Wikipedia:Reference desk/Archives/Science/2021 July 17

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July 17

Beer composition

Beer is an alcoholic drink made by fermenting malt and hops. But what exactly is the finished drink composed of? As far as I have understood, it's a solution of alcohol in water, with microscopic bits of malt and hops floating in it. Am I correct here? Can someone supply a more technical description? JIP | Talk 03:09, 17 July 2021 (UTC)

[1], [2], [3], [4], [5] In short, your intuitive analysis is pretty much accurate: Approximately 90% water, 5% ethyl alchohol (on average), and the rest yeast, hops, malt, and trace amounts of sugars, spices, preservatives and other additives. Snow ^{let's rap} 05:44, 17 July 2021 (UTC)

Note that the "microscopic bits of malt and hops" are in large part proteins, oils and other chemical compounds that have been extracted from malt and hops, or are the products of reactions between them, the "liquor" (brewing-speak for water), the products of the yeast's metabolism and cellular substances, and other added trace constituents. If the beer is "real ale" and therefore unfiltered, there should be an appreciable quantity of still-living and recently expired yeast cells. There will of course also be a good deal of dissolved CO₂, either as a metabolic product of the yeast, or (if the beer is not "real" but pasteurised) added artificially under pressure. (Amounts vary, but typically at least 1 unit of dissolved CO₂ at STP per unit of liquid beer.) {The poster formerly known as 878.81.230.195} 2.122.177.31 (talk) 14:33, 17 July 2021 (UTC)

Now this is a good answer, thank you. So it's a solution of alcohol, proteins, oils and other compounds in water. JIP | Talk 15:53, 17 July 2021 (UTC)

Some of the constituents will be in solution, some in suspension.. Also, there will be (hopefully small) traces of any substance(s) used for fining the beer. {The poster formerly known as 87.81.230.195} 2.122.177.31 (talk) 21:36, 17 July 2021 (UTC)

Heat pumps

"But heat pumps are different, as they don’t generate heat. Instead, they move existing heat energy from outside into your home. This makes them more efficient. Since they deliver more heat energy than the electrical energy they consume." [6]

"It costs about 4.6p per kilowatt hour (kWh) to heat a home with gas. And it’s about 9-16p per kWh using standard electric heaters. A typical air source heat pump might cost about 4.7p per kWh to run. But if you use cheap-rate Economy 7 electricity or an GoElec tariff this could be as low as 2.3p! [7]

If heat pumps "deliver more heat energy than the electrical energy they consume", why would they cost money to run? They should make money.

"Air source heat pumps (ASHPs) absorb heat from the outside air to heat your home and hot water. They can still extract heat when air temperatures are as low as -15°C." [8]

How can it be possible to transfer heat from a lower temperature (-15°C) to a higher temperature (inside your home) without net using energy? Doesn't this violate a fundamental thermodynamic principle?

It DOES use energy in the form of electrical energy. It is identical to using electrical energy to power a refrigerator. Refrigators operate by pumping energy (in the form of heat) out of the cold space and jettisoning it into the surroundings. Neither a heat pump nor a refrigerator violates the first law of thermodynamics or the second law of thermodynamics. Dolphin (t) 22:42, 17 July 2021 (UTC)

Well, clearly not. However, if heat pumps "deliver more heat energy than the electrical energy they consume" then why can a portion of the delivered heat energy not be converted (somehow, in principle anyway) to the electrical energy required, or the energy required to turn the compressor, leaving "free" heat energy left over?

I just don't get why there is any benefit in using electricity to drive a heat pump instead of using the electricity to heat the water directly (let's assume for the purposes that we are heating water), having, if necessary (if the outside temperature is greater than the starting temperature of the water) first let the water naturally reach ambient outside temperature. Again, it seems like "something for nothing". Where is the extra "something" coming from? 2A00:23C8:7B08:6A00:E4E6:ABBB:EB80:8B84 (talk) 08:48, 18 July 2021 (UTC)

A heat pump simply transfers heat energy from elsewhere. I suggest you read our articles on heat pump and latent heat.--Shantavira|^{feed me} 09:57, 18 July 2021 (UTC)

Thanks, but this answer does not really help me answer my specific questions. Also, if one could willy-nilly transfer (heat) energy from one place to another, then we could all boil our kettles at no cost from ambient heat, which is clearly impossible. If anyone is able to shed any light on all of this, it would be very helpful to me. 2A00:23C8:7B08:6A00:145A:9AC7:ECA2:BB33 (talk) 10:38, 18 July 2021 (UTC)

