# What is the way to prevent the cream from settling on the surface of hot milk with least manual labour?

I have seen Cooling a cup of coffee with help of a spoon but that question has a restriction of using only a spoon.

My problem is that I have to boil the milk before pouring it in the bottle for the toddler and I don't want the cream to settle on the surface of the milk because then I have to put the cream aside and give plain milk to the toddler.

I saw that if I constantly keep on string the milk till it cools, I can prevent the cream from settling.

I have tried the spoon method. It works, but it takes a considerable amount of effort to cool 500 ml milk with a spoon.

What is the other way out provided we can all the kitchen utencils (i.e. spoons, spatulas, bowls etc.)?

• "Then I have to put the cream aside and give plain milk..." - can you explain the problem a little more clearly? Are you asking about ways to cool milk quickly, or how to prevent cream from separating? And finally - what is wrong with separated cream? Oct 19 '15 at 16:22
• @Floris I am asking for the method to prevent the cream from forming on the surface of milk while it cools down. Cream lumps do not pass through the hole of the nipple. Oct 20 '15 at 0:13
• I don't know if this will achieve what you want: I have a small pitcher, 8 oz, I think, with a spout that joins the body of the pitcher at the bottom. It's designed to assist in the capturing of liquids that have a layer of fat on the surface. The liquid that pours from the pitcher comes from the bottom rather than the top. Oct 20 '15 at 12:48

This is an attempt at an answer to an older version of the question, where the focus lay only on cooling the milk as fast as possible.

This is only a partial answer - I have no idea whether my "method" is the fastest.

Heat transfer can be done in three ways: convection, conduction and radiation. We cannot really influence radiation, but it doesn't help much anyway (the temperatures are too small). Roughly, thermal conductivity is governed by Fourier's law, which tells us that the local heat flux $q$ is roughly $$q= k\nabla T$$ where $\nabla T$ is the temperature gradient and $k$ is a constant depending on the material where the flux occurs (thermal transmissivity). Therefore, we can influence conduction by

a) bringing the milk into contact with a material with high thermal conductivity (such that conduction is accelerated)

b) cooling this contact material as far down as possible

c) making the contact surface as large as possible

The third is necessary, because we cannot really influence the thermal transmissivity of milk. Even if the material in contact with the milk has infinite thermal transmissivity, this would just mean that regions closest to that material would be cold immediately and regions far away would still be warm and cool according to the conductivity of milk. Of course, we can remedy this by introducing a current, therefore introducing convection (i.e.: we can stirr).

Based on a)-c) and the common sense assumption that you don't want to create a total mess and you don't want to contaminate the milk (albeit what that means can vary), here is a suggestion:

(Maybe) fastest method: Buy a baking tray (copper would be ideal, but I'm not sure these exist. It seems to be best from pure alluminium, if this is safe, but stainless steel seems fine - here is a list of thermal conductivities), put it in your icebox and when you want to cool your milk, get it out, pour the milk onto the baking tray and maybe stirr a little (however, if the baking tray is large enough, the surface area of the 500ml of milk should be enlarged enough) and then recollect the cooled milk in your bottle.

However, there is a problem: While the thermal conductivity of metal is pretty good, its thermal capacity is very low compared to water (and therefore milk). This means that while the temperature exchange will be mostly instantaneous, it might not be enough to cool the whole bottle. Here is a back-of-the-envelope calculation: Heat capacity of water is about four times that of stainless steel (link). Temperature of the ice box is maybe -15°C and the milk would maybe be at 90°C when you get it out of the bottle. Therefore, in order to cool it to 30°C, i.e. the gradient is 60° for the milk and 45° for steel. This would mean that you need more than five times the amount of milk in steel (i.e. 2.5-3kg), which seems too much for a baking tray - and this ignores the fact that the temperatures will not instantly equilibrate, since the flux depends on the gradient.

Thus, you'd maybe need to cool the steel - preferably with water, because this is available (supposedly) in larger cold quantities. One way seems to be to spray the baking tray with cold water from below, but this could create a mess. Another way would be to hold the lower side in a bath of cold water. Therefore, here is the second method:

Method with more cooling power: We use our baking tray as above, but we also have a second vessel (maybe a larger baking tray) with cold water and we hold the baking tray in the cold water and maybe even stirr the cold water.

This however might be very cumbersome. So here's another proposition:

Slower, but enough cooling power: Don't pour the milk into a bottle, put it into some vessel which is very narrow or somehow far away from cylindrical or round form such that the volume to surface ratio is large (maybe some flask or a small bag that you can close) and completely submerge the vessel in cold water. Wave it around a bit and it should be cold quite fast. With this method, you won't get the thermal gradient and conductivity of the baking tray (however, water is still way better at conducting heat than air - see the list linked above), but since you cool water with water, a few litres of cold water will definitely be sufficient to cool the milk without heating up too quickly.

• I used big stainless steel plates in which we eat food, to cool the milk. The temperature drop down speed was shocking. I didn't use any ice water. That wasn't required. I shifted milk from one big plate to another and then after some seconds back to the previous one. This did the trick. The big plates we use for eating are called thali. Oct 23 '15 at 0:16

Cream separates from milk because it forms large globules which rise to the surface. You can prevent this by homogenizing the milk and cream so they form a colloid, in which smaller particles of cream are distributed evenly throughout the milk.

