This is the part where I say he hurt them too much and it's not cute anymore. Also, the cans say coffee. Shouldn't that kill them instead? And he said something about being a single parent before. If he spends all his time studying and messing with these guys, what happens to the kid?
Evidently, in the context of kiriraitaa's story, it doesn't matter what the thing you pour on them is as long as it has sugar in it. Japanese coffee is loaded with the stuff, I hear.
Also he doesn't have a kid, he has a sister and lives with his mom. It's suggested in post #15793 his dad walked out on them. Given the health and mental benefits he's had since starting the torture, just goes to show abusing yukkuris is a good thing.
Japanese coffee (the kind from vending machines and convenience stores, anyway) usually comes extra-sugary, and often with extra caffeine. This one says "milk added" at the bottom of the label, so the bitterness would be cut down to mellow levels. There's more sugar and milk than bitterness in that.