Ripening or maturing of small fatty species such as anchovy and herring with salt, sugar, and sometimes spices, in barrels, in order to obtain characteristic changes in texture and flavour, is a complex process which takes several months. It is thought to involve degradation of proteins and lipids in the fish muscle by enzymes from the digestive tract, especially from the pyloric caeca, and to be seasonally dependent particularly in herring (Clupea harengus). The products are semi-preserves and must be kept chilled. See Salting and Marinating.