FOOD MICROBIOLOGY: STARTER CULTURES
Diagram
Two starter cultures, two temperatures
Mesophilic cultures
Grow best in moderate warmth, about 20 to 45 C. Lactococcus lactis is the classic mesophilic cheese culture.
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Thermophilic cultures
Grow best in heat, about 45 to 70 C. Streptococcus thermophilus is the classic thermophilic culture, used in yogurt and mozzarella.
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The choice
The cheese decides: a cheese made and ripened cool needs a mesophilic culture, and a cheese whose curd is cooked hot needs a thermophilic one.
Figure 1: Mesophilic cultures like moderate warmth and thermophilic cultures like heat. Drawn in our own style from the FTN Food Microbiology notes, Unit 1 and Unit 6, and from Donnelly, Cheese and Microbes, chapter on mesophilic and thermophilic cultures used in traditional cheesemaking.
Introduction
Mesophilic and thermophilic cultures are the two kinds of starter culture used in cheese and fermented foods, and both turn milk into cheese by making acid, but they grow at different temperatures and give different flavours. Mesophilic cultures grow best in moderate warmth, about 20 to 45 C, while thermophilic cultures grow best in heat, about 45 to 70 C. The maker matches the culture to the cheese: a cheese made and ripened cool needs a mesophilic culture, and a cheese whose curd is cooked hot needs a thermophilic one.
The FTN Food Microbiology notes, Unit 1, group microbes by the temperature they prefer, and Unit 6 describes the lactic cultures themselves. Donnelly, Cheese and Microbes, gives the two kinds a whole chapter, on mesophilic and thermophilic cultures used in traditional cheesemaking. This article follows those sources.
Quick comparison
Table 1: Mesophilic cultures compared with thermophilic cultures
| Aspect | Mesophilic cultures | Thermophilic cultures |
|---|---|---|
| Preferred temperature | Moderate warmth, about 20 to 45 C | Heat, about 45 to 70 C |
| Main organisms | Lactococcus lactis and related species | Streptococcus thermophilus, and Lactobacillus species such as Lactobacillus delbrueckii subsp. bulgaricus |
| Foods made | Cheese and cultured butter, made and ripened at moderate temperatures | Yogurt and mozzarella, and cooked-curd cheeses such as Swiss-type cheese |
| What they do | Turn lactose into lactic acid, and make flavour compounds such as diacetyl, which gives butter its aroma | Make acid fast at high temperature, and work in the hot curd where mesophilic cultures would stop |
| Acid speed | Steady acid through the make, at a rate the cheese needs | Fast acid making: Streptococcus thermophilus is a fast acid maker used in yogurt and mozzarella |
| Flavour | Buttery and mild flavours, including diacetyl in butter | The classic yogurt flavour, built with Lactobacillus bulgaricus, which breaks down milk protein into amino acids |
| Mixed cultures | Used alone or in mixes, with the ratio at inoculation controlled so one strain does not crowd out the others | Used in the yogurt pair: Streptococcus thermophilus grows fast and makes acid and formic acid, which helps Lactobacillus bulgaricus, and the pair makes acid faster than either alone |
| If the temperature is wrong | Too hot, and the culture stalls and acid slows, leaving the food near neutral pH longer, which gives illness-causing bacteria time to grow | Too cool, and the culture stalls in the same way, with the same safety cost |
| Keeping the culture | Pure, active and stable: small numbers of subcultures, and frozen or freeze-dried storage | The same: pure, active and stable, with small numbers of subcultures and frozen or freeze-dried storage |
| Weakness | Bacteriophage attack: acid starts normally and then stalls, or a vat never acidifies while the culture looks fine under the microscope | The same phage risk, handled by rotating to an unrelated strain, so the phage has no host |
Scientific explanation
Definition
A mesophilic culture is a starter culture of microbes that grow best at moderate warmth, about 20 to 45 C, and the classic example is Lactococcus lactis, which makes cheese and cultured butter by turning lactose into lactic acid. A thermophilic culture is a starter culture of microbes that grow best in heat, about 45 to 70 C, and the classic example is Streptococcus thermophilus, a fast acid maker used in yogurt and mozzarella.
