Keep rice soaked into water for at least 30 minutes

[jackson6612]

I'm not a science student. Therefore, please keep your replies simple and straightforward so that your help can be fully appreciated. Thanks.

I was told by someone that one should keep rice soaked into water for at least 30 minutes because it activates enzymes.

Similarly, I have been told when a fruit is taken off of a tree it starts rotting because enzymes start their action.

Enzymes are simply chemicals (organic molecules to be precise). Right? When rice is soaked in water, they get activated. Perhaps, because some reaction is kick started by water. Further, water is also needed for germination of rice.

Why a fruit is taken off, why do enzymes get activated? What purpose?

Please guide me.

 

[alxm]

Enzymes are a special kind of proteins (big molecules composed of amino acids chained together) that act as catalysts, converting one chemical compound into another in living things. I'm no expert on nutrition although I am something of an expert on enzymes; at least I've published research about them. But the things you've heard sound suspect to me. (perhaps not a surprise, since there's a lot of speculative nonsense, pseudoscience and outright fraud as soon as it comes to nutrition).

It's true that many/most proteins, including enzymes, 'denaturate', change structure from their ordinary functional structure, if they're dehydrated. And if rehydrated, they may reacquire their original structure, and thus the enzymes might become 'reactivated'. (food-related example: Egg white is mostly proteins in water in their natural state. Cooking the eggs denaturates the proteins, which clump together making it more solid)

But I don't see how that would be useful from any perspective. It makes absolutely no difference from the nutritional perspective whether or not the proteins/enzymes are 'active', because the first thing your body does with them is to chop them up into bits! (using special enzymes called proteases) A process that starts in your saliva. If they survive that, they'd still almost always be denaturated by the acid in your stomach. Quite few proteins survive that (which is why toxic proteins, such as in snake venom, often need to be injected directly into your blood. Snake venom is often safe to drink, not that I'd recommend it)

I can't see either that letting rice enzymes act for 30 minutes (assuming they do) would have any significant impact on anything. A rice grain can't fundamentally change its chemistry in 30 minutes any more than you can. And it's still just converting compounds into other ones; nothing's being added or taken away, and most of those compounds are going to be broken down in your digestive tract soon enough. Water is needed to germinate rice, but again, that's not going to happen in 30 minutes.

The only reason I know of for wetting rice before cooking it, is to rinse off excess starch from the outside, so it sticks together less.

As for fruits, fruits and everything else rots because of microbes. Bacteria and fungi. Like all living things, bacteria and fungi have enzymes in them, but I don't see any more relation between rotting and enzymes than that. Fruit certainly rots just fine when left on a tree. It rots faster on the ground, but that's likely just that it's more exposed to microbes and humidity there.

The freshness of food is almost all about microbiology really. All food conservation/preservation methods really amount to killing microbes or limiting their growth.

 

[jackson6612]

Thank you very much, Alxm. You have answered it pretty well while keeping the language simple. I would not ask further questions until I get an answer to the following:

Enzymes are often compared to catalysts and they are also called
''biological'' catalysts. Catalyst mostly needs two or more reactants to
work on. It only speeds up the reaction between the reactants. The
reaction can also take place in the absence of the catalyst but reaction
rate will be much slower. When there is only one reactant, whose
decomposition results in the formation of new products, a catalyst can
speed up this decomposition.

Starch is a polysaccharide (=made up of many monosaccharide molecules such
as glucose, fructose, and galactose) carbohydrate. A starch molecule is
made up of glucose molecules. A starch molecule can be hydrolyzed
(=reaction will take place in presence of water) into glucose molecules.
Enzyme amylase will convert starch into maltose (=made up of two glucose
molecules) and maltose can be converted into glucose molecules by using
enzyme maltase.

Are enzymes really catalysts? Can a starch molecule convert into glucose
molecules on its own? It is said a catalyst only speeds up a already
happening reaction, then that means starch can change into glucose without
the help of enzymes but at much slower rate. Am I correct? If I'm not
correct, then why aren't I correct?

Thanks a lot your help and time.

 

[epenguin]

Are enzymes really catalysts? Can a starch molecule convert into glucose
molecules on its own? It is said a catalyst only speeds up a already
happening reaction, then that means starch can change into glucose without
the help of enzymes but at much slower rate. Am I correct? If I'm not
correct, then why aren't I correct?

Thanks a lot your help and time.

That is correct. It is a much slower rate. Practically undetectable. It can be speeded up by acid or alkali - non biological catalysts.

 

[alxm]

Are enzymes really catalysts?

