Yogurt from ultrasound treated milk: monitoring of fermentation process and evaluation of product quality characteristics

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1 Yogurt from ultrasound treated milk: monitoring of fermentation process and evaluation of product quality characteristics Panagiotis Sfakianakis & Constantina Tzia Laboratory of Food Chemistry and Technology, School of Chemical Engineering, National Technical University of Athens, 5 Iroon Polytechniou St.,15780 Athens, Greece ( psfakian@central.ntua.gr, tzia@chemeng.ntua.gr) ABSTRACT Ultrasound treatment is suggested as an innovative method for milk homogenization. The application of ultrasound in milk is referred that causes vibration and cavitation phenomena and thus the membrane that surrounds the milk fat globules dissipates and reforms creating smaller globules. The use of ultrasound is of great interest in the processing of liquid food systems, promising improvement of their quality characteristics. Thus, the ultrasound use in milk homogenization may influence the properties of yogurt. Texture improvement, by introducing ultrasound in yogurt processing is also of interest, as it is a fundamental attribute of product. The present study aims to produce yogurt from milk (3.5% fat content and 3.3% protein content) homogenized by ultrasound, at various amplitude levels and to investigate the effect of the particular treatment on the fermentation process and the quality characteristics of final product. As far as the fermentation process is concerned, the ph reduction rate was not affected by the power level of homogenization while the viscosity increase rate was altered. The viscosity of the yogurt was higher for more intensive ultrasound treatment, as well as the firmness and cohesiveness. Sensory evaluation showed that samples homogenized by high power ultrasound had better mouthfeel but weaker taste, while those homogenized by medium power ultrasound were more to the liking of the panelists. Keywords: Ultrasound; Milk Homogenization; Yogurt; Fermentation Process; Quality Characteristics INTRODUCTION Recent studies on food science and technology have shown great interest on non thermal processes, like ultra high pressure processing, pulsed electric and magnetic fields and ultrasound processing. In particular, ultrasound has great potential in food processing and many applications in food industry [1]. Ultrasound procedures are categorized based on the energy amount of the generated sound field, sound power (W) or sound intensity (W*m-2) [2, 3]. Low intensity ultrasound has been used in the food industry since the decade of 1960 for cleaning and for characterization of food and food materials [4]. High intensity ultrasound is capable of altering material properties by generating immense pressure, shear and temperature gradient in the medium through which they further propagate and thus create cavitation phenomena [1]. Therefore, some processes are enchanted by high intensity ultrasound such as emulsification, crystallization, degassing of liquid foods, enzymes inactivation, drying, filtration oxidation reactions, breaking of biological cell walls, activation of immobilized enzymes, extraction of organic compounds form plants and seeds and freezing procedures [1, 2, 4, 5, 6]. A new trend in the field of ultrasound is the application in milk to cause homogenization effects. Milk is a typical example of oil in water emulsion, in which the fat exists in microscopic globules with diameter that varies from 1 to 10 μm. They are surrounded by a membrane that acts as an emulsifier. However, because of interfacial tension the fat globules tend to collide, either by sharing the emulsifier or because the Laplace principle, the larger globules grow at the expanse of the smallest. This phenomenon forces the milk fat to rise to the surface of the milk volume and thus creates the undesirable effect of separation [7]. In order to prevent this effect, milk undergoes homogenization. The basic principle of homogenization is to reduce the size of the milk fat globules by subjecting them to severe conditions, like forcing the milk to flow through a small gap, with diameter 100 μm, and thus creating shear stress that disrupts the fat globules. Another method is the high velocity movement of the milk that creates turbulence into its volume forcing the milk fat globules to impact between them and then disperse. High frequency vibrations (>10 khz) can create homogenization effect also, because of the high intensity (>100 Pa)

