Application

Food

 Introduction

Dairy products is a growing food segment especially for children and health conscious consumers. It is therefore critical to ensure the quality of the textural properties for consumer acceptance. Yoghurt and cheese is the result of a change in milk pH, which induces flocculation of the protein micelles. The flocculated micelles form the gel network. The pH reduction can be induced by addition of bacterial cultures, glucono-δ-lactone (GDL) or lemon juice to milk.

Rheological measurements of dairy products are complex as the gel network is weak and can easily be disturbed by traditional methods like mechanical rheology. Rheolaser Master performs zero shear measurements , since no external stress is applied by the instrument. In this work we follow the yoghurt formation with GDL as a function of time and pH . Elastic properties of joghurts with different protein and milk fat content will also be compared.

 Objective

Analyse the yoghurt formation as a function of time and pH as well as elastic properties of yogurts made from diffrent sources

Device

Rheolaser Master

Rheolaser Master measures the particle Mean Square Displacement ( MSD) , which gives an indication of the viscoelastic properties during gelation. Figure 1 .1 shows the MSD curves of milk gelated with 1 wt% GDL at 40°C.
The MSD curves in the top left are linear. which indicates a liquid behaviour before gelation. At longer times,. micelles flocculate and a network is formed , leading to the typical viscoelastic curves( plateau formation). The gel point is determined with the new Time-Cure-Superposition (TCS) method . TCS  consists in rescaling the MSD curves with factors a and b , which gives a characteristic  v-shaped curve indicating the gel point as shown in Fig 1.2 (see the application note “Gel point determination by TCS” for more details).

This yogurt sample with UHT milk and 1 % GDL gelled at 42 ‘25”. The pH was measured in parallel and the gelation took place at a pH of 5.11, as expected.

 Fig 1: 1 MSD curve evolution as a function of time during yogurt formation.   Fig 1.2 Scaling factors a and b as a function of time.

Influence of protein content
The Rheolaser Master measures the viscoelastic properties such as elasticity index (EI) . The higher the EI , the higher the elasticity/ stiffnes of the dairy product, since the protein (casein in the milk in this case) forms the gel network . The protein content has a direct influence in the elasticity.

Fig 2 below shows the evolution of the EI during gelation for different GDL/ protein ratios.

With an increasing GDL/Protein ratio , more proteins are acidified and able to form the a network. As a consequence ,the EI increases and gelation took place at shorter times. (Gel points were opbtained with the TCS method).

Rheolaser Master offers:
- 1-click experiment & gel point  analysis;
- Measurement at rest ;
- Non intrusive measurement ;
- Fast : 6 measurement positions

Influence of variation in milk quality

One challenge in processing milk into yoghurt and cheese is the variance in the natural character of milk supplied due to changes in feed quality and normal seasonal changes.
As a result the protein or fat content varies slightly from batch to batch. Table 2 shows a comparison of the gel formation of two UHT treated milk batches (UHT A and UHT B) from different suppliers as well as two batches of fresh milk (Fresh A and Fresh B) . Slight differences in EI can be observed between UHT A and B, despite the same fat and protein content in both samples. These differences are due to slight variations on milk quality (protein composition). The influence of the UHT process can also be observed. UHT milk is heated very quickly to very high temperatures while fresh milk is pasteurised to between 60 and 90 °C for a longer time , which leads to different protein conformation’s. Gel formation with fresh milk therefore requires mre time and yoghurts with lower elasticity are obtained.

Learn More                           Requesta quote