It’s A Slippery Subject - Oil

In this new instalment of scientific food blogging we are going to be looking into the taboo world of fat. Whether you believe fats are in your health favour or not we can all agree they do make stuff taste better! A perfectly scientifically valid argument to this point lies with the cocoa bean; aka the chocolate bean. Cocoa in its ground up natural form is rancid (that’s a fat joke, we will come to that) which no amount of healthy antioxidants can make up for. However add in delicious butter and cream to the harsh tasting cocoa, with a dash of sugar and the result makes the world a much better place. Skipping the introductory quips, in this post we are going to explore the differences between oil and fat, and whether these terms can be used interchangeably.  We will also look into the origins of fat, examine the battle royal that is butter vs oil vs margarine, and finally delve into the end of a fats life – its expiration date.

Biological and Culinary Uses

We are told that fats have serious health implications for us so why then are fatty foods so desirable to us? This is a side effect of our evolution; in nature fats are used as an effective energy store. Traditionally fats offered lots of energy that could be stored for times of limited food or for doing intensive work; like running away from dinosaurs. We as the human species have then evolved to like the taste and texture of fats ensuring we eat them; granting us enough energy to get away from any roaming T-rex we may bump into. Yes you could argue that athletes today are more likely to eat a balanced carbohydrate rich diet now, but carbs are not as effective as energy stores as fat /oil per unit weight. For example if we stored our excess energy supply as carbohydrate rather than fat we would be considerably heavier and unable to out run and escape the snapping jaws of death. More interestingly there could be a strong argument made for athletes to return to fat based diets over carbs due to our species evolving using fats for energy. Bread and pasta are rather modern inventions in comparison to eating nuts etc.

Fats have wide applications in food that are not always obvious to us and here we will quickly examine where their culinary genius.  Initially we think of oil as a cooking medium where fats are used to prevent food sticking to pans, while also providing good energy transfer to cook food. Alternatively water could be used to cook food but it would not yield the same results. Water cannot achieve the same high temperatures as fat (due to water evaporating ~100 ^oC at normal earth pressures), prohibiting the formation of intense meaty sweet flavours and the crisp textures associated with oil frying. More subtlety fats trick our brains into thinking food is moister than it is. When chewing food we use saliva to help us swallow it therefore the drier the food the more saliva will be used. Oil lubricates food allowing it to be easily swallowed, making us think a piece of meat is juicer than it actually is. Using this information we can now understand why low fat meats have a small “cooked” window before we consider them overcooked and dry. It also offers an explanation as to why when we eat younger animals and cuts of meat with less fat they initially are considered moist, but the more we chew the drier they become in our mouths. Room temperature solid fats have also been widely used as a preservative, for example in duck confit. When used as a preservative the fat acts as an oxygen barrier, protecting the intended food from becoming oxidised and spoiling. Fats also offer the cook the ability to form tender cakes, flaky pastry and even a method of cleaning their hands.

Fats and oils what’s the difference

So far I have used the terms fats and cooking oils interchangeably but are cooking oils and fats the same? Yes, they are both from the same family of molecules called triglycerides.

The structure of an example triglyceride molecule.

A triglyceride molecule. Tri- means the three chains/fatty acids and the glycerol is group that links the three chains. This raises an interesting question; if all fats are triglycerides then “why are some fats solid and others liquid?” The key to solving this conundrum is saturation. The words saturated and unsaturated have been used to describe the world of fat for a while now with little in the way of explanation for what the terms mean. Here I shall put to rest the organic chemistry jargon. Simply a saturated molecule is one where all the carbon atoms are connected to other carbon atoms by one bond. Subsequently an unsaturated molecule is one where carbon is connected to another carbon atom through multiple bonds. This is drawn below:

Top Left: An unsaturated molecule that has double carbon carbon bonds in its structure. Top Right: A saturated molecule that has single carbon carbon bonds. The letters C and H represent carbon and hydrogen atoms respectively and dashed lines represent the bonds between the atoms. Middle Left: A striped down overall version of the overall molecular structure of an unsaturated chain. Middle Right: A striped down overall version of the overall molecular structure of a saturated chain. Bottom Left: Poor packing of unsaturated fat molecules. Bottom Right: Tight molecular packing of saturated fat molecules.

