Key Analytical Constants for Evaluating Ghee Quality

Ghee is one of the most important dairy products in India, both culturally and economically. But how do we know whether the ghee we buy or produce is pure and free from adulteration? The answer lies in a set of measurable values known as analytical constants. These constants act as a fingerprint for ghee – each one reveals specific information about its fatty acid composition, origin, and purity. Understanding these constants is essential for anyone involved in dairy science, food quality control, or the dairy trade.

What are analytical constants of ghee? 🔗

Analytical constants are standardized physico-chemical measurements that describe the composition and characteristics of a fat or oil. For ghee, these constants are determined under controlled laboratory conditions and compared against accepted standard ranges. When any of these values fall outside the expected range, it signals a possible quality issue – most often, adulteration with cheaper fats or oils. The major analytical constants used for evaluating ghee include the Butyro-refractometer (BR) reading, Reichert-Meissl (RM) value, Polenske value, iodine value, saponification value, and the melting and solidifying points.

Butyro-refractometer (BR) reading 🔗

The Butyro-refractometer reading measures the refractive index of ghee – essentially, how light bends as it passes through a melted ghee sample at 40°C. This reading is directly related to the types and proportions of fatty acids present in ghee.

For pure ghee, the BR reading typically falls in the range of 40.0 to 44.0. According to research conducted at the National Dairy Research Institute (NDRI), Karnal, cow ghee showed a BR reading range of 41.87-43.62, while buffalo ghee ranged from 40.01-43.23. This overlap makes it difficult to distinguish cow and buffalo ghee based solely on BR readings, but deviations beyond the standard range strongly indicate the presence of foreign fats.

When ghee is adulterated with vegetable oils or animal body fats, the refractive index changes because these adulterants have a different fatty acid profile compared to pure milk fat. This makes the BR reading one of the fastest and most commonly used tests for preliminary screening during quality evaluation by FSSAI-approved laboratories.

Why BR reading matters for Agmark grading 🔗

Under the Agmark grading system, ghee samples of all grades – special, general, and standard – must have a BR reading between 40.0 and 43.0 at 40°C. Any sample that falls outside this window will not qualify for the Agmark certification mark, which is essential for commercial sale of graded ghee in India.

Reichert-Meissl (RM) value 🔗

The Reichert-Meissl value is arguably the most important analytical constant for ghee quality. It measures the amount of volatile, water-soluble fatty acids present in ghee – primarily butyric acid and caproic acid. These short-chain fatty acids are unique to milk fat and are present in much higher quantities compared to other animal or vegetable fats.

Pure ghee should have an RM value of not less than 24, as per the latest FSSAI notification from December 2021, which set a uniform minimum RM value across all regions of India (earlier, cotton tract areas had a separate lower limit of 20). Butyric acid contributes about 75% of the RM value, while caproic acid accounts for the remaining 25%.

When ghee is adulterated with non-dairy fats like vegetable oils or animal body fats, the RM value drops significantly. For example, a study on ghee adulteration in Bangladesh found that when soybean oil and coconut oil were mixed into ghee, the RM value dropped to as low as 1.57-4.14 – far below the accepted minimum.

Differences between cow and buffalo ghee 🔗

Research at NDRI showed that the RM value of cow ghee ranged from 27.5 to 31.13, while buffalo ghee showed higher values in the range of 31.91 to 39.99. Buffalo ghee tends to have a slightly higher proportion of butyric acid, which contributes to the higher RM reading. However, both types comfortably exceed the FSSAI minimum of 24 when pure.

Polenske value 🔗

While the RM value captures volatile, water-soluble fatty acids, the Polenske value measures the volatile, water-insoluble fatty acids in ghee. These include fatty acids like caprylic acid, capric acid, and lauric acid, which dissolve in the alcohol layer rather than the water layer during the distillation process.

