Cultures: getting, keeping and sharing them
Many ferments start with a culture that passes from one batch to the next. Which ones can go from hand to hand, which ones you have to buy, and how to look after them.
6 min read First steps Reviewed October 26, 2026 33 facts, each with its source
A culture that passes from batch to batch
Many ferments start with a piece of the last one: a SCOBY, some grains, a spoonful of sourdough starter. In kombucha this is the normal practice1, and in kefir it's an accepted one2. It works because the culture is acidic, and the acidity holds back the microbes we don't want.
That's why BCCDC, British Columbia's public health agency, doesn't recommend it for ferments that aren't acidic3. For those, the culture is bought every time. And a bought culture is activated the way its maker says: BCCDC asks for this with kefir grains4.
Culture by culture
| Culture | Where it comes from | How to keep it |
|---|---|---|
| Kombucha SCOBY | Buy the first one: Virginia Tech asks you not to pass it from person to person5 | In its jar, with 20% kombucha and a cloth on top, for up to 30 days6 |
| Water kefir grains | Live or dehydrated; dehydrated ones are reactivated before use7 | In the fridge, in water with 5% sugar, for up to 30 days8. Never in the freezer9 |
| Milk kefir grains | Shared or bought10 | In fresh milk, in the fridge, for up to 7 days11. Dried or freeze-dried, 12 to 18 months12 |
| Ginger bug | Made at home: it's ginger's own microbes13 | In the fridge, fed once a week (house rule) |
| Sourdough starter | Shared, bought dried or made at home from flour and water14 | Covered in the fridge when not in use, and fed once a week15 |
| Mother of vinegar | Bought as a pure culture16, in stores that supply winemaking equipment17 | Vinegar from one batch starts the next18 |
| Koji spores | Always bought, so they're pure19 | Not reused: toxin-making molds could get in20 |
| Yogurt | A plain yogurt with live cultures21 | Homemade yogurt works as a starter within 5 to 7 days22 |
The guides don't agree on yogurt. The University of Illinois lets you use the last batch22; the NCHFP, the National Center for Home Food Preservation, asks for freshly bought yogurt each time23.
Keeping them alive
A live culture needs to eat and to work. If you use it often, follow the rhythm of its recipe. If you're taking a break, the fridge slows it down, and each culture has its own way of waiting:
- A SCOBY waits in its jar, covered with a cloth, not in an airtight container: without air its yeasts die and the next batch can fail24.
- A ginger bug goes in the fridge and eats once a week. The day before you use it, take it out and feed it (house rule).
- Sourdough starter keeps better dried than frozen: the freezer damages it and it performs poorly afterward25. To dry it, spread it very thin on parchment paper.
- Milk kefir grains prefer milk to water for a short break, and for a long one, dried or frozen.
- Vinegar doesn't need a mother kept on the side: the liquid from a finished batch is enough. Leaving wine open to the air so it starts on its own is risky, because other organisms may grow26.
And for all of them, cleanliness: clean containers and clean hands every time you handle it27.
Sharing a culture
Kefir grains and sourdough starter grow, and the extra can be given away. CSU, Colorado State University Extension, says so for milk kefir grains10 and for sourdough starter28. Sourdough starter travels best dried29.
Fervida suggests doing it this way, and these are house rules:
- Only from a healthy culture that has fermented well in its last few batches, with no mold.
- In a clean jar, with a little of its own liquid: milk for milk kefir grains, sugar water for water kefir grains.
- With a label: what it is, the date and how to feed it.
- To be used soon: the same day, or in the fridge until it starts.
The SCOBY is the exception. The NCHFP lists friends among the places to get one30, but Virginia Tech asks you not to pass it from person to person, and Fervida follows Virginia Tech. Koji spores aren't shared either: they're bought.
When to throw it out
- A SCOBY with blue, green, gray, brown or black fuzz31.
- Sourdough starter with mold, colored or fuzzy. Afterward, clean the jar well before starting another32.
- Kefir grains that grow slowly, change color or smell, or stop fermenting33.
A moldy culture isn't saved by cutting off a piece: it goes, all of it, and it's never given away.
Where every number comes from
Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.
