Hi estherlev,
A fresh dairy yogurt has a typical shelf life ranging from 2 to 4 weeks.
Your shelf life seems excessive for a fresh fermented product but the formulation may be a factor. Also you may be filling aseptically which is ideal.
Yeasts & Moulds tend to be the main cause of shelf life restriction. The standard of your facility and filling machine can significantly affect the level of Y&M contamination.
There a few ways to tackle the problem of Y&M contamination:
Preservative such as potassium sorbate and/or inhibitors
Re-pasteurise the fermented base and fill aseptically or hot fill to create an ambient stable yogurt. (Although this means there are not healthy or bio bacteria in the yogurt – sometimes I do wonder if long life yogurts should really be called yogurt )
Up your hygiene standards, use HEPA filters to provide filtered air overpressure in incubation tanks, storage tanks and filling machines.
Kind regards,
Tony