There are two approaches to braising: brown and white. In brown braising the item is first sautéed or seared to brown the outside, then liquid is added to partially cover, the heat is lowered and the cooking is continued. In white braising there is no searing. White braising is usually used for fish. Chicken is done both ways. Braising is generally used for less tender cuts. The braising liquid should always include acid, which helps tenderise connective tissue. The acid may be vinegar, wine, lemon juice, etc. other ingredients are added to give flavour. In a classic French braise a mirepoix is generally used. In a brown braise the vegetables are sautéed about 10 mins to give them colour and to develop flavour. In a white braise the vegetables are "sweated", sautéed gently over low-heat, often covered. In this case the vegetables are not to brown; the sweating eliminates excess water and brings out more flavour. Protein, which in its natural state is like egg yolk, begins to firm at 120F. Connective tissue, which may be likened to gelatine, begins to soften at 120F. The protein becomes completely firm at 170F; if this happens the only remedy is to continue cooking until the protein softens once more, another hour or more. For this reason pot roasts and other tough cuts are cooked so long: they are first cooked to firm the protein and then to soften it.