We do not know what the manna that sustained the Israelites in the desert really was. It could have been produced by some sort of insect or might have been a type of lichen. The important thing about it was, like everything else, that it was a gift from God.
Memories become very distorted over time.The Israelites, wandering in the desert, began to look back to their sojourn in Egypt with nostalgia, when they had pans of meat to eat and as much bread as they wanted. They seem to have forgotten their servitude and the slaughter of male babies.
St John devoted the whole of chapter 6 to developing the doctrine of the Eucharist. Jesus tried to draw his listeners into distinguishing between perishable and lasting food. Many of them were not able to see beyond the material world or to try to understand the vision Christ was presenting of the gift of himself as the the “Bread of Life”. He knew his time with them was very limited and he wanted to be remembered. We are reminded of him therefore at the words of consecration: “Do this in memory of me.” The memory, however, is not of a past event like the manna but a living presence with us. Like the Israelites in the desert we can forget, and we need constantly to draw to mind his life,death and resurrection.