Why can't we use some of the heat from the surroundings to power the engine that drives the heat pump? Nobody seriously proposes to do that because the proposal violates the second law of thermodynamics which I admit is one of the more challenging concepts in physics. A good place to start with the second law is to see that it says heat can flow spontaneously from a region of higher temperature to one of lower temperature but heat cannot flow spontaneously from a region of lower temperature to one of higher temperature. For heat to go from a lower temperature to a higher temperature it must be pumped and that always requires high-quality energy (such as electricity) from somewhere. Dolphin (t) 12:13, 18 July 2021 (UTC)

The first source you are quoting is somewhat optimistic. Heating a building with an air source heat pump in extremely cold weather takes considerably more energy than in the average British weather conditions of the last few decades. Properly isolating a house against thermal loss may have more effect. Heat pumps cost money to run because one needs to buy the electricity to make them run, and there is no obvious way of selling the heat energy they deliver. Whether air source heat pumps are truly a lower-cost solution depends on the particulars of the systems being compared.  --Lambiam 11:10, 18 July 2021 (UTC)

Re. However, if heat pumps "deliver more heat energy than the electrical energy they consume" then why can a portion of the delivered heat energy not be converted (somehow, in principle anyway) to the electrical energy required, or the energy required to turn the compressor, leaving "free" heat energy left over? You cannot convert low-level heat energy directly into electricity at high efficiency. Electricity power stations create steam (actually superheated steam) to power a turbine generator. If you can work out a way to use the heat radiated from the back of a refrigerator to power the compressor (somehow, in principle anyway) you will probably be the richest person on the planet in short order, solve global warming and provide free, almost limitless energy for generations to come. Seriously though, heat energy and electrical energy are two very different things. It's easy to turn electricity into heat, not so easy to convert heat into electricity. nagualdesign 11:32, 18 July 2021 (UTC)

The second law of thermodynamics page is a long read. All that you need to know for the purpose of calculation is, that for an ideal heat pump, if it takes in a quantity of heat $Q_{1}$ at a temperature $T_{1}$ and discharges a greater quantity of heat $Q_{2}$ at a higher temperature $T_{2}$, then

${\frac {Q_{2}}{Q_{1}}}={\frac {T_{2}}{T_{1}}}$

and (by the conservation of energy), the energy required to drive the heat pump is $W=Q_{2}-Q_{1}$. Here $Q_{1}$, $Q_{2}$ and $W$ can be in any units - kWh say, but $T_{1}$ and $T_{2}$ are the absolute temperature, which is usually measured in Kelvin - i.e. Centigrade + 273.16°.

An ideal heat engine, whether a steam engine or a thermoelectric generator is subject to exactly the same equations, but now $T_{2}$ is lower than $T_{1}$, and $Q_{2}$ is less than $Q_{1}$, so that useful energy is produced.

Thus in the proposed arrangement of a heat pump supplying a heat engine, the heat pump requires all of the the output of the engine to drive it and vice versa. This is perpetual motion of the second kind. A real, imperfect heat engine produces less energy and real heat pump consumes more. catslash (talk) 15:09, 18 July 2021 (UTC)

A heat pump basically air-conditions the outdoors. It pumps cold air from your house into the environment. That means your house gets warmer. Think of a room A, with an air conditioner whose exhaust goes into another room B instead of the outside. The air conditioner motor consumes electric power and dissipates it as heat: let's say it is set up so the dissipated heat also goes into room B. Meanwhile the air conditioner is making room A colder because that's what they do. So room B is getting heat from two sources: the dissipated electric power, and heat extracted from room A, even though A is colder than B. Looking at that from a heating rather than A/C perspective, room B is being heated more efficiently by also pulling heat from room A, than it would by electric resistance alone.

The basic mechanism and efficiency limits of this process can probably be found at Carnot cycle. In the case of real heat pumps that you install in a home (or also air conditioners), the electricity-in to heat-energy-out ratio is called the coefficient of performance (COP). A household heat pump in a not too frigid environment can often have a COP of around 3, depending on temperature differential etc. COP is sometimes converted into other units like energy efficiency ratio (EER) but COP is imho a more natural measurement. 2601:648:8202:350:0:0:0:2B99 (talk) 21:50, 18 July 2021 (UTC)

Why don't weather reports state the wet-bulb temperature?

I've been reading about how wet-bulb temperature is important in a heat wave, but Accuweather, Weather Channel, and Weather Underground don't report the number. Must we calculate it manually?

