Effect of Solutes on Water

It seems that the stars are aligned or something for me to continue my discussion on water and material science. Not only did Dario comment on my Water Activity post asking me to discuss the relationship between glass transitions and water activity, but in a recent Friday Sprog post on Janet’s blog, she discussed phases of matter and a commenter talked about materials that was both solid and liquid. These materials are known as amorphous materials - there seems to confusion as to whether they are solid or liquid because they can have the behavior of both phases. Amorphous materials do not have the regular, ordered pattern of crystalline solids and they typically have more structure than a liquid.

Many foods are amorphous. In fact, very few foods are true crystalline solids or true liquids. Those that are; salt, sucrose, oils; are generally pure compounds. In most foods, water is present with solutes; salts and sugars; and polymers; proteins, complex carbohydrates, and lipids. These interfere with the standard behavior of pure water.

To get you started Janet has a great post about the typical three phases of matter that you meet in high school chemistry class, namely solid, liquid and gas. In that post she represents a great phase diagram:

 

This is a great representation for materials such as water that have a crystalline solid, ice, and when pure easily convert into a liquid. The point of change between ice and water is the melting point if you increase the temperature or the freezing point if you decrease the temperature.

As you add solutes to water the temperature of freezing/melting changes. Freezing occurs at a lower temperature. This is one of the colligative properties and is the reason why, as long as you don’t live anywhere really cold like Minnesota, a dilute solution of salt can be used as an antifreeze, as it freezes at a lower temperature than pure water. Here is a phase diagram from Spark Notes showing both the pure solution and the solution with dissolved solutes:

This is true of whatever solute you use, so sugar would have a similar influence on the freezing temperature. Thus, food freezes at a lower temperature than pure water. If you have ever seen the Alton Brown episode on Thanksgiving the turkey guy talks about the different temperatures and definitions of fresh, hard chilled and frozen turkey. They are also given here. For instance:

Any turkey labeled “fresh” in a food store has never been cooled to a temperature lower than 26°F, which is the point that turkey meat begins to freeze.

And:

[...]in order to be considered “frozen” a turkey must be cooled to a temperature of 0°F or below.

Obviously, hard chilled turkeys are stored between 0 and 26F.

So where does that get us?

Pure water has three phases: ice, water and steam (or solid, liquid and gas for other materials) and the phase change temperatures are influenced by the presence of solutes and polymers present in the liquid. Additionally, as a food freezes, the water typically freezes out, leaving behind a more and more concentrated solute liquid phase. This can result in an change in pH, viscosity amongst other properties. In some instances, the solute-liquid phase might never solidify. If a food is rapidly cooled to below the freezing temperature, a glass may be formed. A glass is an amorphous solid that has a disordered structure while behaving like a highly viscous solid. In some frozen foods, water will freeze to from crystalline ice and the solute-liquid phase will form a glass.

In my next post, I will discuss in detail the effect of sucrose on water and water activity. This is interesting because it is why we can make different candy types.

Basic Concepts: What are Solutions?

In my last Basic Concept post when discussing why reactions occur, I left it rather vague as to what happens when substances dissolve. I suggested that no reaction had taken place because the chemical structure of the solutes had not really changed. Today I want to go into a little more detail as to what happens when solutes and solvents mix and discuss the difference between solution and liquid. We typically think of solutions as being a liquid mixture. But are all liquids solutions and all solutions liquids?

This question is a bit like the rhetoric around the series of statements:

My cat is gray. Are all cats gray?

To be able to answer the question as to the relationship between liquids and solutions we need to define what we mean by liquid and solutions. Janet in a post on phase changes describes:

A liquid has more intermolecular attractions holding the molecules to each other than the gas phase, but fewer than the solid phase. As a result, while the resulting clump of molecules is still nowhere near as compressible as the gas phase of the same substance would be, liquids don’t have rigid shapes, and they take on the shape of the containers you pour them into. The intermediate number of intermolecular associations (between gas and solid) means that the molecules in the liquid are sticking together but have some “wiggle room”.

