The influence of ultra-pasteurization by indirect heating versus direct steam injection on skim and 2% fat milks.

Discipline: sensory; Key words: fluid milk, ultra-pasteurization, direct steam injection, flavour, consumer acceptance.

Milk for drinking is normally pasteurized by employing high temperature (at least 72^oC) for a short period (about 15 sec). For extended shelf life, milk is ultra-pasteurized (UP) by heating to at least 138⁰C for 2 sec. The UP process can be done by indirect heating or by direct steam injection. The influence of these two UP methods on milk flavour has not been widely studied. As a result, the objective of the investigation by Dr A.P. Lee and colleagues was to compare the effect of ultra-pasteurization by means of indirect heating with direct steam injection and with normally pasteurized milk as control in terms of sensory perception by consumers. Their study was published in the Journal of Dairy Science, Volume 100 of 2017, pages 1688 to 1701. The title of the paper was: The influence of ultra-pasteurization by indirect heating versus direct steam injection on skim and 2% fat milks.

Raw skim milk and standardized 2% milk were pasteurized at 140⁰C for 2.3 sec by indirect heating (IND) or by direct steam injection (DSI) or by high temperature, short time (HTST) (78⁰C, 15 sec) and homogenized at 20.7 MPa. The processed milks were stored in light-shielded opaque high-density polyethylene containers at 4⁰C and examined by microbial and descriptive analysis on days 3, 7 and 14. Furosine and serum protein denaturation analyses were performed on day 0 and 14 as an indicator of heat treatment. Consumer acceptance testing was done on day 10 with both adults and children (8 to 13 years of age). The test groups consisted of individuals who at home either drink skim or 2% milk and for testing they were accordingly only provided with skim or 2% milk. The experiment was repeated in triplicate.

The results showed that milks treated by HTST had lower cooked flavour than either UP milks. Milks heated by DSI-UP were characterized by sulphur or eggy and cooked flavours, whereas IND-UP milk had higher sweet aromatic and sweet taste compared with DSI-UP milk. Aromatic flavour intensities of all milks decreased across the 14 days of storage. Furosine concentrations and serum protein denaturation were highest for the IND treatment, followed by the DSI and HTST treatments. Furosine content in both skim and 2% milk increased with time (with implications to stability in extended storage for increased shelf life), but the increase was faster in IND-UP skim milk. Adult and child consumers preferred HTST milk over either UP milk, regardless of fat content. Ultra-pasteurization by IND or DSI did not affect consumer acceptance at 10 days post-processing, but traditional HTST milk were preferred by consumers of all ages.