Saturated molecules adopt a straight line shape that packs together very well which fits the definition of a solid; a tightly packed structure where molecules have limited movement within their arrangement. From this information saturated triglycerides must form solid fats. On the other hand unsaturated triglycerides cause kinks in the carbon chains. These kinks prohibit good packing of the molecules increasing the average distance between the molecules; giving unsaturated molecules lower melting temperatures i.e. they form liquid oils. As opposed to a solid, a liquid is made of molecules that are more freely to move about next to one another enabling us to pour oil at room temperature. Notice I said room temperature because I can buy olive oil that becomes a solid at 4 ^oC (I said can, not does, due to the diversity in oil purity and resulting melting temperature).

Where do we get fats from?

As discussed, fats are fuels giving them an across species origin with each source offering various pros and cons including flavour differences, heating limits and health benefits. Here we will examine three unique ways in which we harvest fat.

1 -   Maybe unexpectedly, seeds have a high concentration of fat which is used as the energy source to start growing. Sunflower oil is prime example of a fat we harvest from seeds and is produced from crushing sunflower seeds. However cheaper sunflower oils may be made by heating up the process leading to a higher yield but often with detriment to the oil quality. This process is heavily seen in producing olive oil where heating the olives to get regular olive oil results in a loss of its delicate flavour. Olives that just get pressed (the good stuff) is denoted as extra virgin olive oil.

2 -   Milk from animals is high in fat to provide energy to a mothers young. But milk is an emulsion with a high concentration of water. To yield a fat i.e. butter we churn milk, which enables fat droplets suspended in the milk to stick together forming a large solid (this is a huge generalisation but its fit for purpose here I think).

3 -   The final fat source is again from animals but rather than collecting their milk, it’s a rather more invasive process where we render fat from their carcase. Animals use internal fat in much the same way we do; for heat insulation and as an energy store for when food is scarcer i.e. in the winter months. To render this heat is used to melt the solid fat in animals - typically this is seen as goose and duck fat around Christmas. Less favourable due to our current palates is beef dripping however similar processes are still used.

There are claims to which fats are better for you but I will not be so bold to say which isn’t and which is. As with the nature of research with more data comes better understanding and I know doubt that one day a definitive answer will be found. I just hope its butter or duck fat.

Now I know some of you that have been paying attention will be saying “hang on, then what’s margarine and where does that come from?” The answer is margarine is fat that has been made to be like butter. Initially there was a competition set out by Napoleon to make a synthetic edible fat due to a lack of oil to meet modern industrial demands. Animal fats were initially used, namely suet, which could give a buttery texture and a small amount of milk to give it palatable taste. However it wasn’t until 1905 we made the swap from processed animals to processed liquid vegetable oils to vet our cheaper buttery fix. As discussed a liquid oil is more unsaturated and therefore to make oil into a semisolid like butter we need to turn those unsaturated double carbon carbon bonds into saturated ones. To do this we heat up the oil and add in high temperature high pressure hydrogen gas, breaking the double bond whilst adding in 2 new hydrogen atoms.

Figure to show the reaction of a unsaturated fat chain to a saturated one.

Now that the oil has the texture of butter we need to make it taste like butter through the addition of diacetyl (the molecule responsible for butter’s flavour) not forgetting some vitamins, salt, preservatives and colour enhancers.

When to Use Which Oils

As discussed oils and fats have high boiling points that allow us to cook food quickly. However when heated in the presence of oxygen the triglycerides begin to break down into smaller molecules, the smaller bits of triglyceride can now leave the bulk oil in the form of smoke. The temperature at which oil begins to smoke is therefore referred to as its smoke point. Oils should not be allowed to smoke due to formation of free fatty acids (a bit of broken down triglyceride) leading to an rancid taste in the oil; which will be transferred to the food. Repeated heating and cooling of the oil will lead to a build-up of free fatty acids that is detrimental to the oil quality.

Figure to show the structure of an example of a saturated free fatty acid.

It is clear then that when deep frying or cooking at very high temperatures, we need an oil/fat that has a high smoke point. However fats themselves are contaminated with other molecules which is often beneficial to their flavour. These contaminants lower the smoke point of the oil, and this can be seen readily in butter. Butter by its self will burn in moderate temperatures but if you melt it first and skim off the solid precipitate we make ghee/clarified butter, allowing a ~75 degrees C temperature increase before it smokes. As a rough guide then lower smoking point fats have more flavour and should be restricted to dressings and low temperature cooking. I have added a smoke point chart below but this is not my work (there is no point re-inventing the wheel) and I have referenced it somewhere in this article…

Table to show common kitchen fat smoke points.