Absolutely. They're the most sophisticated catalysts we know of, far more advanced than anything we've been able to come up with on our own so far.

Can a starch molecule convert into glucose molecules on its own? It is said a catalyst only speeds up a already happening reaction, then that means starch can change into glucose without the help of enzymes but at much slower rate. Am I correct? If I'm not correct, then why aren't I correct?

Sure, but reactions can also go in the opposite direction as well, so the reaction still has to be energetically favorable; which corresponds to reacting faster in one direction than the other. As it were, the hydrolysis (breaking down) of starch into glucose is energetically favored (it costs energy when our bodies store sugar as starch). It's also a reaction that's relatively easily catalyzed; it can be catalyzed by acidity alone.

But as epenguin said, the rates here are extremely different; many orders of magnitude. For instance, diamonds are thermodynamically unstable at room temperature. They would lose energy by becoming graphite instead, and that'll happen if you heat them enough. But at room temperature, without any catalysts, they'll continue to be diamonds for billions and billions of years.

 

[Borek]

Trick is, often there are plenty of possible reactions that can go on their own in a given system. Enzyme very specifically speeds up ONE of them, making all others irrelevant. So it may look as if it was not just a catalyst, but something more.

 

[jackson6612]

Thank you very much, Epenguin, Alxm, Borek, for all the help.

Please remember I'm not a science student - quite a layman in this area. So, please be plain in your replies. Thanks.

perhaps not a surprise, since there's a lot of speculative nonsense, pseudoscience and outright fraud as soon as it comes to nutrition

I'm also an English learner so it would b very kind of you if you could help a little bit in this part too.

"speculative nonsense" - means that though there is some speculations (guesswork) involved which could be right or wrong but most of the time it's utterly nonsense. Am I correct?

"outright fraud" - false claims to increase sale of products. Right?

It's true that many/most proteins, including enzymes, 'denaturate', change structure from their ordinary functional structure, if they're dehydrated. And if rehydrated, they may reacquire their original structure, and thus the enzymes might become 'reactivated'. (food-related example: Egg white is mostly proteins in water in their natural state. Cooking the eggs denaturates the proteins, which clump together making it more solid)

In nutritional terms, does denaturation of egg protein matter? It's still a same molecule with same atoms. In what state, normal or denatured, a protein is more easy to digest?

Quite few proteins survive that (which is why toxic proteins, such as in snake venom, often need to be injected directly into your blood. Snake venom is often safe to drink, not that I'd recommend it)

Thanks for this piece of information. But I'm afraid venom could also be absorbed through salivary glands in mouth. Do I make sense?

I can't see either that letting rice enzymes act for 30 minutes (assuming they do) would have any significant impact on anything. A rice grain can't fundamentally change its chemistry in 30 minutes any more than you can. And it's still just converting compounds into other ones; nothing's being added or taken away, and most of those compounds are going to be broken down in your digestive tract soon enough. Water is needed to germinate rice, but again, that's not going to happen in 30 minutes.

I believe you're right. It's still same molecules. The starch won't convert into a protein.

As for fruits, fruits and everything else rots because of microbes. Bacteria and fungi. Like all living things, bacteria and fungi have enzymes in them, but I don't see any more relation between rotting and enzymes than that. Fruit certainly rots just fine when left on a tree. It rots faster on the ground, but that's likely just that it's more exposed to microbes and humidity there.

So, do you mean taking a fruit off a tree or let it be on a tree doesn't matter? It would rot at the same rate.

Enzyme very specifically speeds up ONE of them, making all others irrelevant. So it may look as if it was not just a catalyst, but something more.

Borek: Are you saying that enzymes differ from catalysts in this particular characteristic?

How do catalysts/enzymes work? Are they involved in some kind of intermediate reactions? Some general and true explanation?

Thanks a lot for all the guidance and time. I'm highly indebted to you for all this.

Best wishes
Jack

 

[Jack the Stri]

In nutritional terms, does denaturation of egg protein matter? It's still a same molecule with same atoms. In what state, normal or denatured, a protein is more easy to digest?

A denatured protein is devoid of its original structure and thus would be easier to digest (since it's more accessible to the enzyme). However, if denatured proteins clump together, this could produce the opposite effect. Most proteins do not clump together in an ordered way though, so any places where the protease cuts would not be 'hidden' systematically. In this case there is a physical effect to consider: boiling and chewing the hard egg white will cause it to be present in smaller chunks in your stomach and saliva and thus be easier to digest, as opposed to the viscous gooey matter that unboiled egg white is.