You can buy a hand-held homogenizer for \$500. Or, if that's too much to spend, try a home juicer to see if it can reduce the cream globules to colloid size and disperse them evenly throughout the milk. If you run the milk and cream through a juicer prior to boiling, it may remain in a colloid state long enough to come to a boil without separating. This should be tested to see if the juicer turns the milk fat to butter, or succeeds in reducing it to a colloid.

If the milk fat globules are ruptured, they may be partially digested by the enzyme lipase in the milk, which could lead to rancidity. However, lipase is deactivated by heat, so the milk should be boiled either prior to or immediately upon homogenization.

• Using the juicer to homogenize was going to be my approach. I didn't know about the role of lipase and the effect of boiling. Oct 20 '15 at 10:20

The scientific process you need to complete is homogenization.

In your case when the fat {cream} separates from the water in the milk by boiling. Once boiled you want to break down the fat quickly so it re-emulsifies with the water. Homogenization breaks the fat into smaller sizes so it no longer separates from the water. A small hand held ultrasonic homogenizer would work best in your case. But since it is not in ones usual set of kitchen appliances I would try the following.

Try using egg beaters at a slow speed not fast because you don't want to put too much air into the liquids. Put the boiling whole milk in a bowl and beat it slowly to break down the fat. A whisk or spoon could also work. This will break up the fat\cream and keep it in suspension with the milk. Placing it in a cooling bowl will also cause the heat to dissipate more quickly due to the cold water heat sink. Just make sure you don't beat it too fast and probably don't want to cool it until fully emulsified. This should help you transfer the manual task of stirring to a more mechanized approach in shorter time frame. Once the milk is warm, fill your bottle, test for temperature and feed your toddler. Also make sure the toddler is 12 months or older. Whole Cow milk has a number of proteins and minerals that can damage a younger child's\babies kidneys and in turn lead to a number of unwanted health complications.

Using an egg whisk at slow speed could do the same but with a little more manual work. Certainly/Probably more effective than a spoon. You need to get the fat back emulsified in the solution. If it is too cold the fat won't break up efficiently so homogenization will work better when the liquids are hot to warm not cold.

Also another point you may not have thought of. You probably do not need to boil the whole milk to begin with since the whole milk was probably already pasteurized {and all of the pathogens and microbes killed} prior to sale. Just warm it up to a comfortable level for the toddler. This is usually what one wants to do.

Homogenization or homogenisation is any of several processes used to make a mixture of two mutually non-soluble liquids the same throughout. This is achieved by turning one of the liquids into a state consisting of extremely small particles distributed uniformly throughout the other liquid. A typical example is the homogenization of milk, where the milk fat globules are reduced in size and dispersed uniformly through the rest of the milk.

In Kitchen Parlance :

Mixing is a general term that includes stirring, beating, blending, binding, creaming, whipping and folding. In mixing, two or more ingredients are evenly dispersed in one another until they become one product. Each mixing method gives a different texture and character to the baked good. The implements used, such as blades, whisks, spoons, etc., themselves make a difference. They have a great impact on what happens during mixing.

STIR: This method is the simplest, as it involves mixing all the ingredients together with a utensil, usually a spoon, using a circular motion.

BEAT: The ingredients are moved vigorously in a back and forth, up and down, and around and around motion until they are smooth. An electric mixer is often used to beat the ingredients together. {Just don't add to much air by beating to vigorously and fast.}

BLEND: Ingredients are mixed so thoroughly they become one.

One last suggestion. One might try cheese cloth to achieve an alternative homogenization process. A homogenization process usually tries to keep much of the air out and forces the fat globules through small screen like openings to break the fat down so it is small enough to stay in solution. Cheese cloth with small apertures may work as the screen. Placing the boiled milk in the cheese cloth shaped like a cone or pouring it over the cheese cloth and a bowl and paying attention not to get burnt one could conceivably squeeze or force the liquids the fat and milk water through the cloth {of proper size, it is usually available in a number of sizes} and then stir up the resultant mixture in the bowl into a suitable mixture to fill the baby bottle for feeding. Since the process would take some time to complete enough heat should dissipate so that the temperature is sufficiently warm. If not place the bottle in an ice bath till proper temperature is reached.

• Beating the crap out of the cream+milk mixture may not work, and must be carefully tested: beating the crap out of cream is in fact the usual way for producing butter. I am not sure why the discrepancy exists but it may be a simple delayed-phase-change depending on the density of the lipids, so that if you gradually add skim milk to hand-churned butter maybe it will return to being whole milk, but then if you skim off the cream and whip again it returns to being butter. It might also be that at higher temperature cream whips to milk and at lower temperature it whips to butter. Oct 19 '15 at 20:26
• @ChrisDrost: Yours is a really good point. Homogenization works by forcing the milk fat through tiny apertures under high pressure. This reduces the globules to colloid size. My suggestion to use a juicer may result in butter! Oct 20 '15 at 0:19
• As I pointed out above I mentioned not to beat it to fast or vigorous. One only wants to break down the fat to smaller globules so it emulsifies keeping it from turning to butter. Oct 20 '15 at 14:09

Use a magnetic stirrer and hotplate. That way you will not let the cream/fat separate out during heating. After heating and stirring at same time, once boiling is reached, place heated milk in bottle and place in ice water bath to cool quickly to appropriate temperature.

See link how to DIY http://www.instructables.com/id/Magnetic-Stirrer-w-Hotplate-for-30/