Thursday, 4 September 2014

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?


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 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:

or another blog that has discussed pectin gelation quite well over at:

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