This calculator helps you design a fermented dairy batch with the right fat, protein, and sugar levels to support probiotic bacteria growth — and then estimates how many colony-forming units (CFU) you’ll end up with after fermentation.

It was built with Australian home fermenters in mind. Unlike the US, Australia doesn’t have “half and half” — that familiar American product that sits halfway between milk and cream. Many probiotic yogurt recipes (particularly those following the L. Reuteri protocol popularised by Dr. William Davis) call for half and half as the base. Since we can’t buy it off the shelf here, the calculator lets you combine whole milk, cream, evaporated milk, and other ingredients to hit the same macro targets yourself.

---

The Two Sections

🧪 Growth Medium Composition

This is where you enter your ingredients. Each ingredient can be toggled on or off with its checkbox. When checked, you can enter the volume (in mL or grams for powder) and adjust the fat, protein, and sugar values to match the specific product you’re using — since these vary between brands.

Available ingredients:

• Whole Milk
• Evaporated Milk
• Pouring Cream
• Whole Milk Powder
• Water
• Prebiotic Fibre (Inulin or Potato Starch)

🧫 Fermentation Parameters

This is where you configure your bacteria strain, starting CFU, incubation time, and temperature. The calculator uses a logistic growth model — meaning growth accelerates early then slows as it approaches the carrying capacity of the medium, which is more realistic than simple exponential growth.

---

Target Ranges

After hitting Calculate, the composition results are colour coded:

• 🟢 Green — within the optimal range
• 🟡 Yellow — below optimal
• 🟠 Orange — above optimal

The targets used are:

Macro	Optimal Range
Fat	9–12%
Protein	3.5–5%
Sugar	4–6%

These ranges are aimed at providing a rich substrate for probiotic bacteria while keeping sugar low enough to avoid feeding unwanted organisms.

---

Worked Examples

Example 1 — The Classic L. Reuteri Yogurt (Australian Style)

L. Reuteri is the most popular strain for home fermented yogurt, largely due to the work of Dr. William Davis whose protocol calls for a long 36-hour fermentation at body temperature to achieve very high CFU counts. His recipes typically use half and half as the base — which simply isn’t available in Australia.

The good news is that combining whole milk and pouring cream gets you right into the same fat range. Here’s a straightforward 1 litre batch:

Ingredient	Amount
Whole Milk	700 mL
Pouring Cream (35% fat)	300 mL

Fermentation settings:

• Strain: L. Reuteri
• Starting CFU: 10 billion
• Time: 36 hours
• Temperature: 37°C (use the Use Optimal button)

Result: ~11.5% fat, ~3.4% protein, ~3.9% sugar — sitting right in the optimal zone. L. Reuteri thrives in high-fat dairy and the 36-hour fermentation gives it plenty of time to reach a very high final CFU count.

---

Example 2 — L. Reuteri Large Batch with Evaporated Milk

For a thicker, more set yogurt with better body, adding evaporated milk boosts the protein without dramatically changing the fat profile. This is a great approach if you find the basic milk and cream version too thin.

Ingredient	Amount
Whole Milk	2000 mL
Evaporated Milk	384 mL
Pouring Cream	600 mL

Fermentation settings:

• Strain: L. Reuteri
• Starting CFU: 10 billion
• Time: 36 hours
• Temperature: 37°C

Result: Fat lands around 10.8%, protein around 4.1%, sugar around 4.8% — all green. The higher protein content from the evaporated milk gives the yogurt a noticeably better texture while keeping everything in the optimal fermentation range. This is roughly the default batch the calculator opens with.

---

Example 3 — L. Casei Shirota Using Yakult® as Your Starter

L. Casei Shirota is the probiotic strain found in Yakult®, the small red-topped bottles available at Woolworths and Coles. Rather than sourcing a commercial starter powder, you can use a bottle of Yakult directly as your inoculant — it contains billions of live L. Casei Shirota cells and works very well as a starter culture, making it one of the most accessible and affordable ways to get started.