Working principle
Both kinds of culture work the same way: the microbes are added to the milk, they turn lactose into lactic acid, the acid sets the curd and protects the food, and the flavour builds as they work. The difference is the temperature at which they do it. Lactococcus lactis and related species make cheese and cultured butter at moderate warmth, 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, where the curd is hot. In yogurt the two thermophilic strains help each other: the streptococcus grows fast and makes acid and formic acid, which helps the lactobacillus, and the lactobacillus breaks down milk protein into amino acids that feed the streptococcus, so the pair makes acid faster and makes more flavour than either alone.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
The test for a starter culture is an activity test in milk: the culture is added to milk, and the acid curve is watched. If the acid curve is flat, the starter is suspect, and a plaque test on a plate can confirm phage by the clear spots where phage killed the bacteria. Choosing a lactic culture means matching its acid speed, flavour and tolerance to the food: a cheese culture must make acid at a steady rate through the make, while a sausage culture must work in salt and at cool temperatures.
Factors affecting readings
Temperature is the strongest factor: each culture has the range where it grows best, and outside that range acid slows, which is a safety problem, not only a flavour problem, because slow acid means the food stays near neutral pH longer, giving illness-causing bacteria time to grow before the acid hurdle appears. The ratio at inoculation matters in mixed cultures: if one strain grows much faster, it can crowd out the others, and the flavour changes. Bacteriophage attack is a further factor: a phage attack shows as acid that starts normally and then stalls, and the fix is to rotate to an unrelated strain at once, because a phage attacks one host type.
Formulas and calculations
Worked example for dosing a starter, from the FTN Food Microbiology notes, Unit 6: the inoculum is the amount of starter culture added to a food, usually as a percent of the batch. A 1 percent inoculum into 1,000 litres of milk is 10 litres of bulk starter. The same percent rule applies to mesophilic and thermophilic cultures alike: the dose is a percent of the batch, and the temperature decides which culture is dosed.
Industrial applications
- Mesophilic cultures: making cheese and cultured butter with Lactococcus lactis at moderate warmth.
- Mesophilic cultures: making butter flavour, where diacetyl from the culture gives butter its aroma.
- Mesophilic cultures: making fermented sausages, where the culture must work in salt and at cool temperatures.
- Thermophilic cultures: making yogurt with Streptococcus thermophilus and Lactobacillus bulgaricus together.
- Thermophilic cultures: making mozzarella, where the curd is hot and the culture must make acid fast.
- Thermophilic cultures: making Swiss-type cheese, where Propionibacterium freudenreichii then works slowly during ripening, making the holes and the nutty flavour.
Advantages and limitations
- Mesophilic cultures, advantage: steady acid at moderate temperatures, with buttery flavours, and they suit the great range of cheeses made and ripened cool.
- Mesophilic cultures, limitation: they stall when the curd is cooked hot, so they cannot serve cooked-curd cheeses, and slow acid is a safety risk.
- Thermophilic cultures, advantage: fast acid making in hot curd, which suits yogurt, mozzarella and cooked-curd cheeses.
- Thermophilic cultures, limitation: they stall when the milk is cool, so they cannot serve cheeses made at moderate temperatures, and they carry the same phage risk as mesophilic cultures.
FAQ
What is the main difference between mesophilic and thermophilic cultures?
The temperature they grow best at: mesophilic cultures like moderate warmth, about 20 to 45 C, while thermophilic cultures like heat, about 45 to 70 C.
Which culture makes yogurt?
A thermophilic pair: Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, which help each other and make acid faster together than either alone.
Which culture makes cheddar-type cheese?
A mesophilic culture, usually Lactococcus lactis, which makes steady acid at moderate temperatures.
Why is slow acid a safety problem, not only a flavour problem?
Slow acid means the food stays near neutral pH longer, giving illness-causing bacteria time to grow before the acid hurdle appears.
What is done when a phage attacks a starter culture?
Rotate to an unrelated strain at once, so the phage has no host, and tighten hygiene, keeping the starter room separate from production.
References and further reading
- FTN subject: Food Microbiology, Unit 1: Microorganisms in Food, Sources and Factors; Unit 6: Starter Cultures and Genetic Importance.
- Catherine W. Donnelly (ed.), Cheese and Microbes, ASM Press, 2014 (Google Books page read)