Chambers Dictionary of Science and Technology defines liquid as:

A state of matter between a solid and a gas, in which the shape of a given mass depends on the containing vessel, the volume being independent. Liquids are almost as incompressible as solids.

The dictionary defines a solution (chem) as a:

Homogeneous mixture of two or more components in a single phase. Often refers specifically to a solution in water.

From this it would imply that solutions are special cases of liquids in the same way gray cats are a special case of cat. I quibble with the dictionary definition of solution (and fortunately so does this site for intro chemistry at OKState) as solutions can be solids in solids, liquids n solids, gases in solids, solids in liquids, liquids in liquids, gases in liquids, and gases in gases.

So having established that solutions are:

1. A mixture of two or more components
2. Typically but not always liquid
3. Homogeneous

When discussing how components become a solutions, it is for convenience sake that the major component of a solution is termed the solvent and the other component(s) is(are) termed the solute(s). This helps chemists discuss what is going on when components of a solution are mixed. These are defined by the dictionary as:

Solvent: The component of a solution which is present in excess, or whose physical state is the same as that of the solution.

Solute: A substance that is dissolved into another.

Solutions typically occur by the solute being dissolved into the solvent. In the case of an aqueous solution of sodium chloride (table salt, NaCl), water is the solvent (aqueous is just the chemistry fancy way of saying in water) and NaCl is the solute.

Interestingly, different solutes dissolve in different ways. As I have posted about before, sodium chloride (NaCl) becomes ions of Na(+) and Cl(-) when mixed with water. This allows the salt to stay in solution because the bipolar nature of water means that water molecules surround the ions:

Note that the partially positive hydrogen atoms of water face the negatively charged Cl ions and the partially negative oxygen atoms face the positively charged Na ions.

Organic molecules such as glucose, sucrose (table sugar) and vitamin C (ascorbic acid) are soluble because they have hydroxyl (OH) side chains. These form hydrogen bonds with water molecules.

The way that water molecules are arranged around the solute is known as solvation. This is defined as the association or combination of molecules of solvent with solute ions or molecules.

Not all organic molecules are able to dissolve in water. The familiar example being oil and water when making a salad dressing. Some organic compounds are able to partially dissolve into water. This makes them sparingly soluble and they partition into the water phase or form micelles with hydrophilic (water loving) groups facing the water molecule and hydrophobic (water hating) groups facing the inside of the micelle. This is how solutions such as milk are formed as suspended in water solvent are proteins and lipids.

Solutions are endlessly fascinating for food scientists and I will discuss more about colloids and suspensions in another post.

Basic Concepts: Why Do Reactions Occur?

I do not know why this thought popped into my head, but it seems that it is a fundamental question in chemistry. Why do certain molecules react and others don’t?

For example, acetic acid (vinegar) and sodium bicarbonate (baking soda) in the classic kitchen lab volcano experiment fizz nicely whereas when you add sucrose (table sugar) to a solution of sodium chloride (table salt) all that happens is that sucrose dissolves into the solution. In the former reaction the acetic acid and sodium bicarbonate reaction to form sodium acetate, carbon dioxide and water. In the latter reaction the molecules do not change, sucrose is no longer a solid but is dissolved into the solution*⁽¹⁾.

According to this website, reactions occur for two reasons:

1) The products have a low energy than the reactants

2) Products are more random (less ordered) than the reactants

Dealing with the first reason. Reactions are either exothermic or endothermic. Thermic means heat and exo means “outside” and endo means “within”. So reactions can occur either by giving off heat (exothermic) or by absorbing heat (endothermic). Endothermic reactions cannot occur unless heat is applied to the system. For example, water will not evaporate without added heat. Exothermic reactions are more likely to occur spontaneously. An example of an exothermic reaction is used in MRE kits by the military. When certain chemicals, e.g. lithium chloride, sodium hydroxide*⁽²⁾, dissolve into water, they give off heat. Thus, their dissolution is exothermic. This is then used in the MRE to heat the provided food.

Unfortunately, we now cannot have any further discussions about why reactions occurring without meeting some thermodynamics. In particular, enthalpy and entropy. Rob Knop at Galactic Interactions has a good post about entropy from a physics perspective.

