Honestly, until fairly recently fruit preserves weren’t exactly hot on my radar. Sure, over the years I’ve made my fair share of various jams and whatnot, but I would never have called it a passion. These days, though, I’ve been channeling my inner pioneer and enjoying the process of putting things in cans and jars.
We make a lot of jam at the hotel. Tea service at the Langham runs all year long, and we swap out the menu all the time, each menu with new jams to serve with scones. It wasn’t too long before I realized I should have a much better idea of what exactly is going on during the preserving process if I was going to make up my own recipes with any success. So I decided to do some research, and the fruit of that labor (get it???) is this post! But before we talk about the food science behind making fruit preserves, let’s get some quick and basic definitions on the table (although a “preserve” is technically its own specific variety, I tend to call the whole family of products “fruit preserves”. Sorry.).
jam – made by cooking broken down fruit (cut, crushed, etc.) with sugar into a semi-solid product that is spreadable.
preserves – similar to jam in texture, but made to preserve the fruit used in its original shape
jelly – made with clarified fruit juice. Clear and physically similar to jello (still spreadable)
marmalade – a clear jelly containing small pieces of fruit and peel (most often citrus based)
No matter how you define the fruit preserve you want to make, it will have two main characteristics to be considered a success. 1. It will be shelf stable at room temperature over a long period of time (while hermetically sealed) and in general have a much longer shelf life than fresh fruit and 2. Will fall somewhere on the spectrum of a gel – from loose to firm.
Jam, jelly, marmalade…all of these products began as a means to preserve fruit beyond it’s normal lifespan. Where savory preserving most often utilizes salt and/or an acidic ph level to minimize bacterial growth, when making fruit preserves sugar is the heavy lifter. Sugar content is a crucial component to making preserves that are safe to hold at room temp. It is hygroscopic, meaning it attracts water. In attracting water molecules sugar separates them from each other (rendering them inactive) and prevents collections of water from forming (those collections known as active water) that would be the perfect breeding ground for bacteria. Not only does sugar keep the fruit preserve safe to eat, as we’ll see in a minute, it helps to create the finished texture.
Physically, preserves are considered a gel. By definition, a gel is a combination of water and various solids, held together through a network of bonded molecules that creates a sort of woven mesh. The mesh encapsulates the water and the combination of the two phases (a phase is a physically distinctive form of matter, in this case solid and liquid) creates the semi-solid gel we associate with jam and jelly. While sugar is the tool used to help achieve shelf life, pectin is the workhorse for gelification.
Pectin is a polysaccharide chain found in the cell walls of plants (a polysaccharide is a collection of two or more different sugar molecules). All fruits contain pectin, but in varying amounts depending on species and level of ripeness. Fruits with high natural pectin content include apples, citrus, quince, gooseberries and black currants. Low pectin fruits include common berries (strawberries, raspberries, blackberries, blueberries), pears and peaches. When making a fruit preserve using low pectin fruit, a high pectin fruit may need to be added in order to reach a proper gel (adding apples to a strawberry jam recipe for instance). Commercial pectin – made from the pith of citrus fruit – can also be added to any fruit preserve recipe and will generally help to shorten cooking time because of its strength.
Under ripe fruit will have a higher concentration of pectin than overripe fruit, but obviously lack the desired flavor and sweetness. Luckily, just as you can balance a low pectin species of fruit with a high pectin species you can combine under and overripe fruit to get the best of both worlds. A general guideline is a ratio of 1:2 of under to overripe fruit, with a minimum of 25% under ripe.
No matter how you get your pectin for your recipe, too much of it will create a tough or rubbery texture in the final product. Not enough pectin and the preserve may never set at all. Generally the ideal quantity of pectin in a preserve recipe is between .5-1% by weight, though this can vary depending on the desired texture and acidity level.
Achieving the right quantity of sugar is just as important as the right quantity of pectin for creating a nice preserve. In general, the final concentration of sugar in the preserve should be between 60-65%. To accurately and consistently test this, a measurement known as brix can be used.
Brix is a measurement of the total percentage of sugar by weight in a water-based solution. Basically this means the higher the percentage of brix, the more concentrated the sugar content. A brix of 80 would represent a solution that is 80% sugar and 20% water.
If the solution contains solids in addition to sugar, than the Brix will only represent an approximate percentage of total solids, because it takes into account all solids, sugar and non-sugar alike. So if you have a solution of sugar in grape juice with a brix of 80, that 80 represents 80% sugar and grape solids and 20% water.
Measuring brix is done using a tool called a refractometer. Thankfully using a refractometer is easier than explaining what it’s measuring. You simply place a sample of the solution on a glass plate (like a slide in a microscope) and hold the refractometer to the light to read the percentage on a scale. A refractometer definitely falls under the category of expensive “specialized equipment”, but if you plan to make a lot of fruit preserves or similar sugar candies, getting one can be a great way to create a more consistent product. If you plan to buy one, be sure to check the range of brix it can calculate to make sure it can read a percentage high or low enough for your purposes.
the science step by step
Now that we’ve defined the major working parts of a fruit preserve recipe, let’s look at a typical jam recipe and explore the science behind each step:
Initially, the fruit, fruit juice and any additional water is gently heated. This extracts the pectin from the cell walls of the fruit. At this stage the pectin chains are generally too short and too diluted in water to form a long, continuous network. The heat also ionizes the pectin creating a negative charge that repels them from one another. Cooking the mixture too long or at too high a heat in this stage will break the pectin down into pieces too small to ever form a strong gel network, so a gentle touch is essential.
Once the pectin has been extracted, sugar is added. While pectin is the mesh that holds everything together, without sugar a preserve would never be possible. The sugar molecules, being hygroscopic, attract water molecules in the solution, allowing the pectin chains to come into closer proximity with one another.
The mixture is brought to a boil. This further reduces the overall water content, allowing the pectin chains to press even closer together. The mixture is cooked to between 104 – 105C (219 – 221F). The temperature of the mixture will only rise as the water content diminishes, and 104 – 105C signifies that enough water has evaporated to create a sugar concentration that will allow the pectin to bond. The final sugar content should be between 60-65% or an approximate brix of 75 degrees (the 75 representing the combined solid content of the sugar and fruit solids).
Acid is added to the mixture. The acid neutralizes the negative charge of the pectin chains creating a positive charge that allows them to bond. If added earlier in the process, the acid would break the pectin down into chains too short to form a continuous network, much like the high heat. The finished preserve it left undisturbed at room temperature, conditions that promote the bonding of the pectin chains to one another.
Success! Sure, the jam, jelly and marmalade recipes you come across will vary a little and the method may change here and there, but they are all trying to achieve the same thing. Understanding the basic science of preserving fruit will help you navigate any recipe you come across, and even make your own!
Now go out into the world, make amazing jam and impress everyone with your new smarts.
Cheers – Chef Scott