The best yogurt starter can be a small amount kept aside from the previous batch you have made.
The starter can define the taste, texture and consistency of the homemade yogurt. It affects the tang based on the bacteria that it contains. You should always use one culture and not mix them.
You can buy starters in the market as well which you give you more options e.g. non dairy or skimmed milk.
The main classification of starters are:
1.) Direct set: These can be used once and set in 8-12 hours
2.) Reusable: These take around 16 hours for the first time and on being resued the time to ferment reduces to less than 8 hours.
The types also are defined by temperatures in which they ferment well. The thermophilic culture needs elevated temperature whereas mesophilic cultures can set at room temperatures and will not need a yogurt maker.
There are different types available based on flavor as well which includes: Bulgarian, Greek, Dairy free, Viili, Filmjolk, Piima, Caspian.
The dairy free starters for vegan yogurts from soy or coconut are available and can be used. This is the best use for the starters where you might not be able to get the culture easily.
The Viili, Filmjolk, Piima, and Caspian Sea Yogurt starters ferment at room temperature. They can be added to cold milk and allowed to sit for recommended time to get the yogurt. There is no need of yogurt makers for this.
The curdling in milk happens by the action of two fermenting agents: Lactobacillus Bulgaricus and Streptococcus Thermophilus. They produce the lactic acid which the flavor to the yogurt.