When a cow is milked, the rapid flow of milk through the streak canal shears off some of the keratin buildup and with it goes some of the bacteria that may be stuck in the material. They are flushed out. The process of milking therefore removes some of the keratin at each milking and this appears to help limit bacterial entry into the udder. Between milkings keratin again is deposited in the streak canal. T he action of the liner on the teat end may prove to be a problem under certain conditions.
Milk normally flows from inside the gland out through the teat opening into the liner.
This is what occurs throughout the milking. If there is a liner slip, squawk or unit fall-off, a rapidly moving stream of air may enter and be directed at the teat end for a brief period. Milk droplets containing bacteria may be carried in this air stream and be projected against the teat end. It is this general mechanism of reverse directed impacts that is thought to explain in part how the milking unit may be involved in mastitis. . . . Any bedding, manure or dirt that sticks to the teat and teat-end carries with it large numbers of environmental bacteria such as the coliforms and environmental streptococci.