Keeping Fat Fresh 

We have already briefly talked about how high temperature heating cycles deteriorate fats; but heating only speeds up this process of oxidation leading to the forming of free fatty acids and a resulting rancid taste. We should then try minimise the fats contact with air to preserve our fats. While being at odds with water through non polar/polar interactions, water and fat still shouldn’t be mixed. Water leads to hydrolysis of the triglycerides again forming free fatty acids and rancidity which again increases with heating. This can be a problem when deep frying. Water/steam from the cooked food is exposed to the oil thereby reducing the life span of the oil in the deep fryer which can lead to increased expenditure. To overcome regular oil changes in your fryer you may wish to swap to a pressurised deep fryer, which increases waters boiling point in the food resulting in more moisture staying in the food (not escaping through steam) and exposing less water to the oil. Never the less oil can still be exposed to light, iron and salt which have all shown to increase the formation of free fatty acids. In other words just change the oil in your fryer regularly! And do not be tempted to mix oils - this has been shown to cause cross polymerisation reactions, especially at high frying temperatures. The products of this are suggested to be carcinogenic.

It’s here that I will call it a day on fats for now. It’s a shame that we ended on a carcinogenic bomb shell but I guess for many of us it’s as true here as it in life. Next week’s topic “The Abattoir” will be much cheerier featuring animations, real footage and first person accounts of its day to day running.  And for chemistry aficionados, yes I did largely throw stereochemistry out of the window for the ease of drawing figures in this article.

References

H. Mcgee, On Food and Cooking, Scribners, New York, 1st ed. 1984

http://jonbarron.org/diet-and-nutrition/healthiest-cooking-oil-chart-smoke-points#.VobPZvmLSM8

http://thetastee1.rssing.com/chan-8502785/all_p1.html

https://en.wikipedia.org/wiki/Smoke_point

A Brief Scientific Introduction to an Egg

Eggs: a Very Very Brief History…

In the kitchen a few types of eggs are readily available; goose, duck, quail but none more so than the chicken. Which leads to the age old question - which came first the chicken or the egg?  A biblical solution to this problem can come from Genesis, where it is said that the father created all the creatures not their embryotic subunits. From a more accurate standpoint there can be only one answer; the egg. Eggs pre-date the earliest chickens whose origins can be found as jungle fowls that existed in South East Asia or India over four thousand years ago. Eggs not only pre date chickens, they pre date birds and are one of the earliest forms of sexual reproduction dating back over 1 billion years!

Egg Ageing

The egg is unique to the cook in that it is the second alkaline source in the kitchen, the first being bicarbonate of soda. The alkaline quality only increases with the age of the egg but to a detrimental effect of the produce. The increased alkalinity of the egg over time is due to the porous nature of the egg shell.

Porous Egg Shell (Scanning Electron Microscope) SEM Image 

We all know that storing eggs within the fridge can lead to the eggs taking on “fridge smell” of whatever is currently being stored in there. This can be used to the cooks’ advantage, for example if eggs are stored in an air tight container with a truffle (the mushroom), the resulting eggs will be perfumed with truffle. Digression aside carbon dioxide gas exits the egg over time; it is this gas that provides the acidic element to an egg and once removed the egg becomes more alkaline. Carbon dioxide is an important acid that our own body must deal with on a day to day basis. When we respire (produce usable energy) we produce carbon dioxide as a side product and when dissolved in a liquid, like water, it increases the liquids acidity. We use homeostasis to prevent our blood from becoming too acidic / alkaline. The egg however has no homeostatic ability therefore the yolk increases from a pH of 6 to 6.6 and the white from 7.7 to 9 and above. While these numbers seem like small changes we must remember that pH is on a log scale therefore a change from 7.7 to 9 is around a 20 fold increase in alkalinity.

Why does a rise in pH lead to a poorer quality egg? Proteins called albumens are spherical balls that are made up of a string of carefully folded amino acids. This ball has negative charges on its surface and similar to magnets, like charges repel one another. In doing so the proteins are kept apart from one another. This property is increased as the egg ages and the pH becomes more basic through the alkaline egg liquid amplifying the negative charge. The stronger the charge the more watery the egg will become due to the proteins not being in close proximity to each other and rubbing past one another.

