Characterization of the physical, microbiological, and chemical properties of sonicated raw bovine milk

Authors: Van Hekken, Diane; Renye, John; Bucci, Anthony; Tomasula, Peggy

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2018
Publication Date: 8/1/2019
Citation: Van Hekken, D.L., Renye Jr, J.A., Bucci, A.J., Tomasula, P.M. 2019. Characterization of the physical, microbiological, and chemical properties of sonicated raw bovine milk. Journal of Dairy Science. 102:6928-6942. https://doi.org/10.3168/jds.2018-15775.
DOI: https://doi.org/10.3168/jds.2018-15775

Interpretive Summary: Ultrasound, sound waves at frequencies above that of human hearing, is widely recognized as a valuable tool in medical practice and imaging. Research conducted by immersing a sonotron in various volumes of milk, operating at much higher power but lower frequencies than that generated for medical applications, has shown that ultrasound is equivalent to standard homogenization and reduces the sizes of the fat droplets into very small, submicron-sized droplets. In this study, we tested a new ultrasonic device that indirectly contacts the milk flowing through a chamber in which rapidly vibrating plates transmit the US waves, acoustic energy, to milk. Our results showed that at short contact times, the milk leaving the chamber had reduced fat droplet sizes, different coagulation properties and showed a reduction in the native microflora of the raw milk tested. This technique has great potential in introducing new concepts in the sensory and textural properties of dairy foods.

Technical Abstract: Innovative processing technologies, such as ultrasonication, can change the properties of milk, allowing for the development of novel-textured, high-moisture dairy foods. Raw milk, standardized to 3% fat and warmed to 42 or 54C, was exposed to continuous, low dual frequency ultrasonication (16/20 kHz, 100 W/cm3) at flow rates of 0.15-, 0.30-, and 0.45-L/min that resulted in resident times within the reaction chamber of 40, 24, and 17 s per pass, respectively. Multiple passes (3, 5, and 7, respectively) were required to obtain a total exposure time of 120 s. Evaluation of fat droplet sizes, enzyme coagulation properties, and microstructure of milk and milk gels was conducted, as well as determining compositional and lipid properties, to determine the potential of milk modified by short exposures of low frequency ultrasonication in making novel enzyme-set curds. Raw milk had the typical broad range of lipid droplet sizes, mode volume-weighted diameter of 2.26 micron, which decreased by < 0.64 micron, with a distribution of< 7% as ultrasonication exposure increased. A distinct decrease in the distribution of the larger particles was observed as the submicron lipid droplets (0.37 micron) accumulated. Confocal microscopy confirmed the largest droplets were selectively reduced in size with a concomitant increase in the number of submicron droplets, which occurred sooner when exposed to shorter bursts of ultrasonication (0.45-L/min treatments) at the higher processing temperature. Ultrasound processing at 42C resulted in faster gelling times and firmer curds at 30 min; however, extended processing at 54C reduced curd firmness and lengthened coagulation time. This showed that thermosonication was more effective in altering protein-lipid interactions, thus the strength of the enzyme-set curds. Scanning electron microscopy revealed a denser curd matrix with the protein strands appearing to be more irregular and clustered than continuous, while transmission electron microscopy showed submicron lipid droplets embedded within the protein strands of the curd matrix. Thermosonication at 0.30- and 0.45-L/min also reduced the total aerobic bacterial count by more than 1 log CFU/ml; and the number of psychrophiles below the limit of detection for this study (10 CFU/ml). Ultrasonication at 120 s had minimal impact on the composition, fatty acid profiles, and lipid heat capacity and enthalpy of the processed milk. These findings suggest that milk modified using continuous ultrasonication, which is more conducive to commercial scale-up, may be useful in developing novel textured curds for cheese or dairy ingredient applications.