Methods for Analyzing Curd Quality

Every batch of curd that leaves a dairy facility has to pass specific chemical tests before it reaches consumers. Two of these tests – fat content determination and acidity measurement – are the backbone of curd quality control. Together, they confirm whether the curd has the right composition, fermentation level, and shelf life to meet food safety standards. Understanding how these tests work, and why they matter, gives a clear picture of what goes into producing consistently high-quality curd.

Why testing curd matters 🔗

Curd quality is not something that can be assessed by sight alone. A batch can look perfectly fine but have compromised fat levels due to poor milk standardization, or carry excess acidity indicating advanced spoilage. Dairy companies apply stringent quality criteria covering composition, freshness, and hygiene – and chemical analysis is the most reliable tool to verify all three. Without systematic testing, product inconsistencies, consumer complaints, and food safety risks go undetected until it is too late.

Physico-chemical parameters such as fat percentage, acidity, and pH provide critical information about processing efficiency, shelf life, and compliance with regulatory benchmarks. For commercial curd specifically, these parameters also determine whether the product delivers the texture and tangy flavor that consumers expect batch after batch.

Determining fat content using the Gerber method 🔗

The Gerber method is a primary chemical test for determining fat content in dairy products, widely used across Europe, India, and much of the world. It was developed and patented by Dr. Niklaus Gerber of Switzerland in 1892 and remains the standard method referenced in international norms including ISO 2446 and the International Dairy Federation Regulation 105. Its enduring popularity comes down to speed, accuracy, and relatively low equipment costs.

Principle of the method 🔗

Fat in curd exists within globules surrounded by a protective protein membrane. To isolate it, the protein coating must be destroyed first. Concentrated sulfuric acid accomplishes this – it oxidizes and hydrolyzes the organic components in the protective coating, dissolves all non-fat solids, and releases the fat. The addition of amyl alcohol then facilitates a clean phase separation, producing a sharp boundary between the fat layer and the acid solution below. Centrifugal force completes the separation, driving the lighter fat upward into the calibrated neck of the butyrometer where it can be measured directly.

Step-by-step procedure for curd 🔗

The procedure for curd testing follows the same validated sequence used for milk fat analysis, adapted for the semi-solid nature of curd. First, 10 ml of concentrated sulfuric acid (specific gravity approximately 1.820-1.825) is measured carefully into the Gerber butyrometer, keeping the tube vertical to prevent premature mixing. Next, 11 ml of the prepared curd sample is added gently along the inner wall of the butyrometer to form a distinct layer over the acid. This is followed by 1 ml of amyl alcohol. This specific sequence – acid first, then sample, then amyl alcohol – ensures the three components form separate layers before mixing begins.

The butyrometer is then sealed with a rubber stopper and shaken vigorously, wrapped in a cloth to manage the heat generated by the exothermic acid reaction. The tube is placed in a water bath at 65°C briefly to stabilize temperature, then transferred to a centrifuge operating at approximately 1,100 rpm for around 4-5 minutes. After centrifugation, the butyrometer is returned to the water bath to keep the fat fluid and readable. The fat percentage is read directly from the graduated neck scale – from the bottom of the fat column to the lower edge of the meniscus.

What the fat result tells you 🔗

A fat content of around 3-4% for cow milk curd is considered within the normal range, indicating properly standardized milk and efficient fat distribution during processing. A clear, well-defined yellow fat column after centrifugation also confirms that protein digestion was complete and separation was efficient. Values falling outside this range may point to issues with raw milk composition, cream standardization errors, or adulteration.

Results should be read quickly before the butyrometer cools, since solidifying fat distorts the meniscus and introduces reading error. Running tests in duplicate is standard practice to ensure reliability. Fat testing is also commonly performed on composite samples to reduce the time and cost of individual batch testing in high-throughput dairy operations.

Measuring acidity by titration 🔗

While fat content speaks to composition, acidity reveals the fermentation status and freshness of curd. During curd production, starter cultures convert lactose into lactic acid. This is the process that gives curd its characteristic tang – but the amount of lactic acid present at the time of testing directly reflects how far fermentation has progressed and whether spoilage has begun.

Why lactic acid is the key indicator 🔗

High levels of lactic acid signal increased microbial activity, which can pose food safety risks and negatively affect product taste, texture, and shelf life. Unlike some microbial byproducts that are destroyed by heat treatment, lactic acid persists through processing, making it a reliable indicator of a product’s fermentation history. Monitoring acidity is therefore one of the most direct ways to assess the quality and freshness of dairy products.

Indian food safety regulations (FSSAI) specify that the acidity of full-cream dahi (curd), expressed as lactic acid, should fall between 0.5% and 1.0%, with a minimum lactic acid bacteria culture count of 1×10⁷ cfu/g. Values above this range indicate over-fermentation or spoilage, while values significantly below it may point to incomplete fermentation or a failing starter culture.

