Weight gain powder that is tasteless - does this exist

My mum, age 99, has recently come out of hospital following a fall and has lost 1.5kg in weight in just a couple of weeks. This is probably because her appetite is like a bird so although she is eating healthily, she is eating about quarter portion size.
I know we can get weight gain supplements from the GP but she hates anything synthetic tasting - she hasn't got to 99 by eating ultra processed food!!
I'm looking for a tasteless powder we can add to her yoghurt, potatoes etc to up the calories/protein through her normal diet. We're already using full fat milk, yoghurt butter etc.
Anyone got any ideas as I done a google and I can't find anything other than chocolate, strawberry or vanilla drinks/powder - yuk!!