AccuWeather has a patented exclusive RealFeel  Temperature, which they really feel is a better measure for how bad it gets in a heat wave. The Weather Channel has a similar FeelsLike temperature.  --Lambiam 10:37, 18 July 2021 (UTC)

Weather reports state the relative humidity and that is more meaningful to most people. If you want the wet-bulb temp you must measure it or use a psychrometric chart. Dolphin (t) 22:56, 17 July 2021 (UTC)

Wet-bulb temperature says 95°F is when sweating stops working which can kill anyone if it's >95°F for more than a few consecutive hours. Eventually this will start happening on Earth and wet-bulb temperature should be more in the news. In theory if dew point stayed above lung temperature the lung will collect dew like cold object until you drown. Sagittarian Milky Way (talk) 00:08, 18 July 2021 (UTC)

Also probably because it's generally no longer measured. In the days before electronic weather stations, the only reliable way to get humidity data was with wet and dry bulb thermometers. Now, electronic humidity sensors are the norm, so it's no longer necessary to observe physical thermometers.--Phil Holmes (talk) 09:34, 18 July 2021 (UTC)

Wet bulb temperature is merely one of the "inputs" needed to calculate relative humidity which is the actual meaningful information. Roger (Dodger67) (talk) 09:41, 18 July 2021 (UTC)

See also Heat index and Apparent temperature.  --Lambiam 10:26, 18 July 2021 (UTC)

Yeasts on fruits

I've read that practically all fruits bear yeasts. Sourdough recipes, for example, call for taking fruit peels or raisins. Some of the fungi spoil fruits, though, and I know not to eat spoilt foods. There are also other diseases, including cancer, caused by certain fungi, I think. How safe, then, are the yeasts on fruits? Imagine Reason (talk) 22:59, 17 July 2021 (UTC)

Within the fungus kingdom, the (unicellular) yeasts are distinct from the (multicellular) molds. Several species of yeast in the Candida genus, including species that are part of the normal oral and intestinal flora of healthy individuals, can under certain conditions cause an infection known as candidiasis. Yeasts are mainly known for their health benefits,^[9] but some species thus far generally considered safe (e.g. Saccharomyces cerevisiae) are now suspected of being potentially pathogenic.^[10] When baking bread, however, all yeast cells in the dough are killed.  --Lambiam 10:19, 18 July 2021 (UTC)

When people try to grow their own yeasts, what are the chances they won't be growing molds as well? Just killing those would not render the food safe, I think? Imagine Reason (talk) 14:59, 18 July 2021 (UTC)

In general, the vast majority of environmental yeasts (and bacteria) are not capable of causing human disease. They are trying to grow and reproduce on/in a very specific plant substrate. In the few cases where there are problems (aflatoxins, for example), public health authorities detect the problem, and step in to require the food manufacturers to do testing and mitigation. See Hazard analysis and risk-based preventive controls for how this is implemented in the US. Abductive (reasoning) 15:13, 18 July 2021 (UTC)

Aflatoxins are produced by molds, not yeasts. There are in fact indications that S. cerevisiae may offer some protection against aflatoxins.^[11]  --Lambiam 19:08, 18 July 2021 (UTC)

I am aware of that, it is an example of something that they caught. Abductive (reasoning) 03:37, 19 July 2021 (UTC)

Bear in mind, Imagine Reason that we (humanity) have been doing this sort of thing for literally thousands of years, and have found by trial and error how to do it properly and what does and doesn't normally give rise to any health problems.

As an aside, you might find the articles Lambic and Noble rot interesting. {The poster formerly known as 87.81.230.195} 2.122.177.31 (talk) 17:13, 18 July 2021 (UTC)

I don't take comfort in traditions. Smoked meats and other food preps may have worked for most of humanity history, when starvation was a bigger worry than cancer risks, but it's not considered safe anymore. Imagine Reason (talk) 20:43, 18 July 2021 (UTC)

According to Smoked meat, "One study has shown an association between the frequency of consumption of smoked foods and intestinal cancer. However, the study was restricted to a small Slovenian population in Hungary, where the local smoke curing process produces levels of contaminants roughly eight times as high as standard processes elsewhere." That's not very convincing. Cullen³²⁸ Let's discuss it 22:03, 18 July 2021 (UTC)

Smoke causes carcinogenic polycyclic aromatic hydrocarbons (PAHs). I don't eat grilled or smoked meat or fish. Thankfully my palate doesn't drive me to seek out such pleasures. Imagine Reason (talk) 23:12, 18 July 2021 (UTC)

I'll give another example of tradition being more harmful than good: Alcohol. I rarely drink it now that the science has swung against it. Third example: Chinese medicine. Imagine Reason (talk) 23:32, 18 July 2021 (UTC)

Also bacon and other processed meats. I rarely eat them at all since reading the report several years ago. nagualdesign 18:43, 19 July 2021 (UTC)

Give me some offals! Or not. Imagine Reason (talk) 00:46, 22 July 2021 (UTC)