Thanks for this piece of information. But I'm afraid venom could also be absorbed through salivary glands in mouth. Do I make sense?

While salivary glands do absorb some proteins, it is unlikely that they would to that with snake venom. But never say never.

So, do you mean taking a fruit off a tree or let it be on a tree doesn't matter? It would rot at the same rate.

This matters for the reason that a preserving molecule, ethylene, is released at faster rates when the fruit is plucked. I must say I disagree with alxm that rot is solely due to other micro-organisms (e.g. why apples get 'bruised'). The decline of homeostasis (or the 'preserving' of the internal environment) can very well effect this, though I wouldn't say that it's the most important reason (in the long run fast rot is probably more due to micro-organisms).

Borek: Are you saying that enzymes differ from catalysts in this particular characteristic?

In principle they catalysts like any other catalyst. Their structure does make them more specific than a small-molecule catalyst or a metal like platinum though.

How do catalysts/enzymes work? Are they involved in some kind of intermediate reactions? Some general and true explanation?

This requires some theory on activation energy for reactions. You can picture it as a ball that has to roll over a hill to get to the other side: even if the other side of the hill is lower (i.e. the energy difference is negative), you still need energy to get over the hill. The way a reaction will swing is then dictated by the difference in height (energy), and the speed by the activation energy (height of the hill). What catalysts do is lower the height of the hill so it requires less energy to get from one side to the other, thus speeding up the reaction. If a catalysts lowers the energy too much, however, the molecule can get stuck in the 'hole' created!

For the sake of completeness, the balance of a reaction is not only dependent on the energy difference, it also depends on the concentration of the reagent and reacted molecule. To visualise this, it might be better to visualise the whole concept as a vibrating plate with a bump in the middle: if the molecules that end up on the left after bouncing from the right are taken away, eventually all molecules will be gone from the right side. Adding more energy (increasing the vibration intensity) will cause this to happen faster.

 

[epenguin]

Borek: Are you saying that enzymes differ from catalysts in this particular characteristic?

How do catalysts/enzymes work? Are they involved in some kind of intermediate reactions? Some general and true explanation?

Thanks a lot for all the guidance and time. I'm highly indebted to you for all this.

Best wishes
Jack

He is saying they are just catalysts but whereas other catalysts, for instance hydrogen ions or platinum surfaces will catalyse a quite large number of reactions an enzyme typically will catalyse only one (or a very small range). E.g. hydrolysis catalysed by a given enzyme of type called 'esterase' will hydrolyse only esters, and then only a limited range of structures, say esters with the acid moiety 3-5 carbons long and others not or very slowly, while another enzyme will hydrolyse say certain aromatic esters, another phosphate esters, another amides and so on. How it achieves this 'specificity' was indicated by the chemist Fischer long ago in his 'lock and key' simile. The structure of the enzyme is complementary to that of the substance whose reaction is catalysed, like a lock and key. This he concluded on the basis of then known facts of specificity though no protein structures were then known. Now many are and there is vastly more detail known about this and we can see the fit between the structures.

And it achieves its specific rate acceleration by being even more complementary to the 'transition state' - jackthetri's 'top of the hill'. Mechanisms are largely like those of other solution catalysis with added refinement. Eg H⁺ ions may be transferred to a substrate, or nucleophiles may attack but the acid or nucleophilic groups are just positioned to do this when the reactant is bound to protein instead of depending on chance encounters in solution.

But you will find this in any up to date book of biochemistry or on proteins in your bookshop. (You really need ones with 3-D pics). Textbooks on enzymes by e.g. Fersht but you need a basis of chemistry for them.

 

[Ygggdrasil]

How do catalysts/enzymes work? Are they involved in some kind of intermediate reactions? Some general and true explanation.

In a general chemical reaction, you can think of two molecules colliding. When these molecules collide, one of two things can happen: they can bounce off of each other or they can react. Two factors govern whether they react or bounce off each other. First, in order for the molecules to react, they must hit each other in the proper orientation for the reaction to occur. For example, if molecule A adds onto a specific part of molecule B, it must collide with that area on molecule B with the proper trajectory in order to react. Hitting a different area of molecule B would not result in a reaction.

Second, the molecules must collide with enough energy to react. This requirement comes because you must break some bonds in the original molecules to form the new bonds in the product. This is why higher temperatures lead to faster reaction rates. At higher temperatures, molecules move faster and have more kinetic energy. Therefore, the collisions between molecules become more energetic. The average amount of energy required for a collision to result in a reaction is called the activation energy.

