Oil-based polyurethane will turn your cabinets a little bit yellow right away and will get worse over time. If you want to avoid yellowing, use high-quality, durable waterborne poly with UV protection instead. When properly applied, they will protect your kitchen cabinets from naturally turning yellow, as they tend to do from UV exposure. You can also prevent poly from turning yellow by allowing less sunlight into your kitchen via window sheers or UV films. Also, avoid using wood fillers with yellow or orange tones, as they will turn any polyurethane yellow. Finally, some types of wood are naturally yellow, which intensifies over time, and you won't be able to change that even with polyurethane.