- Positive correlation between viscosity of concentrate and fat content
- Installation at multiple points in the process
- Instant adjustments to maintain product quality and maximise yield
Milk powder is produced by the process of “spray-drying”, which is widely used in the food and chemicals industries for making products as diverse as baby foods, coffee, soup mixes and dyestuffs.
A process diagram depicting a typical spray-drying process:
The liquid product concentrate is pumped to the atomiser, which reduces the liquid to a spray of droplets of a controlled size, generally from 10 to 500 micrometres depending on the type of product. The droplets are sprayed into a stream of hot gas, which is usually dried air, unless the liquid is oxygen-sensitive or a flammable solvent such as ethanol, when nitrogen is used. The tiny droplets lose moisture very rapidly in the drying air, after which the dry powder is separated from the moist air in cyclones by centrifugal action. Pre-concentrating the liquid improves the efficiency of the relatively expensive drying stage and thereby increases productivity.
The physical properties of the products are intimately associated with the powder structure generated during spray drying. It is possible to control many of the factors that influence powder structure in order to obtain the desired properties, but to do so effectively demands continuous on-line monitoring of the feed liquid in order to keep the product within specification. Manual sampling and measurement introduces delays that can cause lengthy start-up and shutdowns and uncertainties that lead to poor batch-to-batch comparability.
The efficiency of drying is largely determined by the degree of atomisation, which is directly proportional to the viscosity of the feed solution. Thus, the viscosity of the milk concentrate at the atomisation step has a crucial effect on the characteristics of the resulting spray-dried product.
Measuring viscosity inline is therefore ideal for monitoring the spray-drying process. Because viscosity is measured continuously in real time with high sensitivity and excellent repeatability, changes are immediately detected.
There are three stages in the powdered milk process where XL7 viscometers are typically installed:
- Pre-evaporator
- Post-evaporator
- Atomisation
Stage (i) suits a standard XL7 with Tri Clamp fitting. This is elbow mounted in the evaporator pipeline. Stage (ii) and (iii) require high pressure variants.
For one installation, a custom Ø10mm sensor was supplied to suit an Ø18mm pipeline, to provide critical insight into the concentrate prior to atomisation. The viscosity of the concentrate can also provide critical insight in determining fat content, finished powder/particle size and thus lifetime of the milk powder.
Importantly, measurements are unaffected by changes in the flow rate or by the amount of suspended solids in the concentrate, which could be as high as 40%. The single solid-rod sensor has a hygienic clean-in-place design, presenting minimal possibilities of fouling and eliminating “dead” spaces where microorganisms could propagate.
Before acquiring the XL7 viscometer, one company had tried to use conductivity as a measure of milk viscosity, but found that the technique was unreliable and inaccurate. Now, with the XL7 providing continuous, real-time monitoring of milk viscosity, production staff can make instant adjustments to maintain product quality.
“We use the online viscometer to measure product interfaces and percentage solids. The live measurement means we can minimise yield losses.” Robin Sandman, Engineering Manager, H.J. Heinz.