By Harold McGee
Reduced creams have several disadvantages. They take time and attention to prepare, develop a cooked flavor, and are very rich, sometimes overly so for the balance of a given sauce. A useful alternative to reduced creams is crème fraîche, a version of heavy cream whose consistency has been thickened not by boiling down, but by fermentation. The acid produced by lactic bacteria causes the casein proteins in the water phase to cluster together and form a network that immobilizes the water. Some strains of bacteria also secrete long carbohydrate molecules that further thicken the water phase and act as stabilizers. Used in place of reduced cream, crème fraîche requires no preparation, is less rich, and has a fresher flavor. Thanks to its low protein content, it tolerates temperatures that would curdle sour cream.