Entropy explains the second reason why reactions occur. Entropy is considered to be the disorder or randomness of a system and is related to the second law of thermodynamics. That is the law that most of us remember as order –> disorder. Or in my case an explanation as to why my bedroom was messy (neat –> messy as messy was the more disordered state). The more random or disordered the higher the entropy, which is, by the way, represented by S.

Enthalpy is a little tricker to understand. It is related to the heat energy of the system and is represented by H or ΔH where Δ is used to represent “a change in”. In the first reason for why reactions occur, enthalpy can be considered as an explanation. If:

ΔH = H_products - H_reactants

Then exothermic reactions have a negative enthalpy as the energy of the products is lower than the reactants and endothermic reactions have a positive enthalpy.

Entropy and enthalpy are linked, thus if the enthalpy of the system is such that the reaction appears to endothermic, the reaction can only occur if the entropy is increased. They are related by the equation for Gibb’s Free Energy:

ΔG = ΔH -TΔS

For a reaction to occur, ΔG needs to be negative.

So why do reactions occur:

1) The enthalphy of the products is lower than the enthalpy of the reactants

2) The entropy is higher in the final system than in the initial

3) ΔG of a system is negative

So that’s the best I can do. If I’ve got anything wrong*⁽³⁾ or you need further explanation, please let me know in the comments.

*Notes

(1) Dissolution will have to be a topic of another blog. Janet at Adventures of Science and Ethics has a great post about phase change which is part of what is happening. For the pedantic chemists reading this post, dissolution can be exothermic or endothermic. I am discussing noisy and obvious reactions in this post.

(2) MRE heaters, known as flameless ration heaters are unlikely to contain sodium hydroxide as that is caustic soda. According to this website they contain iron, magnesium and sodium.

(3) I blame all errors on my dyslexia, which means exo and endo, positive and negative, left and right can, and do, have opposing definitions. I meant the other left (exo, positive) etc.

Further links

Why do reactions occur?

Magical Properties of Water - Part 3

Encouraged by the commentators on Magical Water Part 2, I am revisiting a year old New Scientist article on the wonders of water. When I read it last April (I actually subscribe to NS, having read it for over twenty years it is hard to give it up) I was disturbed as the article seemed to concentrate a lot on Massru Emoto’s work on how emotion alters water crystal shape. His work was used in that weird movie “what the bleep!?” which left me very uncomfortable. I was also left feeling uncomfortable after reading the New Scientist article. Amongst the new age woo, there were statements by scientists, such as Dr Felix Frank who did research on the role of water in food shelf life and stability. I even had the opportunity to meet Dr Frank as I was asked to co-chair a session on the “Role of Water Functionality” at one of the Institute of Food Technologists’ annual meetings. I just have assume that Dr Frank has either joined the ranks of great scientists tripping over the great wonders of woo. The greatest member of that group being Dr Linus Pauling with his ideas about Vitamin C. Or he has been misused by New Scientist. Admittedly his comments are not too far out but it is the company that he is keeping that bothered me.

I digress. One of the reasons I didn’t get round to writing this article last April, other than day to day distractions, is that Orac chose Massru Emoto and his studies for one of his Friday Doses of Woo. All covered, I thought, by Orac. Additionally while searching for information on water and, in particular the zero point energy, I found this great site on water which is going to save me a lot of time, but also discouraged me from writing on water for a while.

I revisited the issues of water on health when I bought a bottle of water in North Carolina in January and then spent two posts debunking the claims made by Essentia.

The New Scientist article strongly implied that there might be something in the new age theories by using scientific studies, and talking about the quantum effects of water and the fact that water is necessary for many proteins and even DNA (gosh, the molecule of life) to function correctly. I also got discouraged because they talked about the zero-point vibration* properties of water as if this was really important and I had never heard of it before and couldn’t find anything out about it that I understood. I even asked people for help on my last magical water posts and it was confirmed that zero point energy is obviously serious woo. They even mention hydrogen bonding and the unique behavior of frozen water in hushed terms as if it was something new that no one understood.