Titration procedure for curd acidity 🔗

The standard method uses acid-base titration with sodium hydroxide (NaOH) as the titrant and phenolphthalein as the colour indicator. The titratable acidity test measures the volume of alkali required to shift the pH of the sample from its initial value to approximately 8.3 – the colour change point of phenolphthalein.

The procedure involves the following steps:

• Sample preparation: A measured quantity of curd (typically 10 g) is dissolved in a small volume of distilled water to create a homogeneous solution.
• Indicator addition: A few drops of phenolphthalein solution are added to the sample in a white-background flask for clear colour observation.
• Titration: The burette is filled with 0.1 N NaOH solution, which is added drop by drop while stirring continuously. The endpoint is reached when a faint pink colour persists for 10-15 seconds without further addition of alkali.
• Calculation: The volume of NaOH used, multiplied by 0.009 and divided by the sample weight, gives the percentage of lactic acid.

Interpreting acidity results 🔗

Titratable acidity ranging from 0.72-0.81% lactic acid, with pH values between 4.5 and 4.6, indicates normal lactic fermentation by starter cultures such as Lactobacillus and Streptococcus thermophilus. These values confirm the curd is neither under-fermented nor over-acidified and has good shelf stability. When acidity climbs beyond the acceptable range, the curd will typically develop an excessively sour taste, grainy texture, and shortened shelf life – all signs of quality deterioration that would disqualify it from distribution.

Precautions for accurate results 🔗

Both tests demand strict attention to technique. For the Gerber method, the sulfuric acid used must have the correct specific gravity – acid that is too concentrated attacks the amyl alcohol at elevated temperatures, producing compounds that distort fat readings. The centrifuge must be properly balanced, and the butyrometer must be free of any residue from previous tests that could interfere with fat separation. Any undissolved curdy white material remaining before centrifugation indicates incomplete protein digestion and unreliable results.

For acidity titration, expired reagents and contaminated glassware are the most common sources of error. The NaOH solution must be standardized regularly, as it absorbs carbon dioxide from air and loses strength over time. Endpoint detection requires consistent lighting and careful observation – over-titrating past the faint pink endpoint artificially inflates the acidity reading. Running samples in duplicate and cross-checking against control standards are good practices in any quality control laboratory.

Temperature control during sample handling also matters for both tests. Lactic acid produced from lactose fermentation remains unchanged by high-heat treatments, but acidity can continue to develop in samples left at room temperature before testing. Curd samples should be tested promptly after collection and kept refrigerated if there is any delay.

How these tests support broader quality assurance 🔗

Fat content and acidity testing do not operate in isolation. They are part of a broader analytical framework that includes moisture measurement, solids-not-fat (SNF) analysis, microbiological testing, and sensory evaluation. Together, these parameters paint a complete picture of whether a batch of curd is safe, nutritious, and consistent enough for sale.

Curd samples that meet the expected fat, acidity, moisture, SNF, and total solids benchmarks are confirmed to be nutritionally adequate and safe for consumption. From a business perspective, consistent test results also reduce product recalls, build consumer trust, and support regulatory compliance – all critical factors in a competitive dairy market. Companies that prioritize milk sourcing and quality control measures are better positioned to produce curd with superior taste and texture, driving long-term market performance.

What do you think? If a curd sample passes the fat content test but shows acidity significantly above the acceptable range, should it still be released for sale – and what factors would you weigh in that decision? How might automated acidity monitoring during fermentation change the way dairy producers manage curd quality in real time?

References

1. https://essfeed.com/how-milk-sourcing-and-quality-control-affect-curd-texture-and-taste-how-milk-sourcing-and-quality-control-affect-curd-texture-and-taste/
2. https://rjwave.org/ijedr/papers/IJEDR2601558.pdf
3. https://en.wikipedia.org/wiki/Gerber_method
4. https://law.resource.org/pub/eac/ibr/eas.164.2006.pdf
5. https://funke-gerber.de/en/n-gerber-method/method
6. https://ortoalresa.com/en/determination-of-fat-content-in-milk-and-milk-products-for-quality-control/
7. https://www.camlab.co.uk/blog/total-fat-analysis-in-milk-using-the-gerber-method
8. https://agrimoon.com/determination-of-fat-in-milk/
9. https://www.cdrfoodlab.com/cdrfoodlab/learn-more/lactic-acid-milk-test
10. https://labtekservices.co.uk/milk-acidity-test-for-quality-and-freshness/
11. https://chadhasales.com/dairy-product-safety-standards/
12. https://www.dairyknowledge.in/dkp/content/titrable-acidity-milk
13. https://adpi.org/methodsofanalysis/analytical-method-007/