For pure ghee, the Polenske value generally ranges between 1.0 and 2.0. The NDRI study reported cow ghee Polenske values of 1.30-1.90 and buffalo ghee values of 1.10-1.50. Interestingly, cow ghee tends to have a slightly higher Polenske value than buffalo ghee.

The Polenske value is especially useful for detecting adulteration with coconut oil, which has a naturally high proportion of medium-chain fatty acids. Coconut oil adulteration causes the Polenske value to rise above the normal range, making it a tell-tale indicator for this specific type of fraud.

Iodine value 🔗

The iodine value measures the degree of unsaturation in the fatty acids of ghee – in other words, how many double bonds are present in the fat molecules. A higher iodine value indicates a greater proportion of unsaturated fatty acids.

According to research published in the Indian Journal of Dairy Science, the iodine value for pure cow ghee ranges from approximately 35.53 to 41.24, with an average of 38.69. Buffalo ghee, being more saturated, shows a lower range of 30.14 to 36.48, with an average of 34.10. Both types show seasonal variation – the lowest iodine values occur in winter (January) and the highest in summer (July), reflecting changes in the animals’ feed and metabolism.

The iodine value is a standard range of about 26 to 38 for mixed ghee under normal conditions. Adulteration with highly unsaturated oils like soybean oil or groundnut oil pushes the iodine value well above this range. Palm olein, for instance, has an iodine value nearly 1.7 times that of pure ghee, making it detectable at higher adulteration levels.

Saponification value 🔗

The saponification value indicates the average molecular weight of the fatty acids present in ghee. It is defined as the milligrams of potassium hydroxide (KOH) required to saponify one gram of fat. For ghee, this value normally ranges between 220 and 230.

Ghee has a relatively high saponification value compared to most other fats because it contains a significant proportion of short-chain and medium-chain fatty acids, which have lower molecular weights. When ghee is mixed with fats that have predominantly long-chain fatty acids (such as vegetable oils or animal body fats), the saponification value tends to decrease. This makes it another useful parameter for adulteration screening.

Melting and solidifying points 🔗

Unlike pure chemical compounds, ghee is a complex mixture of many different triglycerides. This means ghee does not have a sharp, single melting point or solidifying point. Instead, it transitions gradually over a range of temperatures.

Melting point 🔗

The melting point of ghee – the temperature at which it transitions from solid to liquid – varies depending on the source animal and season. Cow ghee generally melts at a lower temperature range of 28°C to 41°C, while buffalo ghee has a higher melting range of 32°C to 43.5°C. Research has confirmed that buffalo ghee shows a significantly higher average melting point (around 33.6°C) compared to cow ghee (around 32.2°C). The higher melting point of buffalo ghee is due to its greater proportion of long-chain saturated fatty acids, which form stronger crystal structures.

Solidifying point 🔗

The solidifying point is the temperature at which ghee transitions from liquid back to solid. For buffalo ghee, this ranges from 16°C to 28°C, while cow ghee solidifies between 15°C and 23.5°C. The difference between the melting and solidifying points is expected for a complex fat mixture. Any significant deviation from these ranges suggests possible adulteration.

Differences between cow ghee and buffalo ghee 🔗

The analytical constants differ between cow and buffalo ghee primarily because of differences in their fatty acid composition. Here is a comparative overview of key values:

BR reading at 40°C: Cow ghee typically reads 41.87-43.62, while buffalo ghee ranges from 40.01-43.23. The values overlap considerably, making this parameter less useful for distinguishing the two on its own.

RM value: Buffalo ghee tends to be higher (31.91-39.99) compared to cow ghee (27.5-31.13), reflecting a slightly higher butyric acid content in buffalo milk fat.

Polenske value: Cow ghee (1.30-1.90) is slightly higher than buffalo ghee (1.10-1.50).

Iodine value: Cow ghee has a higher average iodine value (~38.69) than buffalo ghee (~34.10), because cow milk fat contains more unsaturated fatty acids.

Melting point: Buffalo ghee melts at a higher temperature (32-43.5°C) than cow ghee (28-41°C), giving buffalo ghee its characteristically firmer and more granular texture.

