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Any liquid can go bad as the water is just too irresistible for mold, fungi, and bacteria. Even though it is usually safe to brew your coffee at room temperature, you should store it in the fridge. This will keep it fresh and drinkable for about a week.
Yes! Ice cubes made from your leftover cold brew are a great way to cool your coffee without diluting it. After freezing, store in an airtight container for up to six months for best taste.
William Ukers, in his exhaustive All About Coffee (1922) describes a British practice of brewing at room temperature overnight, and then very gently heating the strained coffee in the morning. When I first heard about cold brew about 20 years ago, I was advised to make a concentrate and then dilute it with hot water. Either way, it can be done--but whether you will enjoy the taste only you can answer!
There is no hard and fast rule, but most people like 1 part coffee to 4 parts water. That is, 1 cup of coffee with 4 cups of water. This should yield a concentrate that can be diluted about 50/50 with water.
Yes and no. If you mean does it have a lower level of acid in the coffee, then yes, around 65% less acid than hot brewed coffee. Acidic can also be used as a descriptor for flavor however, usually referring to a bright and astringent flavor.
Some people say it will keep up to 2 weeks in the fridge. The idea behind this statement is that the volatile oils that can go rancid in hot coffee do not show up in cold brew. That said, from a food safety standpoint, I would recommend no longer than a week--if it lasts that long!
Caffeine content is not a constant thing when you brew at home. Caffeine content in store bought products vary quite a bit from brand to brand. When you make a concentrate, you use a lot more coffee than you would for a hot cup so as a concentrate, yes the caffeine is going to be higher. When you dilute it down to drink, you get back down into the range of hot coffee. In short, it depends.
Yes, believe it or not. I am actually working on some recipes that do just that (see Cold Brew Ice Cream). So far, in my research I have come across some indications that if you are going to do this, using whole beans may be the way to go. One consideration is you are going to extract some different flavors this way because milk has some fat in it. When you brew in water, only water soluble compounds are extracted. Milk will pull some of the fat soluble compounds too. If you do try this--please remember to brew in the fridge!