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1
In kombucha, starting with part of the last batch is normal
“backslopping is normal practice for this fermentation”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.11: Kombucha and Jun (September 2023, updated December 2024) · read on October 25, 2026
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2
Reusing kefir grains is an accepted practice
“Grains are re-used in subsequent batches of kefir (a type of backslopping) and this process is acceptable.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.12: Kefir, milk and water (December 2024) · read on October 25, 2026
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3
In ferments that aren't acidic, it's not recommended
“In summary, backslopping in non-acid fermentations or in fermentations where the culture creates low acid conditions are not recommended.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026
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4
If you buy them, follow the maker's activation instructions
“If kefir grains are commercially purchased, operators should follow the activation process described in the Product Specification Sheet.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.12: Kefir, milk and water (December 2024) · read on October 25, 2026
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5
Virginia Tech: buy the first SCOBY, and don't pass it from person to person
“It is highly recommended that you purchase your SCOBY commercially for first use. Do not pass SCOBY cultures from one person to another.”
Virginia Cooperative Extension (Virginia Tech) · What do I need to know to sell KOMBUCHA at the farmers market? (FST-297, last reviewed November 2025) · read on October 20, 2026
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6
In its jar, with 20% kombucha, for up to 30 days
“The SCOBY can be stored in the fermentation vessel with about 20% of previously brewed kombucha for up to 30 days.”
University of Georgia Cooperative Extension (National Center for Home Food Preservation) · Crafting Kombucha at Home: Frequently Asked Questions (2026) · read on October 20, 2026
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7
Frozen or dehydrated grains have to be reactivated
“New kefir grains (frozen or dehydrated) will need to be reactivated before use.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.12: Kefir, milk and water (December 2024) · read on October 25, 2026
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8
30 days in the fridge, in water with 5% sugar
“One group was stored in sterile 5% (w/v) sugared water at +4°C for 30 days”
Food Science & Nutrition (Gökırmaklı, Şatır & Guzel-Seydim), via PubMed Central · Microbial viability and nutritional content of water kefir grains under different storage conditions (2024) · read on October 25, 2026
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9
Freezing damages them for good
“Laureys et al. (2017) reported that freezing and thawing WKG resulted in irreversible damage.”
Food Science & Nutrition (Gökırmaklı, Şatır & Guzel-Seydim), via PubMed Central · Microbial viability and nutritional content of water kefir grains under different storage conditions (2024) · read on October 25, 2026
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10
Milk kefir grains are shared between friends or bought
“Kefir grains may be shared between friends or purchased from online retailers or some health food stores.”
Colorado State University Extension (FoodSmart Colorado) · Understanding and Making Kefir · read on October 20, 2026
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11
In fresh milk, up to 7 days in the fridge
“When re-using milk kefir grains, store in fresh milk for up to 7 days, or in water for up to 3 days at refrigeration temperatures for optimal activity.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.12: Kefir, milk and water (December 2024) · read on October 25, 2026
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12
Dried or freeze-dried, 12 to 18 months
“For longer term storage, milk kefir grains that are air-dried or freeze-dried and kept refrigerated or frozen can retain microbial activity for 12-18 months.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.12: Kefir, milk and water (December 2024) · read on October 25, 2026
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13
A ginger bug is a culture of ginger's own microbes
“which is a fermented culture of microorganisms native to ginger”
Revista de Ciências Agroveterinárias (UDESC; Alencar et al., Federal University of Amazonas) · Kinetic and sensory analysis of ginger beer produced by diversified fermentation methods (v.24 n.3, 2025) · read on October 25, 2026
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14
Sourdough starter is shared, bought dried or made from flour and water
“You can obtain a starter from a friend, purchase a commercial dried starter, or make your starter with flour and water.”
University of Illinois Extension · Sourdough Bread: 101 (modified December 2025) · read on October 25, 2026
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15
Covered in the fridge when not in use, and fed weekly
“Keep your starter covered in an air-tight container and refrigerated when not in use. Ideally, feed and use the starter weekly.”
Colorado State University Extension (FoodSmart Colorado) · Sourdough Basics: Caring for your Starter and Making Bread (reviewed October 2025) · read on October 25, 2026
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16
A pure culture, the mother of vinegar, is recommended
“It is recommended to order pure cultures of bacteria, sometimes called the mother of vinegar”
NC State Extension · Vinegar Making (July 2022) · read on October 25, 2026
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17
It's sold in stores that supply winemaking equipment
“They can be purchased in any store that supplies wine-making equipment.”