A Cartoon Displaying how the Intermolecular Distance Between Proteins Changes with Age, Salt and pH  

A further effect of egg ageing is where the yolk becomes more fragile over time. The white contains a higher water percentage which over time diffuses into the yolk, increasing the yolks volume causing it to swell and making it more likely to burst the thin membrane containing the yolk.

Cooking and Culinary Uses

There are three major culinary uses of eggs other than just for the joy of eating them. These include: the stabilisation of emulsions such as mayonnaise; the thickening of liquids into gels and semisolids (things likes custards take advantage of this property); and finally eggs are used to create light textures in cakes etc. through the use of foams. Here we will examine an egg’s ability to set and thicken other liquids and we will focus on emulsions and foams another day.

Other than the obvious high water content, we have briefly talked about how proteins are a major component within eggs and how they determine the structural properties of the egg. Proteins are long chains of amino acid bricks making up a long sequence  / chain. As discussed these chains fold into specific shapes than can for all intents and purposes be thought of as a ball. This ball shape needs to be unfolded in order to cook the egg. Luckily for the cook there are a few ways we can do this; either through the addition of heat, acid or salt. The most common practice of cooking eggs is though the addition of heat. Heat provides energy which breaks apart the internal bonds holding the folded protein ball shape together. Once the weak internal bonds are broken, stronger bonds between proteins can form linking different albumin proteins together. When proteins begin to bind together they trap water within a net structure and this net structure provides a solid texture to the cooked egg, which reflects light making egg whites…well white. The texture of the egg can still be modified through further cooking; additional heat energy allows more protein protein bonding to occur which squeezes out the trapped water; resulting in a firmer texture.

A Cartoon Displaying Three Changes Proteins go Through when Cooked

This brings us nicely to curdling. Curdling is the process where proteins in a net structure separate out from a liquid such as the water in custard. Curdling is caused by proteins bonding very tightly with themselves, squeezing out so much water that it becomes a solid bit of egg protein floating in a liquid.

The addition of other ingredients heavily dictates the temperature the eggs will cook at. Through adding acid (citrus / vinegars) we are adding additional hydrogen atoms which have a positive charge. These small positive charges screen the negative charges (opposites attract) on the surface of the spherical folded proteins. Through reducing this negative charge proteins can come closer together decreasing the time and energy required for unfolding and bonding with other proteins. It is worth noting that pH will also cause some of the protein to unfold. Salt has a similar approach to speeding up the cooking of eggs; the salt when dissolved in water dissociates with itself on a molecular level into its composite charged ions; sodium and chorine. The positive charge of sodium has the same effect as seen with the acidic hydrogen ions.

As cooks we rarely just cook an egg with salt and acid, we add other more delicious ingredients such as cream, milk and sugar. Milk and cream have a high water content and dilute down the proteins so they are less likely to interact with one another. In order to cook and set the egg milk mixture the temperature has to be higher to result in proteins moving  fast enough to raise the possibility of one protein meeting another and bonding together. Sugar on the other hand has a strange effect of coating the proteins with thousands of little sucrose molecules thereby preventing protein interactions and reactions, again raising the cooking temperature.

Cooking Technique

We are informed as cooks that boiling an egg is the simplest form of cooking that we could ever achieve but how is that so? I would say that cooking the bird is easier than its embryotic beginnings. Roasting a bird at 180 ^oC for some length of time will result in it turning brown; once brown all over and when you pull at its legs and they fall off, you know it’s cooked. An egg on the other hand has no reference point to know when it’s cooked or what stage of “cooked” it is at. If you want hard boiled eggs well that’s easy enough, boil the egg for ten minutes. But what if you want that illusive runny yolk? No matter what size the chicken is the reference points stay the same, however the size of the egg drastically changes the cooking time. The problem is that boiling water is too hot with the exception of boiling an egg on top of Everest (where the decrease in pressure on the water’s surface leads to a reduced boiling temperature – however in these conditions you might be better having a cereal bar than a boiled egg). At 100 ° C (roughly 300 kelvin) all the proteins in the egg unfold and link up setting both the white and the yolk. When we cook a boiled egg is a race against time to select the exact point at which enough energy has diffused through the egg to set the white but not so far to ensuring that the yolk remains runny - this sort of cooking just requires practice. A better way of cooking an egg is through varying the temperature. So far we have discussed protein binding as a whole rather than individual protein types but I imagine by now that when I say different proteins have different cooking temperatures you will not be astounded. These different cooking temperatures can be used to the cooks’ advantage when “boiling” an egg. If we select at temperature at which the white is cooked but the yolk is still runny,  we will have cracked it….I’ve done well to say that this is the only egg pun so far. Below is a figure not by me but robbed from (http://www.douglasbaldwin.com/sous-vide.html) showing the temperature at which an egg was cooked at vs its “cookedness”.

