FOOD MICROBIOLOGY: BENEFICIAL MICROBES
Diagram
Two live cultures, two jobs
Starter culture
A known microbe added on purpose to ferment a food: it makes acid, sets the curd, and builds flavour.
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Probiotic culture
A live microbe taken in enough numbers to give a health benefit to the person who takes it: it must be a named strain, alive when eaten, with a shown benefit.
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The test
Every live culture in yogurt is not a probiotic: a yogurt culture that makes good yogurt but has no shown benefit is a starter, not a probiotic.
Figure 1: A starter ferments the food; a probiotic is chosen for a health benefit in the gut. Drawn in our own style from the FTN Food Microbiology notes, Units 6 and 7, and from Charalampopoulos and Rastall, Prebiotics and Probiotics Science and Technology.
Introduction
Starter cultures and probiotic cultures are both live bacteria added to food, but they are chosen for different jobs. A starter culture ferments the food: it makes acid, sets the curd and builds flavour. A probiotic culture is chosen for a health benefit in the gut: it must be a named strain, alive in enough numbers when eaten, with a benefit shown in studies. A yogurt culture that makes good yogurt but has no shown benefit is a starter, not a probiotic, and that is the test that separates the two.
The FTN Food Microbiology notes, Unit 6, define the starter culture, and Unit 7 defines the probiotic and asks whether every live culture in yogurt is one. Charalampopoulos and Rastall, Prebiotics and Probiotics Science and Technology, cover the manufacture of probiotic bacteria. This article follows those sources.
Quick comparison
Table 1: Starter culture compared with probiotic culture
| Aspect | Starter culture | Probiotic culture |
|---|---|---|
| Purpose | To ferment the food | To give a health benefit to the person who takes it |
| Chosen for | Acid speed, flavour and tolerance to the food: salt, temperature and acidity | A shown health benefit, and the ability to survive the journey to the gut |
| Strain | A known microbe or mix of microbes, selected for the fermentation | A defined strain: the strain must be named, because benefits belong to a strain, not to a whole species |
| Must be alive when | Alive and active in the food, making acid through the make | Alive when eaten, in enough numbers at the time of eating |
| Numbers | Dosed as a percent of the batch: a 1 percent inoculum into 1,000 litres of milk is 10 litres of bulk starter | Often millions to billions of live cells per serving, and the product must carry enough live cells at the end of its shelf life, not only on the day it was made |
| Survival needed | Must survive the fermentation and stay pure, active and stable | Must live through making, storage and shelf life, then survive stomach acid and bile, reach the intestine alive, and stay long enough to act |
| Examples | Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in yogurt; Lactococcus lactis in cheese | Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus rhamnosus and Lactobacillus plantarum; Bifidobacterium lactis, Bifidobacterium longum and Bifidobacterium bifidum; Saccharomyces boulardii, a yeast |
| Protection | Kept pure by small numbers of subcultures and frozen or freeze-dried storage | Protected by hardy strains, by adding them after heating, by keeping the food cold, and by coatings or capsules that shield cells from acid |
| Claim | Makes the food: acid, curd and flavour | A health benefit, shown in studies |
| If it fails | Acid slows, and the food stays near neutral pH longer, giving illness-causing bacteria time to grow | Too few live cells at the end of shelf life, and the product cannot deliver the benefit |
Scientific explanation
Definition
A starter culture is a known microbe or mix of microbes added on purpose to ferment a food: the food example in the FTN notes is a yogurt starter of Streptococcus thermophilus and Lactobacillus bulgaricus, which turns milk into yogurt. A probiotic culture is a live microorganism that, when taken in enough numbers, gives a health benefit to the person who takes it: the definition has three parts, the strain must be named, it must be alive when eaten, and its benefit must be shown in studies.
