Fermented Watermelon Rind — Zero-Waste Fermentation
The rind is most of what people throw away. The white rind is high in citrulline, ferments with the same crunch characteristics as cucumber, and holds texture for weeks refrigerated. Zero-waste fermentation that actually tastes good.
Chad Waldman
Analytical Chemist · April 19, 2026
Prep
20 min
Ferment
5–7 days
pH Target
3.4–3.8
Salt
2.5%
Difficulty
Beginner
Most people buy a watermelon, eat the red flesh, and throw away roughly 30% of the total weight in rind. The rind is not filler. It is compositionally distinct from the flesh, and in several ways more nutritionally interesting.
The red flesh is high in simple sugars and lycopene. The white rind is low in sugar and high in citrulline — a non-essential amino acid that converts to arginine in the kidneys. Arginine is the substrate for nitric oxide synthase (eNOS), the enzyme that produces nitric oxide. Nitric oxide is the primary vasodilatory signaling molecule in the endothelium. This is biochemistry confirmed in randomized controlled trials.
Ferment the rind and you get citrulline alongside live lactobacillus in a crunchy, sour, versatile condiment. Zero-waste cooking that doubles as functional food.
Citrulline in the rind — the biochemistry
Citrulline is named after Citrullus — the genus of watermelon — because watermelon is one of the richest dietary sources. The flesh contains citrulline, but the rind contains significantly higher concentrations: estimates range from 3.5 to 28 mg/g dry weight in the rind versus 1–7 mg/g in the flesh.
Once ingested, citrulline undergoes conversion to arginine primarily in the kidneys via the argininosuccinate pathway. Arginine is then available as substrate for eNOS, which produces nitric oxide. NO diffuses into vascular smooth muscle cells, activates guanylate cyclase, raises cGMP, and causes vasodilation. Clinical endpoint: lower arterial blood pressure and improved flow-mediated dilation.
Citrulline
In the rind
3–28 mg/g dry weight in white rind tissue
Arginine
In kidneys
Citrulline → Arginine via argininosuccinate pathway
Nitric Oxide
In endothelium
Arginine → NO via eNOS; vasodilation follows
Fermentation preserves citrulline. The lactic acid environment (pH 3.4–3.8) does not degrade free amino acids. What you start with in the rind is largely what survives into the fermented product.
Ingredients
- Rind from 1/2 watermelon (white part only, green skin peeled; about 1 lb prepared)
- 15g non-iodized salt (weighed)
- 600g filtered water (2.5% brine)
- 4 garlic cloves (smashed)
- 1 tsp whole coriander seeds
- 1 tsp black peppercorns
- Optional fresh mint sprigs or sliced jalapeño
Equipment: wide-mouth quart jar, vegetable peeler, glass weight, pH meter.
How to ferment watermelon rind
1Prepare the rind — peel to white, keep a sliver of pink
Cut the watermelon and remove the red flesh. Leave a thin layer of pink — it adds color and mild sweetness. Peel the dark green exterior skin with a vegetable peeler. The white rind is what you want: firm, dense, low in simple sugars, and structurally similar to cucumber flesh. Cut into 1-inch cubes, spears, or rounds — whatever fits your jar.
Chemist's note
The green outer skin is waxy and prevents brine penetration. Peel it completely. The white rind below is porous and accepts brine readily — it ferments fast, like cucumber, because the cell structure is similar. Citrullus lanatus and Cucumis sativus are both Cucurbitaceae family members.
2Make a 2.5% brine
Dissolve 15 grams of non-iodized salt in 600 grams of filtered water. 2.5% by weight. Watermelon rind is denser and less watery than the flesh — closer to cucumber than to zucchini. 2.5% gives you adequate protection without over-salting what is a relatively delicate-flavored vegetable.
Chemist's note
Unlike zucchini, watermelon rind does not dramatically dilute your brine during fermentation. The cellular structure is tighter, more like cucumber. 2.5% is correct here — any higher and the salt overwhelms the mild, slightly sweet background flavor of the rind.
