From Halopedia, the Halo wiki
The Unggoy, having such a lowly place in Covenant society, lead an existence of servitude; feeding at these nipples is one of their very few real pleasures. Many Unggoy do not enjoy feeding from food nipples: however, they seem to put up with it because it is their main source of nutrition. They appear as a long tube with a nipple at one end to consume from. They contain a blue, bubbling paste that the Unggoy find unappetizing but nutritious. Melissa McKay described the smell as "old socks wrapped in rotting cheese". 
They are described as free standing food-dispensers, with long tubes and receptacles that end in a nipple that the Unggoy use to distribute the food into their gullet.
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