Catalysts can therefore speed up reactions in two general ways: by helping to orient molecules for reaction and by making the bonds in the reactants easier to break, thereby lowering the energy barrier for reactions. Catalysts and enzymes employ a wide variety of ways of achieving these two means and one could write textbooks full of the different mechanisms. So, I'll simply provide a few illuminating examples.

A simple example is a metal surface (such as platinum) acting as a catalyst. A gas (such as H₂) can bind to the surface. Normally, for hydrogen gas to react with another product (e.g. with O₂ to form water or with N₂ to form ammonia), one would have to break the hydrogen-hydrogen bond. Binding of the hydrogen atoms to the platinum atoms in the surface actually weakens the hydrogen-hydrogen bond in the H₂ molecule, making it much easier to break. In fact, some surface catalysts do, in fact, break the hydrogen-hydrogen bond producing bound individual hydrogen atoms on the surface. These bound atoms can then easily recombine with other molecules or atoms on the same surface. Here, it is important that binding to the surface is reversible so that the products can later leave (thus, catalyst binding to the products/reactants must be finely tuned not to be too strong or too weak, but just right).

Enzymes can also form temporary bonds with the reactant molecules which help to weaken and/or break the required bonds for a particular reaction. Proteases, enzymes that digest proteins, are good examples of this. Enzymes are also good at orienting their molecules for reaction since their complex three-dimensional structure allows them to bind to their targets in a very specific way. Indeed, many enzymes that join molecules together (such as the polymerases that add nucleotides to DNA in order to elongate DNA) will bind both reactants then will change shape to move these two molecules together and orient them. This movement positions the reactive end of one molecule right next to the reactive end of the other molecule helping the molecules react in the correct way to give the product that the cell wants.

 

[jackson6612]

Thank you very, very much for all the help, Jack, Epenguin, Yggg. I wish there were a better way to thank you guys. I'm still struggling to grasp the important points, though I have one side question.

What kind of reactions can be catalyzed? I think catalysts play role only in those reaction where molecules are involved and where molecules making up the molecules are tightly bound to each other. One doesn't need a catalyst for the reaction of sodium gas and chlorine gas. There is a contradiction in my own statement. Isn't oxygen a stable gas, then why does it react with hydrogen?!

Perhaps, some catalyst can also be used in reactions of oxygen or chlorine with one of noble gases (or, some other stable gas) to speed up the reaction.

Please guide me on this. Please remember that I'm not a science student.

I wish you all happiness and prosperity.

 

[Jack the Stri]

Well, you don't 'need' a catalyst for any reaction, all a catalyst does is speed up the process, it has no influence on the final outcome. The reason why reactions occur is because they lead to lower enthalpy/higher entropy (or, less heat (inside the system, which you can feel as the system releasing heat)/more chaos). Mathematically speaking, this is given by
$$\Delta G = \Delta H - T \Delta S$$, with G being the free energy, H the enthalpy (heat), and S the entropy (chaos). All systems strive for an as low as possible free energy.
Oxygen reacts with hydrogen gas for the simple reason that the reaction of O₂ + 2H₂ = 2 H₂O leads to a state of less free energy.

 

[jackson6612]

Well, you don't 'need' a catalyst for any reaction, all a catalyst does is speed up the process, it has no influence on the final outcome. The reason why reactions occur is because they lead to lower enthalpy/higher entropy (or, less heat (inside the system, which you can feel as the system releasing heat)/more chaos). Mathematically speaking, this is given by
$$\Delta G = \Delta H - T \Delta S$$, with G being the free energy, H the enthalpy (heat), and S the entropy (chaos). All systems strive for an as low as possible free energy.
Oxygen reacts with hydrogen gas for the simple reason that the reaction of O₂ + 2H₂ = 2 H₂O leads to a state of less free energy.

Jack: Thanks for the help. But I have to tell you that it would have been more convenient for me if you had focused on one particular example.

My question should have been that why some reactions proceed at such fast rate that they don't need catalysts.

Let's focus on the reaction between oxygen and hydrogen. How does binding of an H atom with O atom lead to lower energy and why? Please avoid mathematical equations. Try to explain in terms of atoms. How does the entropy increase after H and O come together?

Please remember that I'm trying to learn science from very basic level and am quite a layman in this area. Thanks for your help and time.

Best wishes
Jackson

 

[Jack the Stri]

The speed of a reaction, say A -> B, at any point in time is given by the difference between the speed of A reacting to B and the speed of B reacting to A (v = v_forward - v_reverse). These speeds are determined by the height of the activation energy, or 'the hill' in my previous post (see also this image).

