@Nam_Bui Softserve ice cream is indeed a bit tricky to prepare from scratch. The formula for a good soft serve ice cream is critical. It should generally have a fat content between 4% and 12%. Lower and the ice cream will be very icy, higher and you'll get a very greasy soft serve.
The quantity of nonfat milk solids (the sugars and proteins, but not the fat, contributed by the dairy in the recipe) should 11% to 14%.
Sugar content should range from 13% to 15%. If you add more sugar, you'll have a product that isn't solid enough and melts too quickly because the sugar depresses the freezing point of the mix.
Stabilizers and emulsifiers are generally used at 0.2% and 0.1% respectively. For hydrocolloid stabilizers, you nearly always need to use a blend. A common blend is xanthan gum, locust bean gum, and kappa-carrageenan. Using just the xanthan or LBG alone will tend to cause the mix to separate as it sits.
A typical blend for a stabilizer is 0.015% kappa carragenan, 0.09% xanthan, and 0.09% LBG.
For emulsification, you can use egg yolks at around 0.6% to 1%. If you want truly egg free, then try 0.1% mono- and di-glycerides (often sold as glice from places like Modernist Pantry) and 0.03% polysorbate 80.
If the soft serve formula is still a bit wet, you can add about 0.1% of calcium sulfate to dry and stiffen the mixture.
So a typical rich soft serve formula might look like:
6% milkfat
13% nonfat milk solids
13% sugar
0.5% stabilizers and emulsifiers
So that the mix has about 32.5% total solids.
This might translate into
91g heavy cream, homogenized (35% fat)
702g whole milk, homogenized (4% fat)
65.6g nonfat skim milk powder
111g sugar
0.5g xanthan gum
0.5g locust bean gum
0.1g kappa carrageenan
8g egg yolk
infused flavors as desired.