Working principle
A starter culture works in the food. Lactic acid cultures are the most used bacterial cultures in food: Lactococcus lactis and related species make cheese and cultured butter by turning lactose into lactic acid, and they also make flavour compounds such as diacetyl, which gives butter its aroma. Lactobacillus species make yogurt, fermented milks and fermented sausages, and some strains make bacteriocins that hold back strangers. Streptococcus thermophilus is a fast acid maker used in yogurt and mozzarella. A probiotic culture works in the eater. For a probiotic to work, it must survive the journey: it must live through making, storage and the shelf life of the food, in numbers high enough to matter, often millions to billions of live cells per serving, then survive stomach acid and bile in the gut, reach the intestine alive, and stay long enough to act. Makers protect probiotics by choosing hardy strains, by adding them after heating, by keeping the food cold, and by using coatings or capsules that shield cells from acid.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
For a starter culture, the test is an activity test in milk: the culture is added to milk, and the acid curve is watched, because a flat acid curve means the starter is suspect. For a probiotic culture, the test is a count of live cells at the end of the shelf life: a product that lists a probiotic must carry enough live cells at the end of its shelf life, not only on the day it was made.
Factors affecting readings
For a starter culture, temperature, time and the ratio at inoculation decide the result, and bacteriophage attack can stall the acid: the fix is to rotate to an unrelated strain at once. For a probiotic culture, heat, storage time, stomach acid and bile decide the result: the cells must be added after heating, kept cold, and shielded from acid, or too few will reach the intestine alive. A genus name alone is not enough for a probiotic, because benefits belong to a strain, not to a whole species: two strains of Lactobacillus acidophilus can behave very differently.
Formulas and calculations
Worked example for a starter dose, from the FTN Food Microbiology notes, Unit 6: a 1 percent inoculum into 1,000 litres of milk is 10 litres of bulk starter.
For a probiotic the books give no formula, so none is given: the number is a count, often millions to billions of live cells per serving, and the product must carry enough live cells at the end of its shelf life.
Industrial applications
- Starter culture: turning milk into yogurt with Streptococcus thermophilus and Lactobacillus bulgaricus.
- Starter culture: making cheese and cultured butter with Lactococcus lactis, which also makes diacetyl for butter aroma.
- Starter culture: making fermented sausages, where the culture must work in salt and at cool temperatures.
- Probiotic culture: fermented milks and supplements with Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus rhamnosus or Lactobacillus plantarum.
- Probiotic culture: yogurts and infant foods with Bifidobacterium lactis, Bifidobacterium longum or Bifidobacterium bifidum.
- Probiotic culture: products with Saccharomyces boulardii, a yeast, and some strains of Bacillus and Streptococcus.
Advantages and limitations
- Starter culture, advantage: it makes the food itself, giving acid, curd and flavour, and it protects the food by making acid fast.
- Starter culture, limitation: it is open to bacteriophage attack, which stalls the acid, and a weak starter leaves the door open to strangers.
- Probiotic culture, advantage: it is chosen for a shown health benefit, with a named strain and live cells in the numbers needed.
- Probiotic culture, limitation: the cells must survive making, storage, shelf life, stomach acid and bile, and a product that lists a probiotic must carry enough live cells at the end of its shelf life.
FAQ
What is the main difference between a starter culture and a probiotic culture?
A starter culture ferments the food, making acid and flavour, while a probiotic culture is chosen for a health benefit in the gut, with a named strain alive in enough numbers when eaten.
Is every live culture in yogurt a probiotic?
No: a probiotic must be a defined strain, alive in enough numbers at the time of eating, with a shown health benefit. A yogurt culture that makes good yogurt but has no shown benefit is a starter, not a probiotic.
Why must a probiotic strain be named?
Because benefits belong to a strain, not to a whole species: two strains of Lactobacillus acidophilus can behave very differently.
How many live cells does a probiotic product need?
Often millions to billions of live cells per serving, and enough at the end of its shelf life, not only on the day it was made.
How are probiotics protected in a food?
By choosing hardy strains, by adding them after heating, by keeping the food cold, and by using coatings or capsules that shield cells from stomach acid.
References and further reading
- FTN subject: Food Microbiology, Unit 6: Starter Cultures and Genetic Importance; Unit 7: Beneficial Use of Microorganisms in Food: Probiotics, Bacteriocins, Nutraceuticals.
- Dimitris Charalampopoulos and Robert A. Rastall (eds.), Prebiotics and Probiotics Science and Technology, Springer, 2009 (Google Books page read)
