Why Sugar and Salt Preserve Food: Murabba, Achaar and Jam
Sugar and salt preserve food for the same reason: they tie up water so tightly that germs can’t use it. That’s why a proper amla murabba or a well-made nimbu achaar lasts a year on the shelf while a “low-sugar” jam grows mould in a fortnight. The ratio does the work, and once mould appears, the jar goes in the bin.
Before you start
- The idea: germs need free water; sugar and salt lock it away
- Jam and murabba: roughly equal weights of fruit and sugar keep sealed jars stable
- Achaar: salt plus acid (lemon, sirka, raw mango) plus a dry jar and clean spoon
- Low-sugar jam: fridge only, use within about 3 weeks
- Mould on jam, murabba or achaar: throw the whole jar away, don’t scrape
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How sugar and salt stop germs: water activity in plain words
Bacteria, yeasts and moulds are tiny living things, and like us, they need water to grow, feed and multiply, whether they land on a slice of mango or a spoon of dahi. Not just any water, though: they need free water, water that isn’t already stuck to something else.
Sugar and salt are both very good at grabbing water. Stir enough of either into fruit or vegetables and two things happen. First, osmosis (water moving towards the saltier or sweeter side) pulls water out of the food and out of any germ that lands on it. Second, the water that’s left is bound up with sugar or salt molecules, so it’s no longer available.
Food scientists call this lower water activity. Think of it as a desert. The water is technically there, but nothing can drink it, so most bacteria stop growing completely and the few that survive just wait. Yeasts and moulds are tougher and can grow in drier conditions than bacteria can, which is exactly why mould, not bacterial slime, is the usual failure on a jar of jam.
A murabba isn’t sweet by accident. The sugar is the lock on the door.
Why low-sugar jam spoils so fast
Cut the sugar in a jam recipe by half and you haven’t just made it less sweet. You’ve raised the free water back into the range where yeasts and moulds are comfortable, and you’ve taken away part of what makes it set.
Fine. Just treat it like fresh food. Low-sugar or “no added sugar” homemade jam belongs in the fridge from day one, in a small jar, used within about three weeks. Shop versions stay safe because they’re made with extra preservatives, special pectins and sealed processing, and even they say “refrigerate after opening” on the label.
The same logic applies to honey (very low in free water, which is why it keeps) and dried fruit like kishmish and khubani (the water has been taken out instead of tied up).
Ratios that keep: murabba, jam and achaar
Jam and murabba
- Weigh, don’t guess. For a shelf-stable jam or murabba, use roughly equal weights of prepared fruit and sugar, for example 500 g chopped fruit and 500 g sugar.
- Cook to a set. Jam is ready when a drop on a cold plate wrinkles when pushed. Murabba syrup is ready at a thick, one-to-two-string consistency (a drop pulled between thumb and finger forms threads).
- Jar it clean and hot. Wash glass jars, then dry them in an oven at 120 °C for 15 minutes or boil them for 10 minutes and dry completely. Fill while the jam is hot, leave a 1 cm gap, and close at once.
- Store and use. Sealed, it keeps in a cool, dark cupboard for many months. Once opened, keep it in the fridge and use a dry spoon.
Our bihi (quince) murabba and jam guide walks through one fruit start to finish.
Achaar
Achaar relies on more than salt. A good desi pickle stacks several barriers together, and that’s why it lasts.
- Salt: most traditional recipes use about 1 part salt to 8 to 10 parts vegetable or fruit by weight, often more for lemon and raw mango.
- Acid: lemon juice, sirka, or the natural sourness of kacha aam and karonda.
- Drying: pieces are dried in sun or air for a few hours to a day so no surface water is left.
- Oil layer: mustard oil heated until it just smokes and then cooled, covering the pieces by about 1 cm, keeps air out.
- Dry everything: jar, spoon and hands. A single wet spoon is the most common reason achaar goes bad.
The fuller safety checklist is in our homemade achar and mould guide.
