自己做yogurt

剛剛跟朋友聊天，他提到他最近的新嗜好：自己做yogurt。
方法聽起來很簡單，只需要牛奶跟一罐超市買的yogurt。跟我們實驗室養酵母菌（或其他菌類）一樣，只要保有一些yogurt菌，就可以不斷的繼續製造新鮮的yogurt。新鮮的yogurt早上加蜂蜜吃應該是一大享受。

這邊是我查到的詳細作法，理想中有一個yogurt maker應該會比叫好控制溫度：

1. Sterilize the milk. Even though your milk has been pasteurized, it will still contain bacteria. Pour a quart of milk into a pot and use a metal spoon. Heat the milk until it is almost boiling. You'll see small bubbles form around the edges and steam beginning to rise. Checking the temperature is a good idea. It should be around 180-185F (82-85C). A candy thermometer comes in handy. Remember to heat slowly and stir often to prevent scorching. A double boiler may also be used.

2. Cool the milk to grow the yogurt. Allow the milk to cool at room temperature or place it in the refrigerator. Stir frequently in order to accurately check the temperature. It should reach 112F (45C). Don't proceed until the milk is below 120F(49C), and don't allow it to go below 90F (32C). 105-110 (41-43) is optimal.
3. Add nonfat dry milk, if desired. If you wish, adding about 1/4cup to 1/2 cup nonfat dry milk at this time will increase the nutritional content of the yogurt. The yogurt will also thicken more easily.
4. Add the starter. All yogurt needs "good" bacteria. The easiest way to do this is to add some existing yogurt. The first time you make your own yogurt, use store-bought plain yogurt. Be certain it has "active cultures" on the label. For each quart of your cooled milk, you'll need 2 tablespoons of yogurt. Let the starter yogurt sit at room temperature while you are waiting for the milk to cool. This will prevent it from being too cold when you add it in. Alternatively, specialty stores may carry freeze-dried bacteria cultures, which are more reliable as starters.
5. Allow the yogurt bacteria to incubate. Pour your milk into clean individual containers. Cover each one tightly with a lid or plastic wrap. Keep the yogurt warm to encourage bacteria growth (between 105F and 122F (41C and 49C) is ideal). Your oven is a great place, just make sure the thermometer is accurate. You'll need to use your candy thermometer and perhaps turn your oven on and then off again periodically or keep the oven light on to warm. Most ovens don't have set temperatures this low. To check the oven temperature, place your thermometer into a bowl of water inside the oven. Wait until the yogurt is thick, about the consistency of pudding. (A simple solution for good results is a large thermos).
* Important: Keep the yogurt still during this process. Jiggling won't ruin it, but it makes it take a lot longer. It can take anywhere from 8-14 hours to incubate. The longer it incubates, the thicker and more tangy the final yogurt will be.
6. Refrigerate the yogurt. Place the yogurt in your fridge for several hours before serving. It will keep for 1-2 weeks. If you are going to use some of it as starter, use it within 5-7 days, so that the bacteria still have growing power. Whey, a thin yellow liquid, will form on the top. You can pour it off or stir it in before eating your yogurt.
7. Add optional flavorings. Experiment until you develop a flavor that your taste buds fancy.

Tips:
* Next time, you can use some from this batch to start the next batch.
* Try whole milk, 2%, 1%, skim, pasteurized, homogenized, organic, raw, diluted evaporated, dry milk powder, cow, goat, soybean, and more!
* Other methods for keeping the yogurt warm are available and numerous. Just use your thermometer and best judgment. Options include hot water in a sink, stove burner, crock-pot, warming tray, heating pad, a sunny window, in your car on a sunny day, etc.
* Placing your yogurt into the freezer to cool it prior to moving it to the refrigerator will result in a smoother consistency.
* If you use skim (or nonfat) milk, add some nonfat dry milk powder for a better, thicker consistency.
* A trick to maintain a consistent low temperature in the oven is to leave the oven light on after preheating it to the desired temperature. The heat generated by the light will usually be sufficient to maintain the temperature.
* Pie filling in a can makes great flavoring.
* Jams, maple syrups, and ice-cream fudges are good choices of flavorings too!

來源：How to make yogurt.