Some additives have been linked with hyperactivity and aggression in young children. Permissable ones, in my opinion, include canned beans, baked beans and tomatoes. They can get stuck in the baby's throat and cause choking. Heating oil to the required temperatures alters the chemical structure, making it potentially harmful. It's better to stick to foods you know will go down well - and avoid battles of will.
All children go through the stage when they learn the power of the word no.
If this veto is used over food you may be able to nip it in the bud by offering a choice of two equally nutritious items instead of one that they can veto. Encourage your toddler to feed herself from an early age. The toddler will enjoy being independent and competence will grow with practice. If there's a problem over food, the secret is not to get emotional about it either because you're worried about your toddler's health or because it's hurtful to have your food refused. It simply isn't worth making an issue over food or allowing a difficult situation to develop.