One 65 mL bottle of Yakult contains approximately 6.5 billion CFU, so use that as your starting CFU value in the calculator.

Ingredient	Amount
Whole Milk	1500 mL
Evaporated Milk	375 mL
Pouring Cream	300 mL

Fermentation settings:

• Strain: L. Casei Shirota
• Starting CFU: 6.5 billion (one bottle of Yakult)
• Time: 36 hours
• Temperature: 37°C (use the Use Optimal button)

Result: L. Casei Shirota is a good sugar utiliser and handles dairy very well. After 36 hours you’ll end up with a mild, slightly tangy yogurt with a very high CFU count — at a fraction of the cost of commercial probiotic supplements.

Note that Yakult itself is quite high in sugar (about 11g per bottle), but this is actually a non-issue for two reasons. First, you’re diluting it across roughly 2 litres of dairy so the contribution to the starting sugar percentage is minimal. More importantly, L. Casei Shirota is a strong sugar fermenter — it will actively consume the sugars in the medium (including whatever came from the Yakult bottle) and convert them into lactic acid over the course of fermentation. By the time your 36 hours are up, the finished yogurt will have a significantly lower sugar content than the raw starting mix. The calculator shows the composition of the medium going in, not what comes out the other end.

---

Example 4 — B. Longum with Inulin for Prebiotic Support

B. Longum is a slower-growing strain but it is exceptionally good at fermenting prebiotic fibres — particularly inulin. Adding inulin to a B. Longum batch provides a significant substrate boost and supports the broader gut microbiome through short-chain fatty acid production during fermentation.

Ingredient	Amount
Whole Milk	2000 mL
Evaporated Milk	384 mL
Pouring Cream	600 mL
Inulin	30 g

Fermentation settings:

• Strain: B. Longum
• Starting CFU: 10 billion
• Time: 36 hours
• Temperature: 37°C (use the Use Optimal button)

Result: The inulin provides a 1.30× substrate boost for B. Longum — the largest fibre boost of any strain in the calculator. Despite being a slower divider, the extended fermentation time and the prebiotic advantage mean B. Longum still produces a high-CFU end product. Try comparing the result with and without inulin by toggling the amount between 0 and 30g to see the difference.

---

Tips

Use the “Use Optimal” button. Each bacteria strain has a different ideal temperature — L. Bulgaricus and S. Thermophilus prefer 42°C, while most Lactobacillus strains prefer 37°C. Clicking Use Optimal fills in the right temperature automatically. You can still override it manually if your incubator runs slightly warm or cool.

The recommended temperature is shown next to the label. Even before clicking the button, you’ll see the strain’s optimal temp displayed in green next to the Temperature field — handy as a quick reference.

Adjust the macro values to match your actual product. The default values are reasonable averages, but full-cream milk from Pauls vs. a home-brand can differ noticeably in fat content. Check the nutrition panel on your carton and update the fields accordingly for a more accurate result.

Prebiotic fibre only boosts CFU if you enter an amount. Selecting Inulin or Potato Starch from the dropdown reveals an amount field. If you leave it at zero, no boost is applied. Different strains respond differently — B. Longum gets the biggest lift from inulin (1.30×), while L. Gasseri also responds well (1.20×).

The CFU estimate is a model, not a lab result. It uses established growth parameters and a logistic growth curve, but real-world results will vary based on starter culture viability, actual incubator temperature consistency, and individual batch conditions. Use the numbers as a guide for comparing recipes and strains rather than as absolute values.

---

Understanding the CFU Results

After calculating, the fermentation results show:

• Starting CFU — your inoculation amount
• After X hrs — estimated total CFU at the end of incubation
• Final CFU / mL — how concentrated the culture is
• Doublings achieved — how many times the population doubled
• CFU per 125 mL serving — a practical per-serve estimate
• Temperature efficiency — how close your set temperature is to the strain’s optimal, expressed as a percentage

If your temperature is outside the viable range for the selected strain, the calculator will warn you and return zero growth.