Research at NDRI also identified the Kirschner value – which measures volatile, water-soluble fatty acids that can be extracted with silver salts – as a promising parameter. Buffalo ghee showed significantly higher Kirschner values (26.84-33.96) compared to cow ghee (20.74-24.14), making it a potential tool for differentiation.

Role of analytical constants in detecting adulteration 🔗

Adulteration of ghee with cheaper fats like vegetable oils, vanaspati (hydrogenated fat), animal body fats, or mineral oil is a persistent problem in the dairy industry. Analytical constants serve as the first line of defence in detecting such fraud.

For instance, mixing vegetable oil into ghee raises the BR reading and iodine value while lowering the RM value. Adding coconut oil increases the Polenske value abnormally. The presence of animal body fats like tallow or lard can alter the melting and solidifying points. Each adulterant creates a distinct pattern of changes across the constants, helping analysts identify not just whether adulteration has occurred but also what substance was used.

However, it is worth noting that these traditional methods have limitations at lower adulteration levels – often failing to detect adulterants present at less than 10-15%. This is why modern techniques like gas chromatography for triglyceride profiling and Raman spectroscopy are increasingly being used alongside traditional physico-chemical analysis.

Regulatory standards for ghee in India 🔗

In India, ghee quality is governed by two major regulatory frameworks. The Food Safety and Standards Authority of India (FSSAI) mandates that ghee must contain at least 99.5% milk fat with a maximum of 0.5% moisture. The FSSAI regulations also require that the Baudouin test (which detects vanaspati adulteration) be negative, and that the BR reading, RM value, and Polenske value fall within prescribed limits.

The Agmark grading system, managed by the Directorate of Marketing and Inspection, classifies ghee into three grades – Special, General, and Standard. All grades require a BR reading of 40.0-43.0, an RM value of not less than 28, and a Polenske value of 1.0-2.0. Free fatty acid content (as oleic acid) must not exceed 3.0% for standard grade ghee.

Factors that cause natural variation in analytical constants 🔗

It is important to understand that the analytical constants of ghee are not fixed numbers – they vary naturally based on several factors. The species of animal (cow vs. buffalo) is a primary factor, as discussed above. Beyond that, the feed and fodder consumed by the animal plays a significant role. Cattle fed cotton seed, for example, produce ghee with lower RM values – which is why separate standards existed for cotton tract areas until 2021.

Seasonal changes also affect ghee composition. Green fodder available during the rainy season increases the proportion of unsaturated fatty acids, raising the iodine value and lowering the melting point. In winter, when animals consume more dry fodder, the reverse trend is observed. The method of preparation – whether desi (traditional fermented cream method), creamery butter method, or direct cream method – also influences the final analytical profile of ghee.

What do you think? Given that analytical constants vary naturally with season, feed, and animal breed, how should regulatory authorities set standards that are strict enough to detect adulteration yet flexible enough to accommodate natural variation? Do you think modern spectroscopic methods will eventually replace traditional physico-chemical testing for ghee quality evaluation?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC7774801/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC9304484/
3. https://www.fssai.gov.in/upload/uploadfiles/files/MILK_AND_MILK_PRODUCTS.pdf
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC9579228/
5. https://www.researchgate.net/publication/311494429_Iodine_value_integrated_with_solvent_fractionation_technique_as_a_tool_for_detecting_palm_olein_and_sheep_body_fat_adulteration_in_ghee_clarified_milk_fat
6. https://www.sciencedirect.com/science/article/abs/pii/S0956713525003500
7. https://www.sciencedirect.com/science/article/pii/092144889390040O/pdf
8. https://www.academia.edu/69564858/Physico_Chemical_Properties_Analysis_Based_Approaches_to_Ascertain_the_Purity_of_Ghee_A_Mini_Review
9. https://foodsafetyhelpline.com/butter-ghee-milk-fats/