If you want the long answer, read on.
- I am required to cool the milk before selling it. This is actually a really good thing, because when milk is transported before being fully cooled, it stays at a warmer temperature longer than if it is fully cooled and then transported. By law, the milk must be cooled to 40*F within two hours. I do this by putting it into an ice-water bath inside my fridge. If milk is put straight into the refrigerator right after being milked, without the ice-water bath, it will take 8-10 hours to get to 40*F or lower. In order for milk to stay fresh as long as possible, milk must be cooled as quickly as possible.
- I'm a very busy human being, with all sorts of variables that change from day to day, with teenagers and babies to look after. Because of this, I don't always milk at the same time every day. I milk twice a day, but it's not an exact 12-hour schedule. Because of this, sometimes I am milking at 11 PM at night, and the milk won't be guaranteed to be fully cooled until 1 AM (though, in my experience, it is usually cooled to 40*F within 30 - 60 minutes). Most people don't want to pick up their milk in the wee hours of the morning, and being that I live at the milk pick-up site, I am thankful that most people don't want to get their milk at such an hour!
- Sometimes, in order to get a full jar of milk for a customer, I have to use milk from the morning and evening and combine it together. It's not a good idea to add warm milk to already cooled milk, so I don't do that. It would increase the bacterial growth and could cause the milk not to taste as fresh. So, I wait until the evening milk is also cooled and then I add the cooled evening milk to the cooled morning milk (always using the oldest jar as the date of the milk, because when two milks are combined, all of it is as old as the oldest milk). I've never stayed awake until 1 AM, until the evening milk is cooled, to do this consolidating of milk. I always do it the next morning, hopefully before the customer gets there. That brings me to my next point.
- The advantage to getting milk that is day-old for you as a customer is convenience. You don't have to check with me to be sure the milk is bottled and cooled before you pick it up. You know you can pick up your milk ANY time the day you have reserved (but hopefully if I had to combine AM and PM milk, I've done it before you get there... usually done by 9 AM). The advantage for me is that I don't have to have customers calling me and asking if I've milked the cow yet. Gosh, my life is really, really crazy sometimes, and that would probably push me overboard on one of those insane days.
- To compare the milk I sell to raw milk which can be purchased at other local food markets, I am sure it is much fresher, being day-old milk, than you can get at any store. I have purchased milk from these local markets in the past, and have been very disappointed in the taste of it, even right after purchase. Once I was at the store when the milk producer was bringing milk into the store. So, I asked about the sell-by date on the milk. How old was that milk when it reached its that date? The producer told me that it was up to two days old when it came out of the bulk tank, and they put the date 10 days out from the date it was pulled from the bulk tank. So, it could be up to 12 days old before it is sold, and I guess they expected it to last 19 or more days by the time the consumer actually drinks it. Wow. I never bought raw milk from one of our local markets again, and that explained clearly why I was so disappointed.
Raw milk doesn't act the same way that pasteurized milk does. When it ages, it goes sour instead of rotting. So, this means even if you have milk that isn't tasting as fresh as you like to drink, it can be clabbered, at room temperature, which has a variety of culinary uses. For drinking, I guarantee the milk for 7 days from the date on the jar. That doesn't mean it will taste the same on day 5 as it did on day 2. Milk is an extremely perishable food, and it will change flavor as it sits in the fridge. You will notice slight changes over the course of the week, but it will be drinkable. It will be drinkable, as long as it has been kept between 33*F and 40*F (cooler in this range is better than warmer) and has not been transferred into any less-than-perfectly-clean containers (see how to properly wash milk containers). Other perishable foods also change while in your refrigerator. Anyone who has ever had fresh-from-the-garden vegetables, or fresh-caught fish knows that vegetables or fish on ice for several days are not the same as they are when you first obtain them. They aren't inedible, but they have changed.
Just in case you were thinking you might use a milk jug from store-bought milk, I highly advise against that. For starters, the milk jugs from the store are shaped in such a way that you can never really get them all the way clean. Also, the type of plastic that is used is very porous and cannot be fully sanitized for a second use. Plastic milk jugs are really for one-time use only. If you would like to transfer your milk to a pitcher, I recommend the ones that are available at Cash n Carry. They are shatter-proof, and have clear, straight, smooth, non-porous sides that can be cleaned. Before you use the pitcher, wash it using my instructions, and you should have no problem with your milk going sour too soon.
Having said that I guarantee the milk to last 7 days before going sour, I must qualify that with my own personal experience and the experience of several customers. I have milk in my fridge right now that is 10 days old, and I'm drinking it. It tastes just fine. In fact, most of the milk my family drinks is at least 5-7 days old before we rotate through it. I guess we are slow milk-drinkers sometimes. It tastes good. Very sweet. I have had customers drink milk that was 14, 16, and 19 days old and they have reported that it tasted fine. The point I am making is that the milk is very likely fine to drink, even after the 7 day guarantee, and it's a matter of your personal preference.
So now you know why your milk is day-old when you pick it up, and how long you can expect it to last. Enjoy your milk! <glork, gulp, ahhh> ...the best stuff on earth...