Two yogurt strains to make the yogurt of your choice. Though they differ a bit in the lactic strains, adjusting temperature and ripening time can also affect flavor and texture.
Heat the milk to 180-185f (82-85c), then cool to inoculation temperature. Usually your yogurt will taste nominal in 4-8 hours. To make Greek Yogurt, strain the yogurt through a fine
mesh food grade nylon bag.

YC-180 50 dose packet can be used with 70 gal. (250 liters) milk.

The culture will produce yoghurt with medium flavor, high viscosity and medium postacidification.
Suitable for cup set, stirred and drinking yogurt. Very mild flavor, smooth and heavy body. No post acidification. This is a great blend, use an appropiate amount to make one gallon or less.

http://www.thecheesemaker.com/pdf/Yogurt_PI YC-180 100249.pdf
http://www.thecheesemaker.com/pdf/kosher_cert_yogurt_YC-180_2013.pdf

 YF-L812 (thick set) 50 dose packet can be used with 70 gal. (250 liters) milk.

The culture will produce yoghurt with very mild flavor, extra high viscosity and very
low post-acidification. Suitable for cup set, stirred and drinking yogurt. This is a great blend, use an appropiate amount to make one gallon or less

Industry:
Remove culture from the freezer just prior to use. Sanitize the top of the pouch with
chlorine. Open the pouch and pour the freeze-dried granules directly into the
pasteurized product using slow agitation. Agitate the mixture for 10-15 minutes to
distribute the culture evenly. The recommended incubation temperature is 35-45°C
(95-113°F).

Hobbyist:
Prior to removing a small amount of the culture 1/4-1/2 tsp**(1.2-2.4ml) for 1-2 liters of milk, it is important to shake the culture in the bag so the various lactic strains are well mixed.
Then, using a dry sanitized small spoon, remove the culture and distribute onto the warmed milk to inoculate at (95-113°F) 35-45°C. Or follow the recipe here or in recipes.
Do not allow the culture to come into contact with water, humidity. Fold the foil packet up to help seal it and store in two plastic Zip Lock or similar brand plastic bags in the freezer.