We have known for a long time that many molecules need water as part of their structure. Known as the water of hydration, this water cannot be removed without chemical compounds losing their structure, and in the case of proteins, function.

Considering how little is known about the chemistry of water - it was disappointing to see how much space was wasted on new age theories.

I do actually believe water is amazing. See my scientific posts on water to find out how truly amazing water is without needing to add woo.

—

* I am sure this definition helps:

(′zir·ō ¦pöint vī′brā·shən) (statistical mechanics) The vibrational motion which molecules in a crystal lattice, or particles in any oscillator potential, retain at a temperature of absolute zero; it is quantum-mechanical in origin. Also known as residual vibration.

—

Through the water site, I also found this site debunking many of the pseudoscience claims made for water. A great site to visit for any one wanting information.

Pectin - Introduction

One of my abstracts that was accepted for presentation at the Institute of Food Technologist’s Annual meeting was on low ester pectin. Pectin (link2) is a very intriguing molecule; a polysaccharide made mostly of galacturonic acid units. However, interspersed within the galacturonic acid polymer there are regions where rhamnose replaces the galacturonic acid and neutral sugars branch out of these regions. The galacturonic acid are frequently esterified with a methyl group. Alternatively, they can be amidated which means an amino group is attached. Amidation is typically added commercially to improve its functional behavior.

Pectin is used extensively in the food industry as a gelling agent. Not only is it used in jams and jellies, but also as a thickening agent and protein stabilizer. It is also added to diet sodas to improve mouthfeel.

Pectin is defined as low ester and high ester. In the US, the cut off between them is when 50% of the galacturonic acid groups are esterified. Low ester pectin requires minerals, typically calcium, to form gels.

Reference and source of picture:

Hoefler, A. C. (2003). Effect of calcium concentration, degree of amidation, soluble solids, and carbohydrate type on the gel strength of low ester citrus pectin. Animal and Food Sciences. Newark, DE, University of Delaware.

Beer Aging Part 2

In a previous post I reviewed a recently published article on beer aging. I got so interested that I found a review of the chemical changes occurring in beer written by the same authors. While it might have been helpful do that before writing the previous post, it was writing that post that got me interested in reading more about this topic.

As mentioned, the problem with beer shelf-life is that it isn’t really stable [no food is – most food would be defined a “metastable”. Yep, even Twinkies]. During storage, chemicals continue to react which alters the flavor, both taste and smell. This can be positive; for example, wine and whiskey are left to “age” so that their flavors can develop; In beer, the flavor changes are typically seen negative as the beer no longer tastes as it ought. It might not taste “bad” but it tastes differently to expectation. This is particularly important for branded beers. A regular drinker of Heineken[*] knows what taste to expect and would be disappointed if it tasted differently. This is not just beer, but all food. For example, if are you used to eating Kellogg’s Cornflakes and then try switching to the store labeled corn flakes, the taste just is not the same.

So what happens to beer during storage? In the article reviewed in my previous post, they looked at fifteen different markers which were produced by several different reactions. According to the article I am discussing today (“The chemistry of beer aging – a critical review) the main reactions taking place in packaged beer are:

1) Reactive oxygen species in stored beer

2) Reaction leading to formation of carbonyl compounds

a. Oxidation of higher alcohols

b. Strecker degradation of amino acids[†]

c. Aldol condensation

d. Degradation of hop bitter acids[‡]

e. Oxidation of unsaturated fatty acids, including enzymatic breakdown

f. Formation of (E)-beta-damascenone

3) Acetalization of aldehydes

4) Maillard reaction

5) Synthesis and hydrolysis of volatile esters

6) Formation of dimethyltrisulfide

7) Degradation of polyphenols

Having teased you with that list, I am now going to ignore it completely to discuss inhibition.

The most important way beer in which beer chemistry changes during in storage is through oxidation reactions. In the list above at least three of the staling mechanisms are directly caused by oxidation, if not more. So beer can be protected by limiting the amount of oxygen present when the beer is bottled. This includes both headspace and dissolved oxygen. It is also why you won’t see beer in plastic bottles any time soon[§].

