The role of heat treatment in light oxidation of fluid milk

摘要

Light-oxidized flavor (LOF) resulting from photooxidationof riboflavin following light exposure is one ofthe most common off-flavors in fluid milk. The sensoryperception of LOF has been studied extensively in hightemperature, short time pasteurized (HTST) milk, butfew studies have evaluated ultrapasteurized (UP) milk.The objective of this study was to evaluate the role ofheat treatment in the development of LOF in UP fluidskim milk. Skim milk was processed by HTST or bydirect steam injection (DSI-UP) and subsequently exposedto 2,000-lx light-emitting diode light for varioustimes. Sensory properties were monitored by descriptiveanalysis and threshold tests, and volatile compoundswere evaluated by solid phase microextraction with gaschromatography-mass spectrometry. Dissolved oxygenand riboflavin were determined at each time point usingan oxygen meter and ultra-performance liquid chromatographywith a fluorescence detector, respectively.The entire experiment was performed in triplicate.Typical cardboard and mushroom flavors (LOF) weredetected by trained panelists in HTST milk after 3.5 hof light exposure. In contrast, LOF was not detectedby trained panelists in UP milk until 36 h of light exposure.Similarly, the best estimate threshold for LOFfrom untrained consumers (n = 101) was higher forDSI-UP milk (61.0 h) than for HTST milk (15.2 h).Milks with LOF were characterized by higher relativeabundance of the lipid oxidation compounds hexanaland heptanal. Dissolved oxygen (DO) and riboflavinconcentrations decreased with increased light exposuretime, and the decrease was slower in UP milk comparedwith HTST milk. Initial DO concentration was investigatedas a possible influence in LOF developmentbecause DSI-UP milks had lower initial DO concentrationsthan HTST milks. However, follow-up evaluationsof deaerated HTST milks suggested that DO was nota significant factor in LOF development. These resultsdemonstrate that UP milk is less sensitive to LOF thanHTST milk, possibly due to sensory masking effectsor antioxidant effects of volatile sulfur compounds. Anenhanced understanding of light and storage effects onmilks will assist with best practices when transportingand displaying fluid milk products for sale.