The effect of thermosonication of milk on selected physicochemical and microstructural properties of yoghurt gels during fermentation

The effect of thermosonication of milk on selected physicochemicaland microstructural properties of yoghurt gels during fermentationJoerg Riener, Francesco Noci, Denis A. Cronin , Desmond J. Morgan, James G. Lyng

INTRODUCTIONYoghurt is a coagulated milk product obtained by lactic acid fermentation, through the action of microorganismsBefore the milk is inoculated with the starter culture fat globules are homogenised to enhance yoghurt consistency and prevent serum separation in the final productpre-inoculation heat treatment of milk (eg. 9095C for 10 min), which will bring denaturation of whey proteinsThermosonication provides positively influence textural and other physiochemical properties of yoghurt gels

OBJECTIVESTo compare rheological and other physicochemical properties of yoghurt prepared from conventionally heat treated milk with yoghurt obtained from thermosonicated milkTo examine the influence of fat content on the properties in yoghurts prepared from thermosonicated full, reduced fat and skim milks

MATERIALS AND METHODSFreshly pasteurised homogenised milk samples with different fat contents were obtained from a local dairy processor and processed on the same dayA two stage homogenizer (150 bar first stage, 50 bar second stage) was used to homogenize the milks giving a mean fat globule size of 0.4 m and milks were pasteurised (72C, 15 s) within 12 h of collection

Thermosonication and Yoghurt preparationThermosonication treatments were carried out using an ultrasonic processor (UP 400S, Hielscher, Germany) working at a constant frequency of 24 kHzMilk samples (200 ml) for continuous application of ultrasound with a power output of 400W for 10 minMilk temperature increased to 72C over the first 6 min of sonication and thereafter remained constantSamples of the processed milk (200 ml) were equilibrated at 40C and inoculated with 1 g of commercial yoghurt starter culture

analysisThe protein, fat, total solids, total acidity and ash content of the milks were determined according to AOAC methodsFat-globule size was determined by laser-light scattering using a Malvern MastersizerThe pH change during fermentation was recorded at 10 min intervals at 40C using a pH meterChanges in the rheological properties during culturing were monitored on a Bohlin GEMINI 200 HR NANO Rheometer

To measure the water holding capacity (WHC) an adapted version of the centrifuge method described by Parnell-Clunies et al. (1986) was used

To examine the microstructure a Cryo-SEM (JEOL JSM-5410LV Scanning Microscope, JEOL Instruments, Tokyo, Japan) was used

Results and discussion

CONCLUSIONClear differences was observed rheological and physicochemical properties of yoghurt cultures prepared from conventionally heated and TS milkYoghurts manufactured from TS milk showed increased gel firmness and viscosity as well as improved WHC compared to conventionally produced yoghurts

COMMENTSAuthor should explain about changes happened in fat content of milk after thermosonicationAuthor should explain about the validation of yoghurtThe statement mouthfeel and body in the texture in yoghurt gels result from a protein network formed by casein strings and or clusters which entrap serum and fat globules was given by Lucey et al., (1998) not by Kalab et al., (1983)

THANK YOUSWAPNIL SUTAR 14FBT2015