Supermarket corporate mergers are receiving closer federal scrutiny from anti-trust investigators in response to processor and consumer concern about market domination, potential for price-fixing, and market monopolies. At the heart of the issue are slotting fees, also known as slotting allowances. The term applies to cash paid by food marketers to retailers to gain or expand shelf space in supermarkets. The $460 billion grocery industry contends that these fees are a necessary way to recover th…