First off...the disclaimer... I have never made hard cheeses before. From the research I have done, thistle and nettle rennet give an off taste to aged cheeses. Thistle should not be used for cows milk as it always produces an off flavor. New England Cheese Making is a good resource for information and supplies. The vegetable rennet the sell is cheap and very concentrated. They have a monthly newsletter with all kinds of info and they are good at answer questions if you have any...I have no affiliation with the company. Experiment and let us know what you think.