Blackcurrant Juice Made Gut Bacteria Multiply 49-Fold in Lab Study

If you have been eating berries for the polyphenols but composting the seeds, oils and leaves, a new laboratory study suggests you might be discarding the more useful part. Researchers tested how fruits, juices, seed oils and leaves from blackcurrant and raspberry plants affect the growth of two probiotic bacteria in lab cultures, and the fruit itself was rarely the top performer. In several combinations, material from outside the fruit multiplied bacterial counts far beyond what the fruit managed, in one case nearly 50 times over an untreated control. For anyone following research on which foods actually support gut-friendly bacteria, the results are a reminder that the parts of a plant usually thrown away can carry real biological activity, even though this is still early, test-tube stage work.

Key takeaways:

• Blackcurrant juice increased growth of the probiotic bacterium Lactobacillus plantarum by 49-fold, and raspberry leaves increased growth of Lactobacillus rhamnosus by 32-fold, both compared to bacteria grown without any plant material added.
• Across all sample-and-strain combinations tested, 81% of plant materials boosted bacterial growth beyond the control; seed oils were the exception and actually suppressed it.
• Growth stimulation tracked most closely with total organic acid content, sugar levels and flavan-3-ols (a class of polyphenol), not with which part came from the fruit itself.
• This is in vitro, test-tube research only. No animal or human gut was involved, so it says nothing yet about what eating these foods would do inside you.

Blackcurrant Juice and Raspberry Leaves Outgrew the Fruit

Researchers found that adding blackcurrant juice to a bacterial culture increased growth of Lactobacillus plantarum (a probiotic species used in fermented foods and some supplements) by 49-fold compared to bacteria grown without any plant material, while raspberry leaves increased growth of a second species, Lactobacillus rhamnosus, by 32-fold, according to the study published in European Food Research and Technology. The team, based at institutions in Poland, tested fruits, juices, seed oils and leaves from both blackcurrant and raspberry plants against two Lactobacillus strains and found that in 81% of the sample-and-strain combinations, the plant material stimulated more bacterial growth than a plain broth control. Seed oils were the clear exception: blackcurrant seed oil cut L. plantarum growth by 27%, and raspberry seed oil cut it by 81%, compared to the control. The researchers also measured a wide range of nutrients (polyphenols, vitamins, minerals, amino acids, sugars and organic acids) across all ten plant materials and found that fruit itself rarely contained the highest levels of any given nutrient, a pattern that lines up with the growth results.

Why This Matters if You Rely on Probiotic Foods or Supplements

Lactobacillus plantarum and Lactobacillus rhamnosus are two of the most widely used probiotic species in fermented foods and over-the-counter supplements, and both have been studied for possible roles in supporting the gut lining and modulating immune activity, research that people managing IBS (irritable bowel syndrome) or general digestive complaints often come across when researching probiotics. This study does not test any health outcome; it tests how fast these bacteria multiply in a nutrient broth. But food manufacturers care a great deal about what makes probiotic bacteria grow efficiently, because a better growth substrate can mean cheaper, more potent probiotic products and new uses for berry byproducts that would otherwise go to waste. That is the practical stake here: an ingredient discovery relevant to food production, not a new home remedy.

How the Bacteria Were Tested

The researchers added a fixed amount of dried fruit, dried leaf, seed oil or juice (0.5 grams of solids, or 5 milliliters of juice) directly into flasks of MRS broth, a standard growth medium for lactic acid bacteria, then inoculated each flask with either L. plantarum ATCC 8014 or L. rhamnosus ATCC 9595, both standard reference strains. Cultures incubated for 48 hours at 36°C, a temperature chosen to reflect the human gastrointestinal tract, and bacterial density was measured afterward by counting colony-forming units (CFU, a standard way of quantifying live bacteria) on agar plates, averaged across four replicate cultures per condition. A control culture grown in the same broth with no plant material established the baseline for comparison, and differences were confirmed as statistically significant using ANOVA and a Duncan test (p ≤ 0.05).