Oxidation can also be prevented by the addition of antioxidants. These include sulphite, which is produced by yeasts in beer from sulphate; polyphenols from barley malt and hops; melanoidins produced by the Maillard reaction during heating steps such as malt roasting and wort boiling. Ascorbic acid and chelating agents could also be added after processing.

The prevention of enzymatic oxidation is also important. The enzymes which cause the problems in beer are lipoxygenases. Their activity may be reduced by lowering the pH. Additionally, choosing barleys with low lipoxygenase activity might also help. You could heat the malt to a temperature that would cause the enzyme to be destroyed, but this will destroy desirable enzymes.

Non-oxidative beer aging reactions are very diverse and, therefore, harder to control and inhibit. Many of these reactions were promoted by reducing the beer pH. Sulphite was considered useful as it inhibits the Maillard reaction in addition to, as mentioned above, inhibiting oxidation.

I don’t know about you, but writing this has made me thirsty. See you down the pub!

–

Reference:

Vanderhaegen, B., H. Neven, et al. (2006). “The chemistry of beer aging - a critical review.” Food Chemistry 95(3): 357-381.

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[*] This is no reference on Heineken, it was the only branded beer that I could think of at that moment. Replace with the name of your favorite beer.

[†] I personally consider the Strecker degradation reaction as part of the Maillard reaction.

[‡] This one, to me, should also be a category of its own as the loss of bitter flavor can destroy the taste of beer.

[§] Go on, some one tell me about a beer in a plastic bottle. And then tell me tastes good in six month.s

Why Use Buffers?

This is part of a series on basic chemistry. See my posts on pH, acids and bases, strong and weak acids & buffers for earlier information.

I am going to approach this post by showing how Woo-practitioners (Quacks) who like to talk about the body being too acid and needing become more alkaline are talking nonsense. In fact, it should be a warning sign, if a treatment or diet mentions “restore your alkaline body pH” or “the secrets of an alkaline body” [1] it is distinctly woo. This is a big area for quackery. For example, I did a Google search on “alkaline body” and got approximately 24,000 hits.

So I am going to discuss human physiology.

We all are buffered. It is very important that the pH of the various body liquids remains within a tight range. Different body fluids may be at different pHs, but they are tightly controlled at that pH by buffering [2].

So why is the alkalinity body theory considered woo?
When you eat anything, it is digested [no arguments, right?] passing through the mouth, the stomach, small intestine, large intestine (AKA bowel, colon). Bits, if not most, are absorbed along the way. Different fluids in the digestive system are at different pH.

For example, the mouth typically has a resting pH around 6.3; and the presence of bicarbonate ions in saliva maintains this pH. This is not a true buffered system as the introduction of food can alter the pH and the bicarbonate salts neutralized the acids present (ref).

As food passes into the stomach, gastric juice is released. Gastric juice contains hydrochloric acid with a pH 1-4. Even when the stomach is full, the pH of the stomach will fall that low. It doesn’t matter what you eat, gastric acid will be produced to reduce the pH. Stomach enzymes are ineffective if the pH is higher.

High levels of HCl (pH 3) convert pepsinogen to pepsin, which is active at acid pH and kills ingested bacteria. (ref)

When food leaves the stomach and enters the small intestine, bile and pancreatic juices are released. Both of these are alkaline, partly to neutralize gastric juices, but also to act as a detergent, allowing the lipids to become more dispersed and available to enzymatic activity. Small intestine enzymes require this increase in pH to be able to function.

So given that food goes through all these changes in pH during digestion, it is unlikely that food can alter the pH of the body. Additionally, blood pH is closely regulated with an internal buffering system based around bicarbonate ions as well as serum proteins. I’ve posted on this before using this reference.

Thus, diet has little, if no, effect on the body’s pH. If your body’s pH is out of whack, you probably should check for an underlying illness.

