Question

Which of the following can be used as an index of heat treatment of milk?

• A. Lactose content
• B. Urea content
• C. Malic acid content
• D. Lactulose content

Hint:

Lactulose is formed purely through heat-induced isomerization of lactose. Because its concentration correlates directly with thermal severity, it serves as the classic chemical indicator of heat load in milk.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
When milk is subjected to thermal processing, several chemical, enzymatic, and physical changes occur.
The extent of these changes depends on the temperature and duration of the heating process.
To verify the severity of the heat treatment applied to milk (e.g., pasteurization, UHT treatment, or sterilization), specific chemical indicators are monitored.
An ideal heat indicator is a compound that is absent in raw milk but is formed in quantities directly proportional to the intensity of the heat treatment.

Step 2: Detailed Explanation:
Lactulose is a synthetic disaccharide composed of galactose and fructose.
It is completely absent in raw milk.
During the heating of milk, lactose undergoes an isomerization reaction known as the Lobry de Bruyn-Alberda van Ekenstein rearrangement.
In this reaction, the glucose moiety of lactose isomerizes into fructose, converting lactose to lactulose:
\[ \text{Lactose (Galactose-Glucose)} \xrightarrow{\text{Heat}} \text{Lactulose (Galactose-Fructose)} \]
The concentration of lactulose formed in milk increases logarithmically with temperature and heating time.
Consequently, measuring the lactulose content is a highly reliable and widely accepted method to differentiate between pasteurized, UHT, and in-bottle sterilized milks.
Other milk components like lactose, urea, and malic acid are either stable or present in varying natural concentrations, making them unsuitable as direct heat-treatment indices.

Step 3: Final Answer
Lactulose content is used as a chemical index to evaluate and monitor the severity of heat treatment applied to milk.