I presume you're talking about the food retail sector or different parts of the food supply chain. If it's okay, I can give a general answer, because a large amount of my work has been on the concentration of power among a small number of firms at various points in agri-food supply chains. That is the case all the way from farm inputs—like seeds, chemicals, machinery and fertilizer—to food production and processing, and also the international food trade, the grain-trading companies, as well as on the food retail side.
The concentration of power at these different points in the food supply chain can affect food security, because when there are just a few players in the market, they tend to have what we call “market power”, which enables them to have a greater degree of say over the supply-and-demand conditions within which they operate. This enables them, for example, to pay less to suppliers at the same time that they might charge more to consumers. This can lead to a situation where workers in the food system might be receiving less of the benefits from the system than they would otherwise. Also, consumers might end up paying more out of their own pocketbooks for food. These effects can multiply throughout the food system.
I hope that answers your question. Thank you.