Fouling Dependency of Air in Dairy Processing

Abstract

In this master thesis project, the formation of fouling on heat exchanger surfaces in dairy processing and mainly the influence of air was examined. Based on prior experimental findings regarding the influence of air bubbles in combination with calculations based on solubility properties of air in milk, a hypothesis was stated. Milk can enter the processing system without any undissolved air, with altered equilibrium due to change in temperature and pressure the solubility can decrease. From this phenomenon, air bubbles can be created which are simultaneously filled with steam at the corresponding steam pressure. Assuming the steam is evaporated from the boundary layer of the bubble a local increase in TS would be present, creating a fouling footprint. The influence of dissolved air and processing pressure on the fouling rate was examined in the thesis and the reason behind the creation of fouling is assumed to be drying.

A lab scale tubular heat exchanger UHT was used in the experiments and a part of the thesis is focused on the instrumentation of the machine. The results points in two directions. While processing milk entering with a high level of dissolved air, an increased process pressure led to a decreased initial fouling rate and prolonged induction period. The results from processing milk with low level of dissolved oxygen were inconclusive. Either a point of low enough oxygen level and high enough pressure was reached for nil creation of bubbles, or the milk properties were changed from the pre-processing which intended to lower the oxygen level.

Popular Abstract

How come some milk can be stored outside of the refrigerator for several months without going bad? How is this milk processed and does the air content affect the processing? If these questions have stumbled upon your mind this is the article to point your eyes towards.

Milk has not always been an as obvious choice for a nutritional beverage as it is considered today. Since it is almost an optimal environment for the growth of microorganisms, without proper processing several dangerous organisms can be found in milk. Raw milk has been known to contain pathogens like Salmonella, E. coli, Listeria and Campylobacter and historically, raw milk was a common source of spreading diseases like tuberculosis, typhus and scarlet fever.

To kill the harmful substances milk is heated to a certain temperature for a certain time, called pasteurization. It was from the work performed by Louis Pasteur in the 19th century regarding the lethal effect heat has on microbes the process today commemorates his name. The kind of processing this thesis deals with is called Ultra High Temperature (pasteurization) which is performed in the temperature range of 135-140 °C for a couple of seconds. UHT milk is defined as commercially sterile meaning that the product is free from microbes that can grow in the conditions it is meant to be stored. The product is packed in sterile packages in a sterile environment and can be stored in ambient temperature (25-40 °C depending on location) up to a couple of months. Of course, once the product is opened it spoils in a matter of days just as regular milk.

When heating the milk, a build-up is created on the hot pipes, called fouling, similar to the burnt milk seen in your pan when you forget to stir your béchamel sauce continuously. Fouling consists of fat, proteins and minerals, all originating from the milk. Fouling is money down the drain in the dairy industry since the process must be shut down regularly for cleaning, you lose product to the fouling and the processed milk quality can be worse when fouling is created. There is much to be gained if the phenomenon could be limited only slightly.

Many factors affect the formation of fouling such as temperature difference between the milk and the heating side, the milk quality and composition and the pressure in the heat exchanger. Air content has long been a recognized factor leading to an increased fouling rate, but no one knows exactly why. This thesis examined the influence of air in milk. When the milk is heated in the process it cannot carry as much oxygen and thus are bubbles created in the process. These bubbles are then filled with steam which dries the product on the hot pipes and the fouling formation starts.

From the experimental work performed on a small scale UHT in this thesis it was shown that the fouling rate was slower if the pressure in the process was increased. It was also shown that the period before the fouling starts to form was longer with an increased process pressure. If more experiments are performed in bigger scale maybe the fouling can be controlled in a wider sense which would save money for the dairy industry.

The report also includes the experimental setup created to be able to examine the formation of fouling including advanced monitoring devices such as differential pressure transmitters and oxygen sensors.

• author: Lorin, Erik
• supervisor: Anton Sellberg; Tomas Skoglund
• organization: Chemical Engineering (M.Sc.Eng.)
• course: KETM05 20181
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Agriculture and Food Sciences; Chemistry; Technology and Engineering
• keywords: Fouling, Dairy, UHT, Bubble, Bubble induced fouling, Drying, THE, Dairy Processing, Processing, Bubble Nucleation, Differential Pressure, Oxygen sensors, chemical engineering, kemiteknik
• language: English
• id: 8942975
• date added to LUP: 2018-06-04 16:05:26
• date last changed: 2018-06-04 16:05:26