Factors Affecting Flavor Quality of Bovine Milk and Dairy Products

Abstract

Milk is an important food staple consumed around the world. Fluid milk is consumed as a standalone beverage, used in meal preparation, serves as the fundamental ingredient for fermented foods (yogurt, kefir, cheese, etc.), and is used for processed dairy products such as butter, ice cream, coffee creamer, etc. Indeed, it is this very versatility that necessitates that milk have the highest possible flavor quality to maintain and expand market share of dairy products, as well as provide nutritive benefits and better hedonics for consumers. In this work, two individual studies were undertaken to better understand drivers of dairy quality as a result of volatile and nonvolatile sensory stimuli respectively, and both were directly related to addressing current needs in the dairy industry. The first study focused on the elucidation of potential causes of a documented off-flavor defect, spontaneous oxidized flavor (SOF) in milk. The second study aimed to provide an improved understanding of the contribution of nonvolatile small-molecular weight compounds on textural attributes as related to creaminess perception in dairy products. For the first study, the principal objective was to identify the causative off-flavor compounds in SOF milk. Comparative aroma analysis between a reported SOF milk and clean milk sample without noted off-flavors (control) were investigated using various volatile analytical techniques, including solvent extraction, solvent assisted flavor evaporation (S.A.F.E.), and gas chromatography-olfactometry/mass spectrometry (GC-O/MS), utilizing the OSME method for aroma discrimination. Key differences in the aroma profiles of the off-flavored milk were found in several very strongly rated “green, musty” terpenoid compounds that were completely absent from the control. These compounds were positively identified as endo-borneol, 2-methylisoborneol, and α-terpineol. After quantification studies, the impact of these compounds was validated using aroma recombination analyses. A degree of difference sensory test incorporated a control clean milk (same as reference), a clean milk spiked with day 0 levels of the terpenoid off-flavor compounds (endo-borneol 0.95 µg/L; 2-methylisoborneol 0.0037 µg/L; α-terpineol 0.84 µg/L), and a clean milk spiked with day 14 levels of the same compounds (endo-borneol 0.20 µg/L; 2-methylisoborneol 0.017 µg/L; α-terpineol 0.62 µg/L). The control milk was rated 0.75, the day 0 milk rated 1.67, and the day 14 milk rated 2.0 (α=0.05, critical difference value=1.0897), showing that the addition of the terpenoid “musty” compounds resulted in a significantly different flavor profile when compared to a reference clean milk. Panelists described similar “unclean” perceptions as were found in the preliminary tasting of the original milk samples. As a result, it was confirmed that the identified terpenoid compounds were responsible for flavor defects observed in the initial off-flavor milk. While the milk screened for this study was identified as ‘SOF’ in nature, more research would be needed to verify that this was indicative of SOF or simply another off-flavor issue. Regardless, these results illustrated a benefit to the dairy industry to conduct more directed off-flavor analyses and determine the root causes of product off-flavors on a per sample basis. The aim of the second study was to provide greater understanding of molecules that contribute to texture attributes associated with creaminess perception in dairy products. Heavy whipping cream was selected from several dairy product extracts as a characteristically ‘creamy’ product. Optimized food grade solvent extraction, HPLC multidimensional fractionation, and descriptive analysis using a five-term sensory lexicon and references were employed to identify fractions of interest with the strongest textural attributes. UPLC/MS/MS analyses and NMR afforded the positive identification of five compounds associated with textural attributes, falling into three categories: vitamin complex (orotic acid, pantothenic acid) hippuric acid (hippuric acid, 2-methylhippuric acid), and sulfate compounds (p-cresol sulfate). Quantitative measurements of the individual compounds were undertaken to assess their presence across a variety of nonfat, reduced fat, and full fat dairy products, and concentrations obtained from skim milk and whole milk were used for further recombination sensory experiments. A control skim milk was compared with a skim milk spiked with the five texture-active compounds at levels found in whole milk (orotic acid 120 mg/kg; pantothenic acid 1.3 mg/kg; hippuric acid 1.6 mg/kg; 2-methylhippuric acid 300 mg/kg; p-cresol sulfate 9.5 mg/kg). The 2-AFC sensory evaluation tests indicated the spiked skim milk had a significantly “creamier, fuller bodied flavor” when compared with the control skim milk (p=1/2, n=20, α=0.01). This verified the sensory relevance of the identified compounds, indicating the contribution of these compounds to creaminess perception. None of these five compounds have been previously reported in literature to exhibit textural effects, nor effects on creaminess quality in dairy. In summary, elucidation of the objectionable flavors present in a purported SOF whole milk sample indicated the cause of off-flavor was from a microbial source or a flavor ‘taint’ not caused by spontaneous oxidation. This indicated the dairy industry would benefit from a more bottom-up approach in identifying responsible objectionable flavor compounds to make informed decisions about minimizing off-flavors in fluid milk. The second study provided novel understanding of compounds that contribute to textural attributes and creaminess perception. Identification of these compounds provides an improved basis for product developers to positively influence creamy texture and target creamy quality enhancement in dairy products. In acknowledging the role that texture compounds play alongside the larger picture of creaminess obtained through previous research, developers may create high-quality nonfat, reduced fat, and full fat dairy products for a variety of markets, consumers, and purposes.