So having sorted out those Quacks, why might we want to use buffers? The body already has buffers especially in the blood. If we want to study body functions outside of the body [known as in vitro] we would need to have a model that mimics the body especially the pH. If we did experiments at a different pH, the enzymes present might not function. For example, salivary amylase, which initiates starch digestion in the mouth, has a pH limit of 5.6-6.9, below and above which it cannot function.

As both plants and animal have natural buffering systems that means that food does too. When I am studying the Maillard reaction, I typically use model systems. This is not ideal, but removes any extraneous factors such as other reactions interfering with the reaction I am want to understand. This model systems are buffered at a pH typically found in food. When the pH is changed, a different reaction mechanism may even take place.

So buffers are important tools in chemistry allowing us to carefully control the pH.

Knowing the pH of a food product is important for food safety, as different micro-organism can or cannot grow when the pH is low. Acid-foods are more resistant to spoilage than low-acid foods, and are regulated differently.

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[1] I cannot bear to put any more links in. Reading the two I posted is giving me a headache. Quick, I had better restore my brain’s alkalinity!

[2] Frustratingly I cannot find a list of body fluids showing their pH. If any body knows for one, please let me know if the comments.

Do herbal teas have antioxidant properties?

Researchers publishing in Food Chemistry (link, sub required (1)) studied the anti-oxidative and anti-hydrogen peroxide (H₂O₂) activities of herbal teas.  The herbs they studied were lavender, rose, chamomile, hibiscus, lemongrass, sage, rosemary, echinacea, thyme, peppermint, gingko, liquorice, and thorn apple. They compared their antioxidant properties and prevent the formation of H₂O₂ with those of green tea.

Herbal teas or tisanes are high in polyphenols, which as free radical scavengers have anti-oxidant properties. I thought I understood the reason why they studied H₂O₂ as in the body a highly level of H₂O₂  could lead to free radical formation and further oxidative damage, but the amounts produced, even by green tea are too small to be significant. In fact, some might even be helpful. Catechin derivatives in the teas have been previously shown to increase the formation of H₂O₂. The pH of the tea was a strong influence on H₂O₂ formation.

They prepared the teas by mixing 0.1 g dried herb with 10 ml H₂O at 100 ^oC and steeping for 10 minutes before filtering.  The filtrate was measured for total polyphenols, radical scavenging activity [for those who care radical-scavenging activity was measured by reaction with DPPH] and H₂O₂ concentration.  The polyphenol concentration and radical-scavenging activity correlated to give a correlation factor of 0.950. Tea made from rose [do they mean rose hip?] was the only tea to have a higher polyphenol content and radical-scavenging activity than green tea.  A factor put down to the anthocyanins present in rose; which are also responsible for the color.  It is possible that rose has a high level of vitamin C which has anti-oxidant activity.

H₂O₂ formation did not correlate with either radical scavenging activity or polyphenol activity but may be related to pH as shown by the fact that echinacea tea had the highest pH and the highest H₂O₂ concentration.  During incubation the H₂O₂ concentration increased only for green tea.  If thorn apple (perhaps that it rose hip?) and hibiscus reduced the formation of H₂O₂ when mixed with green tea.

There was a similar article in the same addition of Food Chemistry (link sub req. (2)) which was looking at the antioxidant properties of extracts from large thyme (Thymus pulegioides rather than Thymus vulgaris).

References

1) Aoshima, H., S. Hirata, et al. (2007). “Antioxidative and anti-hydrogen peroxide activities of various herbal teas.” Food Chemistry 103(2): 617-622. Link

2) Loziene, K., P. R. Venskutonis, et al. (2007). “Radical scavenging and antibacterial properties of the extracts from different Thymus pulegioides L. chemotypes.” Food Chemistry 103(2): 546-559. Link

Beer Aging

This is based on an original research article published in Food Chemistry (1).

Buffers

In my last basic concepts post, I discussed strong and weak acids. This is also true of bases (alkalines). When dissolved in water, weak bases and acids do not fully dissociate, causing an equilibrium to exist between the acid (HA) and acid ion (A^-) (also known as the conjugate base):

HA + H₂O ⇌ H₃O⁺ + A⁻

ref (1)