Temperature vs Egg "Cookedness"

From the image we can see how egg yolks change with increased temperature to a more firm consistency. However the whites are still not the perfect texture and this is what is referred to as an inverted egg: runny whites and set yolk. Years of research has been conducted on the egg which demonstrates that to cook a perfect egg we must blanch eggs at 95 -100 ° C  for 2 – 3 minutes to set the white, then the egg needs to be cooked at a more moderate 62.2 ° C for an hour to cook the yolk. I told you it was easier to cook the chicken.

As a final note on boiling eggs there is the matter of egg smell and discolouration. When boiling an egg sulphur atoms are released from the albumin proteins in the egg whites. These free sulphur atoms then pick up hydrogen atoms which form hydrogen sulfide gas (H₂S_(g)) which is the smell of cooked eggs. Over cooking results in more gas and a stronger egg smell. A rotten egg smell on the other hand is the result of the same gas but in a much higher quantity. Hydrogen sulphide is also responsible for the greyish colour around the yolk. As the egg is heated, pressure increases in the egg resulting in gas moving closer to the centre of the egg, which reacts with the iron present within the yolk making iron sulfide - the grey part surrounding the yolk of hard boiled eggs. Should you worry about overcooking your eggs? Yes hydrogen sulfide in very high concentrations can cause respiratory distress, nausea and death. It’s probably why rotten eggs smell so bad to us, as our evolutionally background makes sure we avoid them.

Poaching eggs for some is a method best avoided but with a bit of science knowhow success could be just lurking around the corner. To the water we could add acid in the form of vinegar which will promote cooking through helping to null the negative charge of proteins. In turn the frontier layer of proteins will coagulate quickly aiding the egg to stay in a more spherical shape – too much acid will affect the taste of the cooked egg. Salt could be added to the simmering water which again will help coagulate the outer layer of egg as previously discussed. However the main influencing factor in the quality of the poached egg is the quality of the egg to begin with. Unfortunately not all eggs are born equal and these are sorted into three types of egg quality: AA, A and B. AA eggs have the highest concentration of protein which is detected through a process called candling, whereas A and B eggs have less protein respectively. The higher concentrations of protein mean the egg white is thicker / viscus; meaning the egg while being poached, is less likely to float and fall apart during cooking. Also, as previously discussed, age is an important factor in determining egg viscosity; the older the egg the more difficult it is to poach.

Omelettes and scrambled eggs are made using milk, cream and butter however the cook must be careful as to the volume of liquid added. Too much liquid will dilute down the proteins so much so that they cannot interact and bond with each other to form a set texture. I have read that it is advised that the addition of 2-5 tsp of liquid is optimal loading per egg for scrambled eggs. Omelettes which form a stiffer coagulation will only take 2 – 3 tsp of added liquid. Butter can play a vital role in the overall texture of these products preventing a tough consistency forming. The butter fat prevents lots of bonds between the protein forming yielding a more enjoyable mouth feel and eating experience.

Custards, whether sweet or savoury, are egg gels. Curdling is your nemesis in these grounds so always cook over a gentle heat and resist the temptation to increase the temperature too high. Many a chef and cook has fallen victim to overheating – just remember that all you are trying to do by heating is to gently remove water bringing proteins and added fat globules closer in proximity to one another, and breaking the weak bond within individual proteins. Overheating curdles the egg forming strong bonds between the proteins that cannot be broken.

BOOM! I think that’s enough about eggs for one day but I do hope that you find this as interesting as I did. I will post more about egg science (specifically on how eggs help us form edible emulsions and foams) at a later date. Normal recipe blogging will resume first though.

