Use of Optical Microscopy for the Investigation of Dried Milk Powders under Varying Humidity Conditions

Introduction

The effects of humidity are evident in the stability and shelf-life of a variety of materials, formulations, and processing behaviour, especially food materials. This article discusses the advantages of a direct comparison method of three similar dried milk samples under varying humidity conditions, using optical microscopy in conjunction with the GenRH-A accessory and Mcell from Surface Measurement Systems Ltd.

Experimental Method

The GenRH-A humidity generator comprises a humidity and temperature probe, temperature monitoring display, a rotameter for control of flow volume, and a programmable digital controller with full time closed-loop control. Figure 1 shows the GenRH-A humidity generator.

Figure 1. Surface Measurement Systems’ GenRH-A humidity generator

The Mcell allows efficient use of the GenRH-A humidity generator with a microscope. The sample is placed into the Mcell, which is integrated into the microscope stage as illustrated in Figure 2.

This stage features double-glazed top and bottom windows to allow reflection and transmission illumination of the sample with reduced heat loss under non-ambient conditions.

Figure 2. SMS Mcell microscope accessory for the GenRH line of products

Experimental Results

Demonstration of Humidity Generation

The microscope slide was fed with a narrow line of Coffeemate virtually fat free milk powder. The sample was placed at a right angle to the direction of gas flow and subjected to varying humidity levels of 50% to 95% RH. The images of the sample captured at various humidity conditions are presented in Figure 3, showing the start of deliquescence around 81% RH.

Figure 3. Photographs of the Coffeemate powder at 64%, 81% and 85% RH, taken at 25°C and 40X magnification

As can be seen in Figure 3, the onset of deliquescence was not affected by the sample position, and was at the same time for the particles from the lower, middle, and top regions. The onset of deliquescence was influenced by the particle size as expected, with the dissolution of smaller particles before larger particles.

Simultaneous Measurement of Three Milk Powder Samples

The similar dried samples of three different milk powders, namely Coffeemate Original, Coffeemate Virtually Fat Free, and freeze-dried, skimmed milk, were fed into the microscope slide in a fashion at right angle to the direction of gas flow. They were then subjected to varying humidity levels of 30% to 95% RH, and then at 95% RH for 80min.

The images of the three samples at various humidity conditions are depicted in Figure 4. The deliquescence was observed in the Coffeemate Virtually Fat Free sample, followed by the freeze-dried skimmed milk sample and Coffeemate Original.

This means that the stability of Coffeemate Original is much higher than other two samples. Swelling was observed instead of deliquescing, allowing the particles to move only slightly.

Figure 4. Coffeemate Original, Coffemate Fat Free and spray dried skimmed milk powder at 30% RH (a), 92% RH (b), and 95% RH (c)

Water sorption rate of smaller particles is higher, thus dissolving well before the larger particles. As a consequence, it is difficult to accurately define the precise deliquescence point for each type of milk powder using this optical method.

The key advantage of this optical method is the ability to provide direct visual comparison of various samples, thus enabling the studying of batch-to-batch deviations in detail and to gain insights into the impact of individual factors such as particle size. This effective comparison is not possible with gravimetric methods as they provide average data.

Conclusion

The optical method discussed in this article allows simultaneous comparison of multiple samples under varying humidity conditions. This capability enables rapid and accurate monitoring of slight changes between different samples that would otherwise be complex or time intensive to examine if observed individually.

About Surface Measurement Systems Ltd

Surface Measurement Systems Ltd develops and engineers innovative experimental techniques and instrumentation for physico-chemical characterisation of complex solids. We are the world leaders in Dynamic Vapor Sorption technology and Inverse Gas Chromatography instrumentation and solutions, providing professional world-class scientific and technical support for our international customers.


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Last updated: Oct 12, 2023 at 9:48 AM

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