The Science Behind pH Changes When Milk Turns Into Curd
Milk is a staple food item consumed worldwide, and its transformation into curd is one of the oldest known biological processes. This transformation involves significant biochemical changes, particularly in the pH level of the milk. Fresh milk typically has a pH of about 6.7 to 6.5, slightly acidic compared to pure water (pH 7). When it turns into curd, this pH drops significantly, usually reaching between 4.5 to 5.5. This article explores in detail the scientific reasons behind this pH change and its implications.
The Biochemical Transformation Process
1. Initial Composition of Milk
Fresh milk is a complex biological fluid containing:
- Water (87%): The primary component
- Lactose (4.8%): Milk sugar
- Fats (3-4%): Depending on the milk type
- Proteins (3.2%): Mainly casein and whey proteins
- Minerals (0.7%): Calcium, phosphorus, etc.
The initial pH of fresh milk (around 6) is slightly acidic due to the presence of various components including dissolved carbon dioxide, citrates, phosphates, and proteins that can act as buffers.
2. Introduction of Starter Culture
When making curd, we introduce lactic acid bacteria (LAB) as starter cultures. The most common species include:
- Lactobacillus bulgaricus
- Streptococcus thermophilus
- Lactococcus lactis
These bacteria are homofermentative, meaning they produce primarily lactic acid from lactose fermentation. The bacteria multiply rapidly in the milk environment, converting lactose into lactic acid through their metabolic processes.
3. Lactose Fermentation Process
The key biochemical reaction that causes pH change is the fermentation of lactose:
C12H22O11 (lactose) + H2O → 4 C3H6O3 (lactic acid)
This process occurs in several enzymatic steps:
- Lactose is transported into the bacterial cell
- β-galactosidase enzyme breaks lactose into glucose and galactose
- These simple sugars are then metabolized via the glycolytic pathway
- Pyruvate is converted to lactic acid
The accumulation of lactic acid in the milk is what causes the pH to drop.
The Chemistry Behind pH Reduction
Lactic acid (C3H6O3) is a weak organic acid that partially dissociates in water:
C3H6O3 ⇌ C3H5O3– + H+
The release of hydrogen ions (H+) is what makes the solution more acidic, lowering the pH. The pH scale is logarithmic, meaning each whole pH value below 7 is ten times more acidic than the next higher value.
Several factors influence how much the pH drops:
- Bacterial strain: Different LAB produce varying amounts of acid
- Temperature: Optimal growth at 37-45°C increases acid production
- Time: Longer fermentation allows more acid accumulation
- Milk composition: Higher lactose content means more substrate for acid production
pH Changes During the Curdling Process
| Stage | Approximate pH | Observable Changes |
|---|---|---|
| Fresh milk | 6.5-6.7 | Liquid, slightly sweet taste |
| Early fermentation (2-3 hours) | 6.0-5.8 | No visible changes, slight tanginess |
| Active fermentation (4-6 hours) | 5.5-5.0 | Milk begins to thicken, noticeable sour taste |
| Coagulation point | 4.8-4.6 | Visible curd formation, clear separation from whey |
| Mature curd (8-12 hours) | 4.5-4.2 | Firm texture, pronounced sour flavor |
Figure: Typical pH curve during milk fermentation showing the gradual decrease in pH over time
Impact of pH Changes on Milk Components
1. Protein Denaturation
The casein proteins in milk are particularly sensitive to pH changes:
- At milk’s natural pH (6.7), casein exists as micelles (small aggregates)
- As pH drops below 5.3, the micelles lose their stability
- At pH 4.6 (casein’s isoelectric point), the proteins coagulate completely
This is why we observe the milk changing from liquid to semi-solid curd.
2. Mineral Solubility
The decreasing pH affects mineral solubility:
- Calcium phosphate becomes more soluble at lower pH
- This releases calcium ions that help bridge casein molecules
- Contributes to the firmness of the curd
3. Microbial Environment
The acidic environment created serves several purposes:
- Inhibits growth of pathogenic bacteria
- Creates favorable conditions for beneficial LAB
- Acts as a natural preservative for the curd
Conclusion: The Significance of pH in Curd Formation
The transformation of milk into curd is fundamentally a pH-driven process. The drop from pH 6 to about 4.5 is crucial for:
- Creating the characteristic texture of curd through protein coagulation
- Developing the desired tangy flavor profile
- Ensuring food safety by creating an inhospitable environment for harmful microbes
- Improving digestibility as the acidic environment begins protein breakdown
Understanding these pH changes allows for better control of the curd-making process, whether at home or in industrial settings. By manipulating factors like fermentation time, temperature, and bacterial strains, we can create products with varying textures and flavors while maintaining food safety standards.