How We Purify Stevia — The Membrane Sorting Line

MemCon × Wostevia · Membrane Trial — Sample 1 R&D · PILOT

Sorting molecules to bottle pure sweetness

Raw stevia extract is a dark, bitter soup of plant matter, sweetener, salts and water. We pass it through progressively finer filters — each one trapping molecules of a certain size. The plant matter is caught first, then the sweetener itself, then the salts, leaving clean water. This trial captured the sweetener at 85% purity, chemical-free.

100 g/L

Crude extract feed

Membrane stages

~30%

Final solids (concentrated)

85%

Steviol glycoside purity

Watch it happen

The molecular sorting line

Molecules flow in from the left. Each filter traps whatever is too big to pass through it — plant matter first, then the sweetener, then the salts. The sweetener is captured on its own at the 300-Dalton filter; only clean water flows out the far end.

Protein & carbs Sweetener Salts Water

Tip — hover any of the three stage cards below to spotlight that filter in the animation.

Three filters, three jobs

Each membrane sorts by molecule size

Think of a Dalton (Da) as how heavy — and roughly how big — a molecule is. Bigger number, bigger molecule. Each filter has holes of a set size, so we sieve the mixture like gravel, then sand, then silt.

Stage 01 · Ultrafiltration

Catch the big stuff

5,000 Da holes

Removes protein & carbohydrates.

The bulky plant molecules that make the extract dark and bitter are too big to pass, so they're held back and drawn off. The liquid turns from treacle-brown to clear gold.

removed

passes on

Stage 02 · Nanofiltration

Capture the sweetener

300 Da holes

Traps the steviol glycosides — the product.

Now the holes are fine. The sweetener (~1,000 Da) is too big to pass, so it's caught and held here while salts and water slip through. Looping it back across the membrane concentrates it and washes it clean — this is where the product is made.

sweetener captured

salts + water pass

Stage 03 · Reverse osmosis

Recover the water

100 Da holes

Reclaims clean water from the leftovers.

The finest filter of all works on what remains. It traps the last of the salts and lets only pure water through — recovered clean and reused. Nothing goes down the drain.

clean water out

salts rejected

Why sizes work as a sieve: the sweetener (steviol glycosides) sits in a size sweet-spot — big enough to be trapped by the fine filters, small enough to slip past the coarse one. So everything larger and everything smaller can be stripped away, leaving it behind.

What's in the bottle

The four things we're separating

50k
Da

Protein & carbohydrates

~50,000 Daltons · the biggest molecules

The dark, bitter plant matter. It's what gives raw extract its muddy colour.

Removed at Stage 1

1k
Da

Steviol glycosides — the sweetener

~1,000 Daltons · trapped between 5,000 & 300

The natural sweetener: up to 300× sweeter than sugar with zero calories. Too big for the 300 Da filter, so it's captured on its own — the product.

Captured at Stage 2 — the product

100
Da

Mineral salts

~100 Daltons · between 300 & 100

Dissolved minerals that muddy the flavour. They slip past the sweetener filter, then are caught by the finest one and discarded.

Removed at Stage 3

18
Da

Water

18 Daltons · passes every filter

The carrier liquid. Small enough to pass all three membranes — captured clean at the end and reused.

Recovered clean

The hard part

The real challenge isn't filtering — it's recovery

Pushing juice through a membrane is easy. The trick is not losing the sweetener along the way, and stopping the membranes from clogging. Two problems, one elegant fix.

The problem

Fouling & lost yield. Solids build up on the membrane surface, and throughput can fall by up to 80% — dragging yield down with it. A naïve single pass recovers as little as 68% of the sweetener, at only 37% purity. Too much of the product ends up in the waste streams.

The fix — recirculation & diafiltration

Instead of one pass, the concentrate is continuously looped back across the membrane — the recirculation loops on the whiteboard — while clean water is added and re-filtered to rinse the sweetener free of salts. Tuned well, this lifts recovery to 90–95% at 94–98% purity.

90–95%

Sweetener recovered, once the loop is tuned (vs ≤68% single-pass)

up to 80%

Throughput lost to fouling if left unmanaged

94–98%

Purity achievable with diafiltration

One caveat, stated plainly: sorting by size gives high total steviol-glycoside purity, but membranes can't separate one glycoside from another (say Reb A from stevioside). Reaching a single-compound 98% needs a final polishing step such as ion exchange or chromatography. Our 85% TSG is a strong result for a clean, chemical-free, membrane-only process.

85%Total steviol
glycosides (TSG)

From dark crude extract to a clean, captured sweetener

With no harsh chemistry, Sample 1 was purified to 85% total steviol glycosides — the sweetener captured on its own at the nanofiltration stage, salts stripped away, and the water recovered clean for reuse. In these pilot trials the line ran continuously and recovered its own water — an encouraging basis for scale-up as our process-development work continues. This is early-stage R&D, not yet a production process.