In contrast to the soft cheeses, the hard kinds are low in water-content, ripen more slowly and may be kept through much longer periods. They retain their form through a wider range of climatic conditions. They develop flavor slowly and correspondingly deteriorate much more slowly. Such cheeses are in marketable condition over longer periods. In their manufacture the cooking of the curd takes a prominent place. +109. Relation of heat to classes.+--The close relation between the heat applied and the product sought forms the basis of a striking series of graphs (Fig. 12, page 78). These show the changes hour by hour in the heat relation during the making process of a series of widely known forms, each of which is chosen as typical. In some of these forms, heat is applied but once to bring the milk to the renneting temperature typical for the variety. Subsequent manipulations are accompanied by a steady fall in temperature. In other forms, the curd when solid is specially heated or "cooked" to bring about the changes characteristic of the variety. These contrasts are clearly brought out by the graphs which represent practices well recognized for the varieties. The detailed process for these groups is considered in succeeding chapters. CHAPTER VII CHEESES WITH SOUR-MILK FLAVOR The cheeses with flavor of sour milk are probably more widely used than any other group. Historically and to a very large degree at present, they are farm cheeses.[31] No estimate of volume of such production in the household has ever been made. The utilization of surplus milk in this way is of ancient origin. With the introduction of the factory system of handling milk, the manufacture of such cheese in the household was largely dropped. The rise in price of all food substances and increasing appreciation of the food value of milk products have made the recovery of all surplus milk in some form very necessary. The manufacture of cottage, Neufchâtel and cream cheese is one of the best forms of such recovery which may be adapted to utilize any grade from skimmed-milk to cream. Large quantities of skimmed-milk have frequently been lost from the total of human food by the manufacture of casein for industrial uses, and by use as stock feed.