When we were dealing with baby foods, all the books and experts talked about how you could make your own baby food at home easily, except carrots. They all just said "don't even try", some even had a warning that "severe reactions" would occur, and that it was a risk to the baby to feed them home made carrot baby food. Well I've seen the projectile vomit and horrid diapers that result from trying to feed a baby homemade carrot baby food, so there is clearly something to be said for it. But what's so hard about carrots specifically? What are the big companies able to do that we can't do at home? Can anyone explain this at the food science level?
According to http://www.wholesomebabyfood.com/nitratearticle.htm, the problem is with nitrate levels in carrots. If that's true, the same problem will occur with fresh spinach.
As to what the food industry does differently: IIRC, nitrate problems tend to get worse when cooked meals spend time at room temperature. Bacteria from the air will metabolize the nitrate into nitrite, which is poisonous at higher concentrations. This only occurs when (naturally occuring) bacteria are present, at 20-40 degrees celsius. In te food industry, the cooking process kills the bacteria, and the food is packaged airtight while the product is still hot. You can't do that at home.
disclaimer: I dropped out from my chemistry bachelor after one year. The above is incomplete at best.