Raspberries are sounds that babies begin to produce between 4-6 months of age and can be made by blowing air through lax lips (bilabial) or by placing the tongue between the lips and blowing (lingua-labial). Raspberries precede the developmental stage of babbling and result from a child's efforts to gain oral motor control needed for later speech development.
As such, raspberries are a milestone to be celebrated. Granted they are not very appropriate when eating.
I suggest that you have times during the day when you give your full attention to the raspberry play with lots of smiles and encouragement, but during mealtime, discourage the behavior through stark contrast.
Focus on refusing to reward the behavior by turning away and waiting until the raspberries stop before resuming feeding. Be consistent and if necessary use words and facial expressions to increase understanding.
Very young children respond to tone of voice and facial expression even when they cannot understand speech. Saying "no" firmly with a matching "no fun" face helps a youngster to understand that this behavior is not appropriate at this time. Smiling or saying "no" in a playful voice sends mixed messages and the word becomes meaningless.
Be consistent and know that this too shall pass all too quickly.