2 shockwave they emit that disturbs the milk fat globules [8]. Also cavitation phenomena result in homogenization effect, because the rapid formation and collapse of air bubbles in the volume of the milk unleash a hammering force that disrupts the milk fat globule s membrane and the milk fat globules themselves [9]. An interesting remark was that ultrasound does not affect milk fat globules directly but create intense cavitation phenomena that destroy the milk fat globules and create the homogenization effect [9]. In any case of homogenization technique, the results are the reduction of the diameter of the milk fat globules from 10-2 μm to μm. Also homogenization changes the milk fat globules membrane composition; due to their size reduction, the milk fat surface area increases and the molecules of the native milk fat globules membrane become insufficient to contain the new-formed globules, therefore several proteins from the milk serum, mostly casein, are absorbed from the membrane thus the later becomes richer in proteins [10]. Milk homogenization has been studied quite thoroughly, focusing on the effect of homogenization on the formation of milk gels, specifically on yogurt. Yogurt is produced by fermentation of milk with suitable microorganisms (symbiotic cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. Bulgaricus) resulting in reduction of ph and coagulation. Several studies have examined homogenization methods different from the conventional and show quite interesting results [9, 10, 11, 12]. By applying ultrasound treatment at frequency of 24 khz and amplitudes of 200 and 400 W for 10 min and 20 khz with amplitude levels of 90, 225 and 450 W for 1, 6 and 10 min, discovered that high intensity ultrasound is promising homogenization method; specifically high amplitude ultrasound proved to have very good homogenization effect, forming volatile components that contribute to sensory characteristics of the milk. Furthermore the longer the milk exposed to ultrasound, the smaller the size of the milk fat globules. Ultrasound homogenized milk has been used in yogurt production, leading to product with improved water holding capacity, reduced syneresis and higher viscosity. Combination of ultrasound and heating (thermosonication) has been applied to milk that lead to yogurt with superior rheological properties, stronger gel structure, higher water holding capacity and lower syneresis compared with conventionally treated milk. Additionally yogurt from themosonicated milk scored higher acceptability in sensory evaluation than the respective from conventionally treated milk [13]. MATERIALS & METHODS The raw cow milk (non homogenized and non pasteurized) used in the experiments was obtained from a local dairy industry. The starter culture used was an industrial symbiotic culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. Bulgaricus. The milk was standardized at 3.5% fat content and 3.3% SNFC, heated at 60 C, treated by ultrasound for 10 min at frequency 20 khz and amplitudes of 150, 267.5, 375, and 750 W, then pasteurized at 85 C for 20 min, cooled at 46 C, inoculated with the started culture and incubated at 45 C until the ph value reached 4.7. After that the samples were stored at 4 C for 24 h. After the ultrasound treatment, Μilk Fat Globules (MFG) were observed at an optical microscope (Leica DM 750, Lense 100/1.25). During fermentation ph was monitored (WTW phmeter 3310 set 3, Germany) as well as viscosity (Rheoteck Rheometer RC-1 (Germany). At the final product, viscosity (Brookfield DV-II+, Programmable Viscometer, USA, spindle: hellipath set F) and texture properties (hardness, adhesiveness, cohesiveness and gumminess) were measured (Texture Analyser TA-XT2i, Stable Microsystems, UK, Cylindrical probe diameter 25 mm, length 35 mm). RESULTS & DISCUSSION Ultrasound homogenization led to decrease of MFG size. Low amplitude (150W) had not a satisfactory homogenization effect; the MFG size and their distribution were similar to untreated milk, as it is obvious from figures 1-3. Medium amplitude ultrasound (267.5, 375 W) had a good homogenization effect; MFG average diameter was 2 μm (Fig. 3, 4). Higher amplitude (750W) ultrasound reduced the MFG size crucially (Fig. 6), making them barely visible at the optical microscope (100x magnification); their average diameter size was 0.3 μm.