NC State Extension · Vinegar Making (July 2022) · read on October 25, 2026
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18
Vinegar from one batch starts the next
“They can be used to start a new batch of vinegar, so it is not necessary to purchase new starter bacteria”
NC State Extension · Vinegar Making (July 2022) · read on October 25, 2026
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19
Koji spores bought, so they're pure
“Starter culture must be purchased from commercial sources to ensure purity.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
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20
Koji isn't reused: toxin-making molds could get in
“Culture re-use (backslopping) is not recommended in these ferments as wild strains of Aspergillus that contain toxins may be introduced.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
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21
A yogurt with live cultures
“Be sure the product label indicates that it contains a live culture.”
National Center for Home Food Preservation (Nummer, 2002); copy hosted by UC Davis Food Safety · Fermenting Yogurt at Home (October 2002) · read on October 25, 2026
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22
Homemade yogurt works as a starter within 5 to 7 days
“If using a previous batch of homemade yogurt, use it as a starter within 5 to 7 days from when the yogurt was made.”
University of Illinois Extension · Making Yogurt at Home, Best Practice Guide (modified May 2024) · read on October 25, 2026
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23
The NCHFP asks for a freshly bought culture each time
“Use a fresh, recently purchased culture from the grocery store each time you make yogurt.”
National Center for Home Food Preservation (Nummer, 2002); copy hosted by UC Davis Food Safety · Fermenting Yogurt at Home (October 2002) · read on October 25, 2026
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24
In an airtight container, its yeasts die
“Using an airtight container may create an anaerobic environment, causing aerobic yeasts to die and potentially resulting in a failed next batch of kombucha.”
University of Georgia Cooperative Extension (National Center for Home Food Preservation) · Crafting Kombucha at Home: Frequently Asked Questions (2026) · read on October 20, 2026
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25
Frozen, it's damaged and performs poorly: better to dry it
“Yes, but it can damage the cell structure, causing poor performance when thawed. It is better to dehydrate it for long-term storage.”
University of Illinois Extension · Sourdough Bread: 101 (modified December 2025) · read on October 25, 2026
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26
Leaving wine open to the air is risky: other organisms may grow
“Leaving wine exposed to air may sometimes start the process, but it is very risky because some other organisms may grow”
NC State Extension · Vinegar Making (July 2022) · read on October 25, 2026
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27
Clean containers and clean hands when handling the culture
“Sanitary containers and sanitary handling of the SCOBY mother culture are necessary to prevent cross-contamination with undesirable spoilage and pathogenic microbes that may interfere with the successive fermentation.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.11: Kombucha and Jun (September 2023, updated December 2024) · read on October 25, 2026
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28
Extra sourdough starter can be shared
“If there is more removed starter than needed, excess starter can be shared, frozen, used in other recipes, composted, or discarded.”
Colorado State University Extension (FoodSmart Colorado) · Sourdough Basics: Caring for your Starter and Making Bread (reviewed October 2025) · read on October 25, 2026
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29
It can be dried for storing or sharing
“Sourdough starter may be dried for easy storing or sharing.”
University of Alaska Fairbanks Cooperative Extension Service · Sourdough (FNH-00061, revised October 2024) · read on October 25, 2026
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30
The NCHFP lists friends among the places to get a SCOBY
“SCOBYs can be obtained from various sources, including local kombucha brewers, online marketplaces, health food stores, or through friends who brew kombucha.”
University of Georgia Cooperative Extension (National Center for Home Food Preservation) · Crafting Kombucha at Home: Frequently Asked Questions (2026) · read on October 20, 2026
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31
A SCOBY with blue, green, gray, brown or black fuzz goes
“Discard if fuzzy patches of blue, green, grey, brown or black on SCOBY.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.11: Kombucha and Jun (September 2023, updated December 2024) · read on October 25, 2026
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32
Moldy sourdough starter isn't used, and the jar is cleaned well
“Sourdough starter that shows any sign of mold (colored and/or fuzzy) should NOT be used, and the container should be thoroughly cleaned and rinsed before starting over.”
Colorado State University Extension (Farm to Table) · Sourdough Starter Best Practices (March 2020) · read on October 25, 2026
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33
Grains that grow slowly or change color or smell go
“If kefir grains grow slowly, if the grains develop unusual colour, odour, or if grains lose activity, grain function is compromised, and they should be discarded.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.12: Kefir, milk and water (December 2024) · read on October 25, 2026