat. Add 5% glycerin flakes, about 22 grams, and stir until they have melted. Remove the pot from the heat and carefully pour into a heat resistant whipping siphon. If you prefer, you can let the oil cool to room temperature before pouring.
Seal and charge the whipping siphon then refrigerate it for several hours. Once cold, the oil will be ready to dispense but it will last in the refrigerator for several days.