Certain species can cause milk in cooling tanks to develop stringiness as they produce a mucilage type material. Certain bacterial contaminants and their associated enzymes and toxins may even survive pasteurization and create problems in manufactured milk products. One issue is simply the prolonged length of milking times on the large dairies. Smaller dairies may have a 2-3 hour milking time after which the system is thoroughly washed. It is then idle for 8-10 hours between milkings.
Large dairies have just the opposite problem.
B acteria need only a few basic things for survival and growth. Food and water are essential and milk provides these needs. They also favor certain ranges in temperature. While this varies by bacterial species, warm conditions in a milking system during milking encourage bacterial growth. The last element in determining total bacterial growth is time.