I guess many may regard junk food and street food as one and the same. Or many who prefer junk food over street food because junk food has more impeccable packaging and lets face it, street food, specially in India is not prepared in the most clean of places.
One easy way I distinguis the two is the source, junk food is largely branded, prepared in large scale. Street food is local recipes prepared on a limited basis in a place you can see. But there is more to it than the source. Junk food goes through multiple cycles of storing and reheating. For example compare vada pav from a street vendor and the same item from say an airport shop (if at all any airport in India has sense enough to sell something really nice other than overpriced dosas, horrid panner rolls and couc couc salad, ugh). Airports will serve you stale vada pav heated in a microwave. This affects the taste in a very fundamental way. It is not crisp, the spice balance is disturbed. THe street vendor is going to give you fresh items because for him it makes sense to keep the vada half ready and then fry it when you order.You can feel the aloo inside must have gone for a dip in the oil and tastes much better.
I guess the same classification holds no matter what the item is. Pizza or pasta purchased from a roadside vendor is freshly made and hence not junk food but Italian street food. Three day old vadapavs microwaved and served in airports is junk food never mind its very desi origins.
I guess it is the freshness which distinguishes street food from junk food. I know the microwave has wrought a revolution in cooking and made life simpler for a lot of us, but I feel the presence of microwave encourages a lot of food reheating. They cycle of cooking excess food, refrigerating and then reheating gets kind of vicious in the end. Maybe that is why I am happy that I come from a traditional brahmanical family where cooked food in refrigerator is a strict no no. I dont like most of the restrictions imposed on me because of this background but this is one I appreciate. It forces me to cook moderate quantities enough for one day, and makes sure I realize the price for overcooking in the most literal way. A friend of mine and her family even made away with the refrigerator completely, because they realized they ate fresh food all the time, without that. I dont mean, an elaborate three course meal prepared fresh everytime, but even a simple single course meal which is made a few minutes before serving is well worth it.
Which is why I will eat street food over junk any day. And encourage my child to do the same.