What This Means, and Does Not Mean, for Your Gut

If you are managing a gut condition and hoping this points to a specific food or supplement to try, it does not, at least not yet. The lab conditions here (sterile broth, no digestive enzymes, no stomach acid, no competing microbes) are nothing like an actual gut, so a 49-fold increase in a flask cannot be read as a 49-fold increase in your intestines from drinking blackcurrant juice or eating raspberry leaves. What the study does offer is a preliminary signal to food scientists about which plant materials might be worth developing into functional food ingredients or fermentation aids. For now, existing evidence-backed probiotic foods and supplements remain the more grounded choice, and nothing here changes standard guidance on managing IBS, IBD or other gut-related conditions. Polyphenol-rich plant compounds have shown prebiotic-like effects on gut bacteria in other lab and animal work, and this study fits that broader pattern, but it does not establish a proven human benefit.

Where the Evidence Runs Out

This experiment was entirely in vitro (conducted outside a living organism, in a lab dish or flask), with no animal or human trial involved. The link between growth stimulation and specific compounds, such as organic acids, sugars and flavan-3-ols, is correlational rather than directly proven: the researchers found a strong statistical association (for example, total organic acid content correlated with L. plantarum growth at r = 0.95) but did not isolate individual compounds to confirm each one causes the effect on its own. Only two bacterial strains were tested, a small slice of the hundreds of species that make up a real gut community, and the experiment did not simulate digestion, meaning no digestive enzymes or the pH swings a food actually experiences passing through the stomach and intestines.

What Comes Next

The study's authors call for follow-up work that simulates digestive conditions, including enzyme exposure and pH changes, and that tests these plant materials against a broader range of both beneficial and potentially harmful gut bacteria. They also suggest exploring fermented products made directly from these berry byproducts. No timeline was given for that next phase of research, and broader research on valorizing berry byproducts suggests seeds, leaves and pomace are an active area of food-science interest well beyond this one study.

Common Questions

Does this mean raspberry leaves or blackcurrant juice are probiotics? No. Probiotics are live microorganisms themselves; this study tested how plant material feeds bacteria already present in a lab culture, so at most these materials are candidates for prebiotic-style ingredients, something only further research could confirm.

What is Lactobacillus rhamnosus, and where would I encounter it? It is one of the most studied probiotic species, found in some yogurts, fermented foods and supplements, and it has been researched for potential roles in gut and immune support, though this study only measured how fast it grew in a broth.

Why did the seed oils hurt bacterial growth instead of helping? The study did not test the mechanism directly, but the seed oils contained far less sugar and fewer of the organic acids and vitamins found in the juices and leaves, the same compounds the researchers linked to faster bacterial growth elsewhere in the data.

References

1. Martyna Szydłowska, Ludwika Tomaszewska-Hetman, Piotr Juszczyk, et al. (2026). Whole-matrix effects of blackcurrant and raspberry materials: linking compositional diversity with microbiota-supporting potential. European Food Research and Technology. https://doi.org/10.1007/s00217-026-05284-5
2. Amal Aljohani, Noha Rashwan, Shruti Vasani, et al. (2024). The Health Benefits of Probiotic Lactiplantibacillus plantarum: A Systematic Review and Meta-Analysis. Probiotics and Antimicrobial Proteins. https://doi.org/10.1007/s12602-024-10287-3
3. Diana Plamada, Dan Cristian Vodnar (2021). Polyphenols—Gut Microbiota Interrelationship: A Transition to a New Generation of Prebiotics. Nutrients. https://doi.org/10.3390/nu14010137
4. Sandra Pedisić, Zoran Zorić, Maja Repajić, et al. (2025). Valorization of Berry Fruit By-Products: Bioactive Compounds, Extraction, Health Benefits, Encapsulation and Food Applications. Foods. https://doi.org/10.3390/foods14081354