References:

H. Mcgee, On Food and Cooking, Scribners, New York, 1^st ed. 1984

http://blog.khymos.org/2009/04/09/towards-the-perfect-soft-boiled-egg/ (accessed 24/2/15)

http://www.douglasbaldwin.com/sous-vide.html (accessed 24/2/15)

http://www.chefsteps.com/activities/the-egg-calculator?utm_source=twitter&utm_medium=post-the-egg-calculator-tw&utm_campaign=EggCalculatorCampaign (accessed 24/2/15)

http://avogadro.chem.iastate.edu/MSDS/hydrogen_sulfide.pdf (accessed 25/2/15)

10 B*llsh*t Things that Bloggers Say:

10 Bullshit things that chefs, food bloggers, “food critics” etc. say /do –The featured quotes below are all genuine:

1-    “Let the flavours mingle” / “let the flavours do their thing”

This is called marinating, where either flavour molecules diffuse into a piece of meat or where molecules interact with one another for curing etc.
 
2-    “The flavours here all worked really well”
 
All you need to do is say whether the food tasted good as a meal or did it clash? Stop the pretentious middle class filler jargon. Furthermore this statement is rarely followed up by examining what specifically “worked well” and why.
 
3-    “I could have done that at home”
 
Well let’s get the obvious out of the way - through owning a kitchen you are half way there. Yes you could make it at home. You have 90% of the gear that the chef has: an oven, some spoons, a pan or two and probably the very same chef’s recipe book!
 
4-      “The plates were cold”
 
Does anyone warm their plates up at home? No, and that is something you could definitely do at home! And then when the plates are screaming hot you give yourself third degree burns because you touched it even though the waiter / waitress told you not to.
 
5-    “Locks in the moisture”

What a load of nonsense. A well cooked piece of beef, in terms of quality of cooking not burning / ruining a piece of beef, is just that; brilliantly cooked. Not cooked so it locked in the moisture because if that was the case it would be served raw. All forms of cooking in the traditional sense of applying heat or salt etc. causes loss of water.

6-    “#foodporn”  
 
I think this hashtag is just a hipster excuse to say the word porn. Personally I do not think molten gooey cheese pouring out of a crusty, dry, flaky filo pastry parcel is sexy. Unless crusty and dry with sticky cheese is what your into.
 
7-    Eight photos of the same plate of food from different arty angles.
 
Bloggers in particular love to take pictures top down, then at 45 degrees, then at 90 degrees and then to spin the food 90 degrees around and repeat. Don’t get me wrong each photo looks good (usually) but self edit PICK ONE. No? Ok show me the 360 degrees of the plate again. 

8-    Anything cooked in a slow cooker!

Slow cookers maintain an 80 ^oC temperature which, for long slow cooking purposes, is still too hot yielding dry, over cooked meat and rank vegetables that often just get thrown in at the start. The sauce / gravy is always too thin and this needs boiling to concentrate the flavour. And finally what’s wrong with the oven?! Ovens can maintain low temps, and you don’t need to buy a new piece of work top clutter to cook.

9-     Vegan / gluten free / dairy free – fad dietary requirements
 
There is nothing more depressing than a vegan gluten free brownie, trust me I have tried one from a reputable gluten free blogger and it was vile! Don’t get me wrong there are people who have medical reasons and some people who hate the idea of eating animals, but if you’re just doing it because it’s the latest faddy thing you want to jump on the band waggon with, you’re an idiot. Just take a look at the additives put into gluten free products.
 
10-  The use of chicken breast
 
Why chicken breast? The thigh meat is far cheaper and tastier. Breast is usually dry, boring, more expensive and did I mention dry?

Pectin vs Gelatine

In the last two recipes we have used both pectin and gelatine to set a jam and create a jelly in order to make a pannacotta, so what is the difference between pectin and gelatine? And can they be used interchangeably?

Pectin

We will start off by examining pectin. In brief, pectin is a category for lots of polymers with a repeating sequence of D-galacturonic acid monomer blocks. That is some full on jargon so let’s try break it down into something more understandable. Think of the “polymer” as a house where the house is built from “monomer” bricks, where all the bricks are pretty much the same. These monomers are found naturally bonded to one another inside plant cells making the pectin polymers, which can be represented in two ways, as shown below.

Left- full chemical structure of acid monomer (letters denote atom type O=oxygen C=carbon H=hydrogen, single lines are single bonds, double lines are double bonds), Right - a cleaned up tidier structure (all points on the hexagon represent a carbon unless otherwise stated)

Top - joined monomers making the pectin polymer Bottom - cartoon representation of the polymer

As seen above there is more than one type of monomer that can be present in a pectin molecule. It is this wealth of different monomers that create unique pectins which can form gels under different conditions. Here we will discuss the text book gelation method of “smooth” pectin which has a high percentage of ester groups - we will not worry about its “hairy” high percentage carboxylic acid monomer counterpart.

