Girls, we're drinking a mixture of baby food. The mixture worked really…

Girls, we're drinking a mixture of baby food. The mixture worked really well for us and it also makes weight gain good too. The only thing I can't figure out is after feeding and going to wash the bottle, there's such a "stale" smell from the milk. Is this bad formula? Or the bottles? I think this same formula goes into the little stomach... avent bottles. I sterilize the bottles... but the smell goes away but after feeding, it's like that...