Rain barrels are food grade, UV protected and BPA free. Olives, capers and sometimes peppers are stored and cured in these drums. They are designed for food products and in some cases the product may cure in the containers in Mediterranean countries  outdoors for months to years. They are designed to withstand the hot sun and elements. The water you are collecting comes off your roof,  and it will bring with it what ever debris and bacteria that is washes over. The bigger consideration is what is your roof made of any will that contaminate the water. The best method to practice is one of common sense. If your roof is brand new  made of treated shingles, perhaps you may want to let it weather before using the water on your veggies. Or if your roof is old and  made of shingles that are breaking down you may also want to consider holding back on the veggie garden. As far as contaminants from the roof leaching into  the rain as is passes over your roof, the water passes too quickly to absorb it is only washing. If you remain concerned about roof contaminants get your water tested. Some gardeners suggest not using rain barrel water 2 weeks prior to harvesting.