Available at: http://digitalcommons.calpoly.edu/theses/798
Date of Award
MS in Engineering
Biomedical and General Engineering
Dr. Daniel Walsh
Characterization Of Raw Milk Fouling On Plate-Type Heat Exchangers Using Different Alloys And Cow PhenotypeS
Stephen Ernest Nelson
Milk and other dairy products are widely used in many households today. Milk is a popular beverage that is seen as a healthy alternative to other synthetic beverages such as soda pop and other sugar based drinks. It became law that milk, and milk products are pasteurized before its release to the general public (FDA 2003). Pasteurization is a thermal process where the intent is to lower the concentration of microorganisms in the milk to render it safe to drink with heat (Bansal and Chen 2006). With all the thermal processing of the raw milk, this leads to thermal efficiency degradation of the heat exchangers used to pasteurize the milk (Bansal and Chen 2006) due to direct fouling of the heating surface.
The buildup of organic and inorganic matter onto a metal surface from the constant heating of the milk on a stainless steel surface is called fouling. The exact mode on which the fouling layers nucleate and grow is unknown by the date of this writing.
Milk fouling has been around as long as the pasteurization process. (Visser and Jeurnink 1997) Fouling rate is related to a function of variables. Fouling rate is a function of milk type, time, and temperature, age of the milk, seasonal variations, process equipment design and more. The main consensus of milk fouling initiation is that of the whey protein b-Lactoglobulin which constitutes about 0.32% in whole milk (de Jong 1997; Bansal and Chen 2005a; Bansal and Chen 2006). Table III shows the general compositions of the constituents in milk.
In order to look for dependence between milk phenotypes and heated surface alloys, a design of experiment (DOE) was made. The experiment used three types of milk phenotypes to test for fouling differences. Also, four alloy compositions were also tested against the milk phenotypes. This produced a three by four matrix of variable combinations or 3x4 factorial design. It was hoped that these combinations will show a certain, but repeatable process condition which will produce lower fouling rate versus the control milk type. The milk phenotypes used in this experiment are phenotype AB-AB (control), AB-AA, and AB-BB. The phenotype of label before the hyphen was the k-casein phenotype, the label after the hyphen represented the b-Lacto globulin phenotype. The four metal types tested were stainless steel 304 (control), stainless steel 316, stainless steel 430, and titanium 6V 4Al. It was not feasible to change out the plates in the pilot scale milk pasteurizer at the pilot plant at the Dairy Products Technology Center (DPTC), or to make special replacement plates that exposed each metal to be tested on a single heat exchanger plate (AOAC-c 1995).
The manufacturing of a complete laboratory scale milk pasteurizer for the study of milk fouling on metal plates proved to be very successful. The model flow cell heat exchanger produced high enough quality of milk foulant on the test coupons in comparison to the large scale fouling layers found in full scale dairy heat processing equipment. Although generally speaking, there was not a significant technology breakthrough of using different alloys as the material for the plates in milk pasteurizer heat exchangers, a method of creating the milk fouling layer on a smaller scale can be very useful in future works studying milk fouling. The titanium alloy showed a significantly lower fouling rate, this was probably mostly due to the highly passivated surface of the Titanium. It was also seen that the actual breed of cow could have played a significant role in fouling. The new FCHE model was produced to show the viability of creating a biofilm or milk fouling layer on any material provided that it is rigid enough. Microorganisms were also briefly studied on the foulant layer that was produced with the flow cell. This new approach should provide a basis for new and more advanced research of the mechanisms and nature of milk fouling in heat processing equipment.