Obviously, the reverse reaction speed of water decomposing into O₂ and H₂ is very small. This is due to the fact that the energy difference before and after the reaction ($$\Delta G$$) is very large, making the reaction energetically unfavourable (this means that a lot of energy must be invested to make it happen). The reaction happens very fast because the activation energy (height of the hill) is small in comparison to the energy difference: thus, the molecules can easily 'hop' from one side of the reaction to the other, but not back again.

In the particular case of hydrogen and oxygen forming water, the entropy ($$\Delta S$$) actually decreases, which is unfavourable for the reaction. However, this is compensated for by a decrease in enthalpy ($$\Delta H$$), which leads to $$\Delta G$$ being negative, which means a reaction will occur.

As to why the energy is lower when H binds to O than if it remains bound to another H (and similarly for O), we need to briefly dip into electronegativity. Electronegativity describes how 'eager' atoms are to attract electrons. The higher the electronegativity, the easier it is for an atom to draw electrons towards it, and the harder it is for electrons to be drawn away from it. Oxygen is much more electronegative than hydrogen. In O₂, both oxygen atoms pull at the electrons as hard and in the end they have to 'share' the binding electrons, the same is true for hydrogen. In water, however, oxygen pulls at hydrogen's electrons, and since it can pull harder than hydrogen can, the oxygen atom will take more than its 'fair share' of the binding electrons. It is this fulfilling of electronegativity that makes water more stable than H₂ and O₂ separately.

Sorry if I use too much maths, but it's hard to describe thermodynamics without it :)

 

[jackson6612]

Thank you very much, Jack.

Sorry if I use too much maths, but it's hard to describe thermodynamics without it :)

No, no, you shouldn't be sorry about anything. It's my limitations which are to be blamed. But I do have serious problems with pedagogical approach toward science teaching in general. I have seen when someone tries to explain a science and mathematical problem, they start with the things which are abstract. It would be utter nonsense to start to explain kinetic energy with some mathematical equation. You need to start with 'physical' explanation where you try to explain every phenomenon physically. Then comes the part how to mathematize those 'physcal' explanations. Unfortunately, this also happens with mathematics teaching or guideline where teachers or instructors reduce every mathematical problem to a set of formulae. Mathematics can be as intuitive as any other subject if you know how to teach it. Mathematics is there for mathematization of scientific ideas not to explain every idea in mathematical terms. I hope you get my point.

Now I'm going to ask multiple questions and there would be ample of mistakes, so be careful!

I have been to different sites but nowhere I found any intuitive or 'physical' explanation of 'free energy'. As a comparison take thermal energy which I think is same as kinetic energy and accounts for temperature and motion of the molecules or atoms is basis for this energy. So, what is this 'free energy'?

Both catalyzed and uncatalyzed reactions start with 0 free energy and as the reactions proceed they end up with different activation energy. Now my question is: Why was fee energy '0' at the start?

Link: http://sarahssureshots.wikispaces.com/file/view/figure06-14.jpg/105957707/figure06-14.jpg

Is finding a suitable catalyst for a reaction a kind of trial and error struggle/procedure as are much of the things in chemistry?

As you say in case of water entropy decreases. I believe entropy is about how ordered a thing is. A room with 50 persons in it is less 'ordered' as compared to a room with 5 persons in it. 2H2 + O2 --> 2H2O. In this reaction 2 molecules of H combine with 1 molecule of O, 2 molecules in total, to produce 2 molecules of water. This would mean that the product of the reaction is more ordered and less-fussy as compared to the reactants. Do I capture the essence of entropy idea here?

As you say enthalpy is heat. Which I believe is internal energy of atoms. A molecule consisting of 2 or more atoms is also involved in internal motions as compared to linear motion which is basis for kinetic energy (temperature). Here 'internal motions' mean internal vibrations of the atoms constituting a molecule. Less heat/enthalpy would mean less internal vibrations which in turn would result in more stable molecule. Do I have it right?

Please remember that I'm trying to learn science from very basic level. So, please explain it in as much detail as possible. Thank you very much for all the guidance and your time.

Best wishes
Jackson

 

[Ygggdrasil]

Here's a particularly good explanation, intuitive explanation of free energy:

https://gravityandlevity.wordpress....e-plays-skee-ball-the-meaning-of-free-energy/

 

[jackson6612]

Yggg: Thank you very much for the link. It was really helpful. At least now I have a rough idea what 'free energy' is all about. I wish they could write science books in that way!