Pectin polymers are in each cell of the fruit and when heated the cells begin to break down releasing everything in the cells, including the pectin. When making jam we must also add sugar; this preserves the jam while equally playing a part in setting the jam. As the jam boils we remove water molecules in the form of steam making the sugary fruit syrup more concentrated. By the syrup becoming more concentrated the long pectin molecules come closer in contact with one another where they start to knot and begin to tangle together creating a fine 3D molecular net. This net prevents sugar, flavour and water molecules etc. from moving around so easily making the jam viscous and thick in texture, subsequently forming a jelly like substance.

But why is sugar concentration so important when it comes to making a set jam? Sugar (sucrose) is very soluble in water (2 kg will dissolve in 1 litre of water) so we can assume water and sugar get on like a house on fire. Pectin on the other hand is a bit pickier as it struggles to dissolve in water. When the water is boiled off when making the jam the remaining water surrounds the sugar molecules, leaving the pectin to become undissolved. Without water around the pectin molecules they can interact with one another forming weak bonds giving strength to the 3D net. And finally, pectin needs a bit of acid to enable the gel network to form. The added acid ensures that the carbocyclic acid groups on some of the monomers in the pectin polymer are “protonated” - pardon the jargon again. In essence, unless your jam fruit is a bit acidic then the hydrogen atom on the carbocyclic acid will fall off because the rest of the molecule is quite stable without it. However, although the molecule is stable when the hydrogen falls off, it will have a residual negative charge repelling any other negatively charged pectin molecules from approaching; subsequently preventing net formation.

By adding in extra acid (like lemon juice) the syrup will already have (acidic) hydrogen atoms floating around so it is less likely that one will leave the carbocyclic acid group on the pectin molecule.

                        

The concentration of pectin varies across the varieties of fruit but as a basic rule of thumb soft fruits like strawberries, cherries and peaches have low pectin concentrations, whereas apples and citrus rinds have a high concentration of pectin. In the case of our Cherry Jam, the cherries have a low pectin concentration so we had to add in pectin rich sugar.

Gelatine

Gelatine is the product of collagen break down, so it’s pretty safe to assume that all gelatine you use is from animal products, specifically pig skin and ox bones. Collagen is what keeps your skin supple and smooth while also keeping you, well, inside you - without it our skin wouldn't be as flexible, stretchable and your body wouldn't be anywhere near as strong. So what is collagen? It’s a protein made of three polypeptide chains which wind together forming into a triple helix. So in other words it’s made up of three thinner proteins that have twisted together.

To get gelatine from this protein ravel we must break down collagen by applying heat and some acid.  By doing this we chop up the lengths of protein that can be more readily dissolved. You may recall when we made the pannacotta we soaked the gelatine in cold water before adding it to the creamy hot syrup. If we had soaked the gelatine in warm water it would have dissolved and this would have led to the addition of too much water to the pannacotta. Once the creamy gelatine rich syrup had been added to the moulds we let the pannacotta's set in the fridge. Through lowering the temperature there is less thermal energy in the dessert leading to the reformation of weak bonds between the gelatine proteins, which in turn create a 3D net similar to what we saw for pectin, across the entire dessert.

So why do we use pectin for jam and gelatine for pannacotta? Pectin, in order to set, requires a high sugar content and some acid. Pannacotta is not an intensely sweet dessert therefore the sugar content would not be high enough to set the pannacotta. Also adding acid to cream can easily lead to the cream splitting, so rather than a delicate dessert we would have a sweet cheese floating in whey (the liquid part from the cheese making process). Why not use gelatine in jam making? Gelatine wouldn’t give us quite the same jam texture we have grown to love. And while gelatine would lower the calorie content of the jam considerably, because we wouldn’t need all that sugar to enable the pectin to set, the sugar does prevent the jam from spoiling! The sugar creates an osmotic pressure across any bacteria’s cell membrane when it tries to live on the jam i.e. sugar dries out the bacteria. But preserving food is another lecture for another time. 

A final note on gelatine - you cannot use it to set pineapple based dishes. This is because pineapple contains high levels of proteolytic enzymes which break down collagen in meat while also breaking down your gelatine!    

