How it has Changed
Traditionally the making procedure determined that Colby should have a curdy texture with natural openings in the body. The flavor was slightly sweet with a slight salty note. Best of all, the cheese had a mild dairy, milky note.
This was the way things remained until sometime after the mid 1970s, when the Wisconsin Department of Agriculture decided to amend the state standard of identity for Colby cheese, by adding the following sentence "Wisconsin certified premium grade AA Colby and Monterrey (jack) cheese shall be reasonably firm. The cheese may have evenly distributed small mechanical openings or a closed body."
This change, especially the portion highlighted in red, has led to significant changes in the make process of Colby by "modern" dairy producers. Because mechanical openings are no longer required of Colby, many processors are making a cheese that is closer to a mild cheddar and labeling it as Colby.
In addition I think today's cultures are faster. Older cultures were slower single strains, resulting in slower make times. These slower cultures tended to make for a sweeter cheese.
Another significant change is the curd wash. Many large manufacturers today do a curd rinse with no hold after dropping the curd pH down to a 5.60 whereas in the past Colby makers used to drain whey to the curd line while the curd was still sweet at 6.00 pH or higher. Then after the whey was drained to the curd line, water was added to drop the curd temperature to a set target. After 15 minutes, the whey/water was drained off the curd and then the curd was salted. Most of the acid developed in the press.
The most obvious reason this changed was that larger dairy plants understandably did not want to process all that water along with the whey.
As time passed the retailers pressed the makers to heat the milk higher and add twice the original rennet. They also had them cut the curds finer (producing a drier cheese). In all reducing the process to 2 hours. The final result has produced a cheese much closer to a young Cheddar than a true "Old Fashioned" Colby cheese.