The old habits were preservation science without the words. Jars went out in the sun, which drove off moisture. Only one dry wooden spoon was allowed near the achaar. Murabba syrup was boiled again if it began to thin, which lowered its free water back down. Nani couldn’t have named water activity, but she was managing it every summer.
Mould rules: when to throw it away
The old advice was to scrape off the mould and eat the rest. Don’t. On soft, moist foods like jam, murabba and achaar, a mould’s roots (the thread-like part you can’t see) spread well below the fuzzy spot, and some moulds make toxins that heat doesn’t destroy.
- Mould on jam, murabba, chutney or achaar: throw the whole jar away.
- Bulging lid, fizzing, spurting when opened, or a sour, alcoholic smell in jam: fermenting yeast or worse; throw it away without tasting.
- Cloudy brine or soft, slimy pieces in a salt pickle: throw it away.
- A white, powdery film on top of a sour lemon pickle: often harmless yeast, but if you can’t tell yeast from mould, don’t take the chance.
Don’t sniff mould closely either, since spores can irritate the airways.
Myth: sugar preserves, so murabba is healthy
The sugar that makes a murabba keep also makes it, well, mostly sugar. A tablespoon of amla or gajar murabba is a sweet, not a health food, whatever the fruit inside. Keep it to a small piece with a meal, and count it as sugar if you have diabetes.
A second myth: “salt pickles are raw food, so all the vitamins survive.” Some vitamin C does survive in lemon achaar, but salt, sun and months on the shelf lower it, and a spoon of achaar also brings a lot of salt. It’s a relish, not a serving of vegetables.
The same water-locking idea explains olives kept in brine; our zaitoon storage guide covers that. For more of the why behind everyday cooking, visit Kitchen Science, Honestly.
If you have diabetes, count murabba and jam as sugar; if you have high blood pressure or kidney disease, remember that a spoonful of achaar can carry more salt than a whole meal should, and ask your doctor what fits your limit. Garlic, ginger or herbs kept in plain oil without enough acid can grow the germ that causes botulism, so keep those in the fridge and use within a few days. Never store sour achaar or murabba in unlined copper or brass pots.
Sugar, salt and preserving: common questions
Why does sugar preserve food?
Sugar binds water so tightly that bacteria, yeasts and moulds can’t use it. It also pulls water out of any germ by osmosis, so most of them stop growing.
How long does homemade murabba last?
A properly cooked murabba in a clean, sealed jar keeps for many months in a cool cupboard. Once opened, keep it in the fridge and use a dry spoon.
Why did homemade jam get mould?
Usually too little sugar, jam that wasn’t cooked to a set, a jar that wasn’t clean and dry, or a wet spoon. Low-sugar jam must live in the fridge.
Can mould be scraped off jam or achaar?
No. Mould roots spread below the surface in soft foods, and some moulds make toxins. Throw the whole jar away.
How much salt is needed for achaar?
Most traditional recipes use roughly 1 part salt to 8 to 10 parts vegetable by weight, along with acid, sun-drying and an oil layer. Cutting salt a lot means storing the achaar in the fridge.
Is sugar-free jam safe to keep outside the fridge?
Homemade, no. Keep it refrigerated and finish it within about three weeks. Shop-bought versions rely on preservatives and sealing, and still need the fridge once opened.
Why does achaar go bad after some months?
Moisture gets in, often from a wet spoon, humid weather or too little oil on top. Keep pieces covered with oil and use only a dry spoon.
Can people with diabetes eat murabba?
Only as a small sweet, counted in the day’s sugar. The fruit doesn’t cancel out the syrup, so check with your doctor or diabetes educator about portions.
Does honey go bad?
Pure honey stored closed keeps for a very long time because it holds so little free water. Honey that gets watered down or left open can ferment. Never give honey to babies under 1 year.
What is the white layer on lemon pickle?
Often a harmless surface yeast, but it can be hard to tell from mould. If it’s fuzzy, coloured or smells off, throw the jar away.