3 Figure 1. MFG of raw milk Figure 2. MFG of milk homogenized by ultrasound, amplitude 150 W Figure 3. MFG of milk homogenized by ultrasound, amplitude W Figure 4. MFG of milk homogenized by ultrasound, amplitude 375 W Figure 5. MFG of milk homogenized by ultrasound, amplitude W Figure 6. MFG of milk homogenized by ultrasound, amplitude 750 W During fermentation process the ph decrease rate of milk samples, homogenized by ultrasound, was slower compared with the respective process of conventionally homogenized milk, by pressure. Instead of 3 to 4h, the fermentation of ultrasound treated milk lasted 4-5 h. However during fermentation the viscosity increase was rapid and the final viscosity values were high, ranged from 1.4 to 2 Pa*s for medium and high amplitude ultrasound (375, 562.5, 750 W) treated milk. Both ph decrease and viscosity evolution for all samples followed the sigmoidal curve described by the modified Gompertz model [14]: For ph during fermentation time: ph ο, ph = initial and end values of ph respectively μ = maximum rate of ph decrease λ (min) = duration of lag phase For viscosity: μ αο, μ α = initial and end values of viscosity respectively μ = maximum rate of viscosity decrease λ (min) = duration of lag phase In figures 7-11 the decrease of ph and the evolution of viscosity curves during fermentation of milk samples homogenized by ultrasound are shown.

4 Figure 7. ph decrease and Viscosity evolution during fermentation of milk sample homogenized by 150 W Figure 8. ph decrease and Viscosity evolution during fermentation of milk sample homogenized by W Figure 9. ph decrease and Viscosity evolution during fermentation of milk sample homogenized by 375 W Figure 10. ph decrease and Viscosity evolution during fermentation of milk sample homogenized by W Figure 11. ph decrease and Viscosity evolution during fermentation of milk sample homogenized by 750 W The duration of the fermentation process ranged from 300 min (for samples treated by high intensity ultrasound, W and 750 W) to 335 min (sample treated low intensity ultrasound 150 W). Samples from milk treated by high intensity ultrasound presented longer lag phase (λ visc ) and higher viscosity increase rate (μ visc ) during coagulation, in comparison to those treated with low intensity The more intensive the ultrasound treatment, the higher the coagulum s viscosity at the end of the fermentation process. As far as the reduction of ph is concerned all the samples showed a similar behavior in response to the intensity of ultrasound treatment. μ ph did not differ for the various samples, λ ph presented variation. The table shows the values that describe the fermentation process of each sample.

5 Table 1. Values of λ, μ of ph and viscosity curves of fermentation process Ultrasound Duration Amplitude (W) (min) μ ph λ ph (min) μ visc λ visc (min) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 12 In Figure 12 the viscosity and texture characteristics of the final product, yogurt are shown. Figure 12. Viscosity and texture characteristics of yogurt produced by milk homogenized by ultrasound The intensity of the ultrasound treatment had an interesting effect on the characteristics of the yogurt. Yogurt from milk treated by higher intensity ultrasound had high viscosity and harness values. In general viscosity and hardness followed the same pattern, responding to the increase of the amplitude of ultrasound treatment. Cohesiveness and adhesiveness did not present difference for the differently treated samples, with the exception of the sample treated by 150 W ultrasound that was much lower than the rest. CONCLUSION Concluding, high intensity ultrasound had a good homogenization effect, but led to longer fermentation process duration. The yogurt produced from ultrasound treated milk had higher viscosity, stronger coagulum and superior texture characteristics. Differences in the intensity of the ultrasound did not affect the duration of the fermentation or the decrease rate of the ph during fermentation, while increase of the ultrasound intensity led to faster evolution of viscosity and rose the later to higher values. Furthermore, the higher the amplitude of ultrasound of homogenization is, the firmer and more viscous the yogurt will become. REFERENCES [1] Dolatowski Z.J., Stadnik J. & Stasiak D Applications of Utrasound in Food Technology. Acta Sci. Pol. Technol. Alimemnt., 6(3), [2] McClements D.J Advances in the application of ultrasound in food analysis and processing. Trends Food Sci. Techn.,6, [3] Fellows P.J Food processing technology: Principles and practice. Woodhead Publishing. [4] Mason T.J., Paniwnyk L. & Lorimer J.P The uses of ultrasound in food technology. Ultrason Sonochem., 3, [5] Knorr D., Zenker M., Heinz V. & Lee D.-U Applications and potential of ultrasonics in food processing, Trends Food Sci. Techn., 15, [6] Roberts R.T High intensity ultrasonics in food processing. Chem. Ind., 15, [7] Fox P.F Fat Globules in Milk. Elsevier Science Ltd.

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