Browsing by Author "Uusihonko, Jenni"

In UHT-heating (Ultra High Temperature) product is heated above 130 degrees for a few seconds, which destroys all micro-organisms and prolongs shelf life of the product. UHT-heating is divided into direct and indirect heating. The aim of this study was to compare the effects of direct and indirect UHT-heating on the composition and quality of skimmed milk. The properties were also examined with a protein drink, but to a lesser extent. Differences between indirect and direct UHT-heating and off-flavours formed from proteins were examined by sensory evaluations and proteins in the samples were examined by SDS-polyacrylamide gel electrophoresis method (SDS-PAGE), which separates proteins according to their size. Sensory evaluations and electrophoresis analyzes were organized after production and during storage at regular intervals until best before date, storing samples at different storage temperatures. In sensory evaluations the differences between direct and indirect UHT-heating were relatively small from the production to best before date, but mainly direct UHT-heated milks received better or as good evaluation results as indirect UHT-heated milks. Milks stored in cold temperature received better evaluation results than those stored in warm temperature, and storage temperature had a greater effect on milk evaluation results than UHT-heating. Changes (reddish colour, caramel taste) caused by Maillard reaction were observed in both UHT-heated milks after one month of warm storage. In electrophoresis analyzes the protein bands of direct and indirect UHT-heated milks differed immediately after production and during storage. After production protein bands of milks heated by direct UHT-heating method corresponded more to the protein bands of raw milk. The intensity of protein bands decreased how longer and how warmer temperature the milk samples were stored. Whey proteins β-lactoglobulin and α-lactalbumin were visible in milks heated by direct UHT-heating method, but not indirect, while casein protein bands (αs1-casein and β-casein) were intensively visible in electrophoresis analyzes of both heating methods, as well as large protein aggregates (molecular size over 250 kDa) at top of the gel.