For those unfamiliar with the name, Yoshinoya's is a Japanese export, the first fast food chain in America to serve a bowl of rice with toppings. Despite explosive success in Japan, they timidly started with one store in southern California around 1980. Yoshinoya's is ubiquitous in that state, but the company has stubbornly remained corporate instead of franchising. Today, there are many mini-chains of japanese fast food rice bowls throughout America. Even Jack-in-the-Box has introduced them. But Yoshinoya's has even yet to extend to neighboring Arizona. It's a shame.
So I'm sorry, surfol, that I can't help you; Yoshinoya "chicken wings" is news to me. I remember back when I'd freak a Yoshinoya store staff out by ordering their signature beef bowl and then reaching into my pocket to crack a raw egg on top of it.
But, fast food chicken wings, I'm 99% guessing they fry it. Sweetness might be coming from a citrus, probably orange...