Non-Enzymatic Browning Introduction 2

Food tastes best when browned.

Food in always complex unless you are studying something quite simple such as a beverage with few ingredients (vitamin water, anyone?).  Even sucrose has a complex chemistry, more of which I will share in a future post.  So individual NEB reactions cannot be isolated in food.  Quite often intermediates and products from one reaction become intermediates in another reaction, especially in the Maillard reaction. Thus, most food chemistry textbooks use Non-Enzymatic Browning (NEB) as synonymous with the Maillard reaction. However, the other NEB reaction cause browning in food without the use of enzymes.

Both caramelization and lipid oxidation cause browning in certain foods, i.e. sugar-based and fried foods, respectively. Ascorbic acid degradation is significant in food with a low pH (high acidity) especially in citrus juices.  The reaction of flavanoids is important in highly colored foods as the colorful anthocyanins degrade and lose their color.  The reaction of flavanoids may also be important in soy protein, but less because of a color change and more due to a lose of isoflavones.

NEB Intro Part 1

Non Enzymatic Browning

My major interest in food chemistry is how food changes during processing and storage.  I am especially interested in how color changes take place.  The reactions I am interested in are called Non Enzymatic Browning reactions to differentiate them from the browning that occurs when you cut an apple or banana, which involves an enzyme.

Non enzymatic browning (NEB, non enzymic browning) reactions are the most important reactions in food, and, no, I am not biased.   Just image the aroma of melting chocolate, freshly baked bread or  a roasting leg of lamb, the golden color of a croissant, the dark amber color of a well brewed beer; caramels, toast.  These are all caused NEB reactions.

There are five different NEB reactions and I intend over the next few months to write about each of them:

1. Caramelization – browning of sugar, especially sucrose
2. Lipid Oxidation – the oxidation of fats and oils; including rancidity
3. Break down of flavonoids – highly c0lored compounds can also lose their color
4. Degradation of ascorbic acid (Vitamin C) – AsA is unstable even without oxygen
5. The Maillard Reaction –  reaction between carbonyl compounds and amino acids

Numbers (3) and (4) are not typically on a list of NEB reactions, but I did my thesis on ascorbic acid browning and it definitely goes brown without oxygen and without enzymes.  The degradation of flavonoids is one I have added and came to me in flash of inspiration when at a conference.  I am sharing it with you now, so this is new even though I had the idea three or four years ago.

More later…

Simple Sugars: Fructose, glucose and sucrose

Glucose, fructose, sucrose

Simple sugars are carbohydrates. Glucose and fructose are monosaccharides and sucrose is a disaccharide of the two combined with a bond.  Glucose and fructose have the same molecular formula (C₆H₁₂O₆) but glucose has a six member ring and fructose has a five member ring structure.

Fructose is known as the fruit sugar as its make source in the diet is fruits and vegetables. Honey is also a good source.

Glucose is known as grape sugar, blood sugar or corn sugar as these are its riches sources. Listed in food ingredients as dextrose.

Sucrose is the sugar we know as sugar or table sugar. Typically extracted as cane or beet sugar. If sucrose is treated with acid or heat, it hydrolyzes to form glucose and fructose.  This mixture of sucrose, glucose and fructose is also called invert sugar.

Nutritionally, these sugars are the same as they all provide 4 Cal/g. This is true for starch and other digestible carbohydrates too. Of the three sugars, fructose is the sweetest and glucose the least sweet, so typically less fructose can be used than table sugar (sucrose) – if sucrose has a sweetness of one, fructose is 1.7 and glucose 0.74

Fructose is more soluble than other sugars and hard to crystallize because it is more hygroscopic and holds onto water stronger than the others. This means that fructose can be used to extend the shelf life of baked products more than other sugars.

Wikipedia has lots information on sugars, including information on the three I am interested in fructose, glucose and sucrose.

Molecular Gastronomy is Part of Food Science

In a recent issue of Food Technology, the magazine for IFT members, Hervé This responds to the suggestion that molecular gastronomy is part culinary art and part science. He gives a very good summary of the differences between cookery/culinary, food science and food technology:

“Cooking is a technique (sometimes an art) and the objective is to make food.”

“On the other hand, molecular gastronomy is a science. It is performed in a laboratory.”

“Furthermore, science is not technology. Thus, applied science cannot exist. Application involves technology (from techne, doing, and logos, study). When examining mechanisms of phenomena, the goal is not to apply knowledge (application), but rather to produce it.”

He admits that he himself had problems during his thesis of separating out science from technology but he states very strongly that molecular gastronomy is science and molecular cooking is using the results from molecular gastronomy to create new food items or improve old ones. This’ Ph.D. thesis, on Physical Chemistry of Materials, was entitled Molecular and Physical Gastronomy or the equivalent in French.

The confusion between the science, art and technology of food is present in food science. That there does not appear to be a final definition of molecular gastronomy adds to this confusion, especially as chefs have taken over this term, rather than using This’ preferred Molecular Cooking. Khymos gives a good summary of the different definitions.

I do have problems with the fact that Molecular Gastronomy is so trendy and considered to be the saving of the world’s food supply.  [So I exaggerate? What’s the problem?] Many articles about Molecular Gastronomy and the restaurants that practice molecular cookery appear to have never heard of food science.  So I appreciated the fact that This states that molecular gastronomy is part of food science but I struggle to place it within the traditional subject areas of food science.  It overlaps mostly with food chemistry.  At least This’ part of Molecular Gastronomy is heavily physical chemistry based.  The research undertaken is more directly relevant to cooking and culinary arts than much of food chemistry.  For example, my research on the Maillard reaction has few direct practical applications, unless you are willing to mix amino acids and sugars together in your kitchen.  I still would not recommend eating the results of my research.

Within the article he gives an excellent summary of what science is – the idea of testing a hypothesis to give new information which increases our knowledge of a system.   I might even use some of these ideas for teaching.

References

Hervé This Molecular Gastronomy vs. Molecular Cooking Food Technology December 2008 (PDF)

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.

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* 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.

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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!

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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