How long does syrup last after opening?

This is one of the most common questions our customers ask, and it deserves an honest, detailed answer — not just a short instruction on the label.

What preserves the syrup in a sealed bottle, and what happens when it's opened?

We described the production process in our elderflower syrup recipe — we just do it in larger vessels and barrels. Our pasteurisation method is a little different: the syrup is heated to 87°C as it flows through pipes submerged in hot water, on its way to the bottle filler. The hot syrup is then filled into bottles and sealed just seconds later. The bottles are then labelled and placed horizontally on pallets.

This tipping brings the hot syrup into contact with the cap, pasteurising it too. This method is called "Hot fill". Syrup filled this way can last for many years — we state a shelf life of 24 months for herb syrups (elderflower, sage, lavender, mint, ginger and lemongrass) and 13 months for fruit syrups (raspberry, strawberry and lemon). Not because they spoil sooner, but because natural changes in flavour, aroma and colour occur over time. These changes depend largely on storage temperature.

Our syrups contain no preservatives. What preserves them is their own nature: high sugar concentration and acidity.

With the exception of lemon, all our syrups have 50 Brix (the percentage of dry matter — in our case, sugar) — meaning roughly half the contents of the bottle is pure sugar. Such a concentration creates osmotic pressure that is unfavourable for most micro-organisms, drawing water from their cells and preventing them from multiplying. In addition, our syrups have a pH of around 2.3 — comparable to the acidity of lemon juice. At such a pH, virtually no bacteria can survive, let alone grow.

This combination of high Brix and low pH is what experts call "hurdle technology" — multiple barriers that micro-organisms must overcome before they can begin to spoil the product. As long as the bottle is sealed, these barriers are virtually impenetrable.

What changes when we open the bottle?

Opening the bottle lets air in. And in the air — everywhere in the world, in every home, in every kitchen — spores of yeasts and moulds are floating. They are invisible, unavoidable and completely normal.

Most of these micro-organisms can do nothing to your syrup. pH 2.3 and 50 Brix are their enemies. But there is one group that has evolved precisely for such conditions: osmophilic yeasts. The best known is Zygosaccharomyces rouxii — a micro-organism that feels at home in highly acidic, high-sugar solutions. If a spore of this yeast lands in your bottle and finds the right conditions, it will begin to multiply. You will see tiny bubbles, feel a slight fizzing or smell a faintly alcoholic odour. That is the sign that fermentation has begun and the syrup is on its way to spoiling.

Whether this happens depends on luck. It depends on how many spores of this specific yeast are floating in the air of your kitchen, terrace or garden that day. And that is not something you can control.

What you can control is everything else.

What can you do?

The cleanliness of the bottle neck is the most important thing. The bottle neck is the entry point for everything that can get inside. Do not touch it with your fingers, do not lick it. Every contact is a potential introduction of micro-organisms directly into the syrup.

Close the bottle immediately after use. The less time the syrup is exposed to air, the fewer spores have the chance to enter.

Store in the refrigerator. At 5°C, osmophilic yeasts grow ten to twenty times more slowly than at room temperature. Syrup that might start fermenting on a kitchen counter in summer within two weeks can remain perfect in the refrigerator for two to three months. The refrigerator does not kill micro-organisms — it slows them down. I have had bottles where I could no longer remember when I had opened them.

How long does it realistically last?

With good hygiene and refrigerator storage: two to four months is a realistic estimate. At room temperature with good hygiene: three to six weeks.

But these are estimates, not guarantees.

What is also interesting in this story is that different environments have different spores and yeasts. Generally, warm and humid places have more of them and they are more varied, while dry and cold places have fewer. The micro-location also matters. The concentration near greenery and soil is certainly higher than in a living room.

How do you recognise that the syrup has started to spoil?

• Bubbles in the syrup or under the cap that were not there when you opened the bottle
• Fizzing when opening or stirring
• An alcoholic or vinegary smell that was not there before
• A surface film or mould visible to the naked eye

If you notice any of the above, do not consume the syrup. It is not dangerous in terms of pathogenic bacteria — pH 2.3 prevents that — but the flavour and quality are compromised.

In short

Our syrups are robust. High Brix and low pH make them inherently stable. But they are not eternal after opening, and that is perfectly fine — because they are natural products without preservatives.

With a little care — clean bottle neck, closed bottle, refrigerator — they will last longer than you expect.