And for those of you who stuck through this lecture thank you, normal recipe blogging shall resume next week :) 

For further information you can go to the Royal Society  of Chemistry's web page: 
http://www.rsc.org/chemistryworld/podcast/CIIEcompounds/transcripts/pectin.asp

or another blog that has discussed pectin gelation quite well over at:
http://sciexplorer.blogspot.co.uk/2012/08/jam-science-first-let-me-be-clear-i-am.html

Baking Powder vs Baking Soda

(Sodium Bicarbonate)

Both baking powder and baking soda in baking get used interchangeably but should they? To understand this we need to know what their chemical structures are, and why they work as levening agents. To start off a levening agent is a chemical that helps the rise of a batter or dough, creating air pockets that give softer, less dense mouth feel. Baking powder and baking soda contain the same levening agent sodium bicarbonate. Sodium bicarbonate is made from the atoms carbon (C), oxygen (O), hydrogen (H) and sodium (Na) linked together through bonds. However the sodium is held close to the rest of the molecule through a different kind of bond to the rest of the atoms. This bond is referred to an ionic bond. An ionic bond is the same as holding oppositely charged magnets together; they stick.  In order for sodium bicarbonate to be used as a levening agent we must react it with something to cause it to break up into carbon dioxide gas (CO_{2 (g)}) and water (H₂O _(l)). Where the carbon dioxide gas creates “air pockets” in the batter when baked which in turn will lead to a lighter, softer mouth feel when the cake is eaten. 

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  To react the sodium bicarbonate we must use something that is safe to eat and preferably something that is already in the food preventing a change in flavour.  Luckily our cake batter is a liquid due to the water in eggs, milk, buttermilk, etc. which starts the reaction. When the sodium bicarbonate is dissolved the sodium atom is lost because water molecules have a small negative charge, which screens the negative charge of the carbonate part of the molecule. Now that sodium is gone the rest of the molecule is vulnerable for attack by lurking chemicals!

  
  

  

   (Water Shielding the Sodium Atom) 

  
  

  What do I mean by lurking chemicals? I mean acids. Acids are commonly found in food, everything from buttermilk to fruits, but what are acids and what are they doing in our food? Just as easily the sodium was lost in sodium bicarbonate by just dissolving in water; the same can happen when a loosely bonded positively charged hydrogen is hydrated, it falls off the molecule. And that’s all an acid is (for this purpose), a positively charged hydrogen floating around in water.

  However if this positive hydrogen finds the negative bicarbonate they bond much more strongly together than the sodium did because hydrogen is much much smaller than the sodium, therefore we can say the charge density on the hydrogen is far greater. But as a consequence of the positive hydrogen bonding with the negative bicarbonate a chain reaction is set off, causing the bicarbonate to decompose, breaking down into water and carbon dioxide. But from what I just said you need strong acid, an ingredient that will lose a hydrogen atom when dissolved in water, but not all cakes have acidic ingredients in them like buttermilk or fruit, so where is the acid source? And this is where the difference between baking powder and baking soda lays. Baking soda is pure sodium bicarbonate and baking powder is sodium bicarbonate and an acid in the form of cream of tartar. In baking powder the acid and sodium bicarbonate are dry therefore cannot react; they must be dissolved in water to allow a reaction to occur.  Now we understand the difference between baking soda and baking powder we can use them when appropriate. If you know you have a strong acid in your batter like buttermilk you know you can get away with only using baking soda to yield a good rise. However if you know that the batter contains no strong acids it may be worth using baking powder where you know you’re more guaranteed a good rise. 

  
  

  

  (Reaction Mechanism of Sodium Bicarbonate with an Acidic Hydrogen)

  
  

  A draw back that affects both baking powder and baking soda is that as soon as they are dissolved they can react with acid and if the batter is not in the oven you are losing precious carbon dioxide, and therefore rise. To prevent this and further ensure a decent rise you can buy and use double action baking power which contains two acids; cream of tatar  and sodium aluminium phosphate or sodium aluminium sulphate, or sodium acid pyrophosphate. These additional acids do not react with the bicarbonate until a critical high temperature is reached, which in baking terms means you get two rises, ensuring a deliciously light cake! But only if your cake has a neutral pH; if you have buttermilk or an acid in your batter it doesn't matter that your heat sensitive acid will not react until your critical temp, all your bicarbonate will have already be gas and water.

  
  

  Happy Baking!