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I was wondering how companies like Innocent (smoothies) or basically any juice manufacturer keep their liquids fresh so long while my smoothies are bad after a couple of days in the fridge (the water and the fruit seems to separate). I've heard about oxidation and preventing it by adding lemon juice or rosemary to the mix or by sealing the bottle with no air left in it.
So, how is juice/smoothies being conserved by the manufacturers and is there any way that I can do that at home?
You are mixing up terms here. "Preservation" is a food safety term. It is a measure against bacteria colonizing your smoothie. For that, industry can use pasteurization, or PEF (electric destruction of cell membranes), or just quick turnaround of refrigerated smoothies.
As for the oxidation, they use just what you said, lots of antioxidants and little access to air. They may even package them in a low-oxygen environment, although I don't know if they actually do so. Since most customers will balk at added antioxidants in the ingredients list of a smoothie, they either use a significant proportion of fruit with antioxidant properties in the recipe, or sometimes use added vitamins. Many vitamins, for example C and E, are antioxidants.
The last thing you are asking about is texture, or the separation of pulp and water. This is the most difficult thing to achieve at home. It takes industry lots of engineers finetuning their recipe, process and machines to produce a homogenous smoothie or fruit juice which does not separate. At home, you have neither the knowledge nor the resources to work that way. Actually, the cards are somewhat stacked against you, as most smoothie enthusiasts use high speed blenders, which cause quite a bit of separation and oxidation by themselves through their centrifugal action. So, you have to just learn to live with it, or maybe start making your smoothies fresh so they don't have the time to separate.
Pasteurization is probably the answer for some suppliers. That, along with sterile or nearly sterile processing plants and containers would dramatically increase shelf life.
