sugar, syrup, malt sugar, sticky
餳 centers on sweet, sticky substances made from grain: sugar, syrup, and malt sugar. The stickiness is a natural property of these sugary products.
餳 combines the food radical 飠 with 昜, which likely contributes the sound. The character is associated with sweet, sticky grain-based sugars.
The food radical 飠 suggests something edible, and 昜 looks like a container with a sticky substance inside. Imagine a pot of thick, sweet malt syrup: 餳 is sugar or syrup.
For トウ, picture a sticky toffee candy: toffee -> トウ, and the food radical 飠 reminds you it's a sweet treat.