09 Aug Why Some Preserves Last Longer Than Others

Walk down the preserve aisle in any shop and you’ll notice something interesting. Some products have a shelf life of a year or more, whilst others are designed to be enjoyed much sooner. Even after opening, the differences continue. A fruit conserve may keep well in the fridge for several weeks, whereas a lemon curd is best enjoyed much sooner. Many people assume the explanation is simple. If a product lasts a long time, it must contain artificial preservatives. In reality though, that’s not always the answer. The shelf life of a preserve is determined by a combination of traditional preserving methods, carefully developed recipes and modern food science. Understanding those principles helps explain why different products have naturally different shelf lives, even when no artificial preservatives are used.

Preserving Food Before Refrigeration

Long before refrigerators became part of everyday life, people still needed ways to make food last beyond the harvest or the hunt. Preserving food wasn’t simply convenient, it was essential. Different foods required different approaches. Meat was salted, cured or smoked. Vegetables were pickled or fermented. Hard cheeses matured in cool cellars where they developed flavour whilst remaining stable for months. Fruit was cooked with sugar to create conserves, jams and marmalades that could be enjoyed long after the growing season had ended. Although these techniques look very different, they all work towards the same goal: creating conditions in which the microorganisms responsible for spoilage struggle to grow.

What Makes Food Spoil?

Fresh food naturally contains bacteria, yeasts and moulds. Many are completely harmless, but given the right conditions they multiply, eventually causing food to spoil. Like every living organism, microorganisms need certain things to grow:

• Water
• Favourable temperatures
• Nutrients
• The right level of acidity

Successful food preservation is about making those conditions less favourable.

The Science Behind Preserving Food

One of the biggest misconceptions about preserving is that there is a single secret ingredient or technique responsible for a long shelf life. However, it’s usually several factors working together. Food scientists often describe this as the hurdle concept. Rather than relying on one barrier to stop microorganisms growing, several smaller hurdles combine to make survival increasingly difficult. Those hurdles may include:

• Sugar levels
• Acidity levels
• Heat treatment (e.g. boiling for a specific amount of time)
• Hygienic manufacturing
• Sterilised jars
• An airtight vacuum seal

Individually, each contributes something. Together, they create a product that is more likely to remain safe and stable throughout its intended shelf life.

Sugar Does More Than Make Preserves Sweet

Sugar plays an important role in preservation. When making our preserves, we measure the concentration of dissolved solids using a refractometer, which gives us a reading in degrees Brix (°Bx). This helps us monitor sugar concentration and consistency between batches.
Sugar also binds with water, reducing the amount of water available for microorganisms to use. Food scientists measure this as water activity (aW). Water activity isn’t the same as moisture content. A preserve may contain plenty of water, but if much of that water is bound to sugar it becomes much harder for bacteria, yeasts and moulds to grow. This is one of the reasons why traditional fruit conserves naturally have excellent keeping qualities.

Acidity Matters

Acidity provides another important hurdle. Food scientists measure acidity using pH. For those of you who aren’t scientists, you may remember using litmus paper or pH indicator solutions at school, where the colour changed depending on how acidic or alkaline a liquid was.

Many fruits are naturally acidic, whilst ingredients such as lemon juice or vinegar can reduce the pH even further. Lower pH values (higher acidity) create conditions that are much less favourable for many harmful bacteria, making it much more difficult for them to survive and multiply. Combined with the other preservation methods we’ve already discussed, acidity plays a major role in determining how long a preserve remains safe and stable.

Heat Treatment and Sealing Jars

Cooking develops flavour, but it also performs another vital function. Heating destroys many of the microorganisms that would otherwise contribute to spoilage. Filling sterilised jars whilst the preserve is still hot and sealing them immediately creates a vacuum as the product cools. This helps maintain an airtight seal, preventing microorganisms from entering the jar after filling and protecting the contents until it is opened. Like sugar and acidity, heat treatment forms one of several hurdles that work together to keep preserves safe and stable.

Every Preserve Presents a Different Challenge

One of the more challenging ranges we have developed at Nichols Pickles & Preserves is our curds. As well as the challenge of creating a recipe that delivered the flavour and texture we wanted, without using artificial preservatives, we had to achieve a shelf life we could confidently stand behind. That means being confident it will be fine on a shop shelf, or in someone’s cupboard.

Unlike a traditional fruit conserve, curds contain butter and eggs. Foods containing eggs and dairy are generally considered higher risk because they provide a much more favourable environment for microorganisms than a high-sugar fruit preserve. At the same time, curds cannot simply be boiled harder to compensate for these challenges. Too much heat causes the proteins in the eggs to coagulate (clump together), leaving the curd grainy or even scrambled. That means every stage of recipe development and production matters.

When we cook our lemon and lime curds, the factors we can control include water activity (aW), pH, heat treatment, hygienic manufacturing, sterilisation of jars, and careful filling and sealing. None of these factors work in isolation. Together, they create the conditions needed to produce a safe, stable product without compromising flavour or texture. Although these same principles apply to every product we make, developing a traditional fruit conserve is generally much more forgiving. Higher sugar concentrations, more intensive boiling and naturally lower water activity create conditions in which spoilage microorganisms find it much more difficult to survive and multiply.

Does a Long Shelf Life Mean Artificial Preservatives Have Been Added?

Not at all.

Many traditional preserves achieve excellent shelf lives without artificial preservatives. Instead, they rely on carefully balanced recipes, the science of preservation and good manufacturing practice. We make a conscious choice not to use artificial preservatives. Instead, we rely on traditional preserving methods supported by modern food science and independent laboratory testing.

How Do We Know a Shelf Life is Correct?

Shelf life should never be an educated guess. That’s why our products are independently laboratory tested to verify their safety, stability and shelf life. Laboratories assess a range of factors that influence how well a product keeps. These include measurements such as pH and water activity (aW), as well as microbiological testing throughout the product’s intended shelf life. Depending on the product, this includes monitoring organisms such as yeasts and moulds, both before and after opening.

For those of you who are a little nerdy (like us), microbiological results are commonly reported as colony-forming units (CFU). Put simply, CFU is a measure of the number of viable microorganisms capable of growing under laboratory conditions. By monitoring these levels over time, laboratories can verify that a product remains safe and stable throughout its declared shelf life.

Laboratory testing isn’t simply about demonstrating compliance. It’s about giving ourselves, our trade customers and our retail customers confidence that the shelf life printed on every jar is supported by evidence rather than assumption. We also prefer to err on the side of caution. For example, our laboratory testing has shown that our fruit curds remain safe and stable for 28 days after opening. However, we recommend consuming them within 21 days. We’d rather provide a sensible margin of safety than encourage customers to keep a product for longer than necessary.

Tradition Supported by Science

The principles of food preservation have changed remarkably little over the centuries. Whether food is salted, pickled, fermented or turned into a fruit conserve, the aim remains the same: create conditions in which spoilage microorganisms struggle to grow whilst preserving flavour for as long as possible. For us, those traditional principles are supported by modern food science. Every recipe is carefully developed, every batch is produced using rigorous hygiene standards, and every product is independently laboratory tested before reaching our customers. It’s a combination of tradition, science and careful craftsmanship that allows us to produce preserves without artificial preservatives whilst giving customers confidence in every jar.