All bottled soft drinks containing thirty per cent or more natural fruit or natural vegetable juice as hereinafter defined, and all bottled natural liquid milk drinks containing thirty per cent or more of natural liquid milk, as hereinafter defined, shall be exempt from the tax imposed by this article. Provided, that this exemption shall not apply to any fruit, or vegetable juice drink, to which is added any one or more of the following: Any coloring, artificial flavoring or preservative. Sugar or salt or vitamins shall not be construed as an artificial flavor or preservative.