酸奶的制作过程,用英语写一篇短文.

下面这篇文章足够精细,你不妨精简一下.
How to Make Homemade Yogurt
Here is a step-by-step guide to making homemade yogurt. Esoteric Teaching walks you through the steps and gives you tips that can be used during the process. There are several different ways to make yogurt at home and this is just one of the ways to make your own yogurt.
Ingredients
1 quart milk (skim, 1%, 2%, or whole)
1/3 cup nonfat dry milk (optional; will result in a thicker yogurt with a higher protein content)
2 to 3 tablespoons plain yogurt with active cultures
You can use store-bought yogurt as a starter for your first batch; once you have produced your own yogurt, you can use a portion of your stock to culture subsequent batches
Sweeteners and flavorings, if desired
See the flavorings section for ideas
Equipment
Double boiler
Metal spoon (which you can sterilize)
Candy thermometer
1 quart-size container or 4 or 5 cup-size containers to hold finished yogurt, with lids
Plastic wrap if container(s) do not have lids
Dish towel
One of the following: slow cooker; oven; heating pad and cutting board; or cooler
Refrigerator
Step 1: Sterilize Your Equipment
Make sure everything you're going to use to make your yogurt is extremely clean!
The easiest way to ensure that your equipment (saucepan, bowls, heatproof yogurt containers, utensils, etc.) is scrupulously clean is to run it through a dishwasher with a heated drying cycle.
Alternatively, wash your containers and tools well in warm water with dish detergent and let the dishes air-dry.3
Then fill containers with boiling water and let them stand until you are ready to fill them with yogurt.
Step 2: Heat Your Milk
Even if you use pasteurized milk, it will require sterilization due to the bacteria it contains.
Fill the bottom of a double boiler with water and bring to simmer over medium heat.
Pour milk into top of double boiler.
Add nonfat powdered dry milk if using.
Heat milk slowly, stirring continuously with a metal spoon, until small bubbles gather around the perimeter and steam rises from the surface.
The temperature of the milk should be between 180 degrees F and 200 degrees F.
Do not allow milk to boil.
Keep milk at this temperature for approximately 10 minutes, stirring frequently with a metal spoon.
Be sure to stir up from the bottom of the pot so the milk at the bottom doesn't scorch.
Step 3: Add the Culture
Once it has been sterilized, you must bring the temperature of the milk down so that the heat doesn't kill off the beneficial bacteria in the warmed starter culture.
Bring your starter out of the refrigerator so it can reach room temperature.
Remove the top pan of the double boiler and place in the refrigerator.
Alternatively, place it in a pan of very cold water, making sure that no water rises over the side and mixes with the milk.
Keep checking the temperature of the milk every few minutes.
You don't want to add the starter until the milk has dropped to below 120 degrees F, but do not let the milk fall below 100 degrees F.
The ideal temperature at which to add the starter is approximately 110 to 112 degrees F.
When the milk reaches 115 degrees F, remove from refrigerator or water bath and stir.
Remove about a cup of the milk.
Stir 3 tablespoons plain yogurt with live active cultures into the cup of milk.
Stir this back into the pot of sterilized milk.
Your milk has now been cultured, or pitched.4
Cover the cultured milk.
Step 4: Incubate the Bacteria
Incubating the beneficial bacteria in your yogurt is the trickiest part of the whole procedure. If the temperature of the milk mixture is too low, the bacteria won't grow enough to produce yogurt. If the temperature is too high, however, it will kill the bacteria. There are a few methods you can use to regulate the temperature of your milk mixture as it turns to yogurt. Whichever method you choose, be sure to keep an eye on the temperature (use your thermometer!) to make sure it stays within the necessary parameters. Also, for the best results, your developing yogurt should be kept as still and undisturbed as possible.
Oven Method
Turn oven to 170 degrees F.
When oven comes up to temperature, turn oven off, wait 5 minutes, and place milk mixture inside.
For ideal incubation, maintain a temperature of about 110 degrees F inside the oven.
Use an oven thermometer or your candy thermometer to monitor the temperature inside the oven.
Do not let the oven temperature drop below 100 degrees F.
Let incubate for 5 to 7 hours.
Slow-Cooker Method
Preheat the empty slow cooker] (with the crockery insert in place) on the warm or low setting for 10 minutes.
Place covered containers of yogurt in the slow cooker insert.
Turn off the slow cooker and cover with a folded bath towel to insulate.
Turn the slow cooker on warm or low for 5 minutes every hour to maintain temperature.
Let incubate for about 5 to 6 hours.
Heating Pad Method
Place a dry cutting board on a sturdy table or countertop.
Set heating pad on top of cutting board and plug it in.5
Turn the heating pad to medium heat.
Place bowl of milk mixture on top of the heating pad.
Let sit, undisturbed, for at least 6 or 7 hours.
Cooler Method
If you have a well-insulated cooler that retains heat well, you can try this method.
Fill 4 liter- or quart-size bottles with very hot tap (about 130 to 140 degrees) water.
Place bottles inside cooler.
Pour cultured milk into one large or several small containers with lids and place them inside the cooler.
Place a few folded sheets of newspaper or a folded towel over the bottles to keep the heat in, shut the lid, and do not disturb for 5 to 7 hours.
Step 5: When Yogurt Is Done
After the estimated incubation time has elapsed, check your milk to see if it has turned into yogurt. The longer the yogurt remains warm and still, the thicker and tarter it will become.
If the yogurt appears to be set and feels slightly firm, it's done.
It will continue to thicken as it cools.
If you haven't already done so, spoon the yogurt out of the top of the double boiler and into the sterilized storage containers (e.g., a large mason jar or several individual-size yogurt cups).
Cover each container of yogurt with a tight-fitting lid or plastic wrap.
Store in the refrigerator for no more than 10 days.3
Tips and Warnings
Flavorings
You can use whatever you wish to flavor your homemade yogurt.
Try adding fruit jam or jelly, honey, molasses, brown sugar, cut-up fruit, fresh or frozen berries, crushed pineapple, espresso powder, flavored extracts, or whatever else you find appealing.
Be creative with your combinations!
Texture
The texture of homemade yogurt can be quite different from the store-bought variety. Often, it is thinner and more liquid. If you prefer a thicker, denser yogurt, you can thicken it with a bit of unflavored gelatin.3
Sprinkle 1 teaspoon of gelatin over the milk before heating it.
Let stand for 5 minutes, stir, then heat.
Heat for 30 minutes, stirring, to dissolve gelatin completely.
Milk Varieties
You can use cow's milk, goat's milk, or sheep's milk to make yogurt.
All fat contents are acceptable, from cream to skim.
For safety's sake, use only pasteurized milk.
Starter Culture
You can reserve 2 to 3 tablespoons of your yogurt to use as a starter in your next batch. Just be sure to use the starter within 5 to 7 days.
Yogurt Makers
Though, as you've seen above, you can make yogurt using equipment you probably already own, there are also yogurt makers you can use to make your own yogurt. If you purchase a yogurt maker the steps will be similar to those above, though make sure to follow the manufacturer's instructions.
Food Safety
Although the beneficial bacteria in yogurt is most welcome, it's also possible, through contamination via utensils or cooking implements, to introduce harmful bacteria into your milk.
If you detect any kind of off odor, color, or taste in your yogurt, do not eat it.
To avoid contamination, don't forget to sterilize your containers and your cooking equipment.