3Pack with aromatics
Layer the rind into a wide-mouth quart jar with 4 smashed garlic cloves, 1 teaspoon whole coriander seeds, 1 teaspoon black peppercorns, and optionally fresh mint sprigs or a jalapeño. Watermelon rind pairs particularly well with warm spices — coriander, fennel seed, or a small cinnamon stick all work. Go savory, spicy, or herbal.
Chemist's note
Traditional Southern American rind preserves use cinnamon, clove, and allspice. The lacto-fermented version is lighter on sweetness and richer in lactic acid tang. I go with coriander and mint for summer, chili and garlic for a fridge staple I actually use.
4Submerge and seal
Pour brine over the rind until everything is submerged by at least 1 inch. Watermelon rind is denser than asparagus or zucchini — it will not float dramatically, but weight it down anyway. A glass weight or brine-filled bag works. Seal loosely or use an airlock. Store at 68–75°F out of direct sunlight.
Chemist's note
The brine will turn slightly pink from residual flesh on the rind — this is normal. A deeper pink means you left more flesh on, which is fine. It adds a subtle sweetness that the lactic acid will eventually balance out.
5Ferment 5–7 days, taste, then refrigerate
Leave at 68–75°F for 5–7 days. Watermelon rind ferments slightly slower than cucumber because it has less sugar in the rind tissue. Taste starting at day 5. You want: sour tang, retained crunch, and earthy-mild rind flavor. pH target is 3.4–3.8. Refrigerate when flavor is right. Keeps 4–6 weeks refrigerated.
Chemist's note
One of the best things about this ferment is longevity. Unlike zucchini, the dense white rind maintains texture for 4–6 weeks in the refrigerator. The citrulline content is largely preserved — lactic acid environments do not degrade free amino acids.
The science
A randomized controlled trial (n=11) showed that microencapsulated watermelon rind containing 4g of L-citrulline significantly improved flow-mediated dilation and increased plasma L-arginine within 60 minutes of ingestion. The rind — typically discarded — demonstrated acute endothelial benefits, validating the citrulline concentration in the white portion of the fruit.
European Journal of Clinical Nutrition, 2022 · PMID: 36109652→
A 2026 systematic review of 12 RCTs (n=360 postmenopausal women) found that citrulline supplementation — directly or via watermelon products — produced reductions in systolic blood pressure of up to 9 mmHg in hypertensive participants. Watermelon-derived citrulline consistently converted to arginine and reached plasma as a functional nitric oxide precursor.
BMC Women's Health, 2026 · PMID: 41588439→
A critical review of vasoactive dietary compounds confirmed that L-citrulline at 5.6g or more produces superior endothelial effects compared to arginine supplementation because citrulline bypasses first-pass intestinal and hepatic arginase metabolism and reaches the kidney for more efficient arginine synthesis. Watermelon rind is among the highest natural dietary sources.
Nutrients, 2023 · PMID: 37299579→
Troubleshooting
Rind is soft after 5 days
Likely left the green skin on — it prevents brine penetration but causes uneven softening at cut edges — or used rind from a very ripe melon where the white tissue is already breaking down. Choose a firm, not-overripe melon. The rind should be dense before it goes in the jar.
Pink brine — is that safe?
Yes. The pink color comes from residual red flesh pigment leeching into the brine. It is normal and harmless. The brine will smell pleasantly sour and slightly sweet by day 5.
Bland flavor after fermentation
Watermelon rind has a mild base flavor and requires good aromatics. Try adding more garlic, a whole star anise, or a tablespoon of fresh ginger. The rind is a blank canvas — the lactic acid tang is the acid backbone, but the aromatic profile is yours.
Moldy surface
Exposure above the brine line. Weight the rind down properly and ensure all pieces are fully submerged. Rind pieces near the surface can float if not properly compressed.
Tools for this recipe
You bought the watermelon. You paid for the whole thing. The rind is 30% of what you paid for. Throw it in a jar with salt water, wait 5–7 days, and you have a crunchy, sour, citrulline-rich condiment that would cost $12 at a specialty grocery store. This is the most obvious zero-waste ferment in the kitchen and almost nobody does it.
I'm Chad. Your chemist.