powder has become very important for the product diversification in dairy industry.

Transcription

1 Chapter 5 SUMMARY A D CO CLUSIO S The production of skim milk powder, casein powder, whey protein concentrate powder has become very important for the product diversification in dairy industry. India holds first rank in respect of total milk production in the world. Over 55% of total milk production in Indian subcontinent consists of buffalo milk and India is the largest producer of buffalo milk in the world. Unlike other countries, where cow milk is used, these products in Indian subcontinent are mainly processed from buffalo milk. The present study aimed at characterizing and quantifying the engineering properties and moisture sorption parameters of skim milk powder, dried acid casein and whey protein concentrate powder produced from buffalo milk, with a view to exploit the competitive advantages of buffalo milk for value addition and developments of new products with special attributes. The products used in this study were processed from buffalo skim milk following the standard procedures and processing parameters, in the Experimental Dairy Plant at National Dairy Research Institute, Karnal and at Dairy Science College, Hebbel, Bangalore. Composition of these products was determined accurately. Freshly prepared products were analysed for their functional engineering properties and experimental evaluation of sorption characteristics. 5.1 Bulk density and particle density were maximum in dried casein followed by skim milk powder and whey protein concentrate powder. The loose and packed bulk densities and the particle density of freshly prepared buffalo skim milk powder were 0.35, 0.53 and 1.29 g/cm 3, of dried casein as 0.65, 0.87 and 1.46 g/cm 3 and of whey protein concentrate powder as 0.33, 0.49 and 1.19 g/cm

2 5.2 The occluded air content and interstitial air content of the buffalo skim milk powder immediately after manufacturing was evaluated to be 9.72 cm 3/ 100 g and cm 3/ 100 g. The values of above properties of dried casein and whey protein concentrate powder were found to be 6.72, and 8.70, cm 3/ 100 g respectively. The occluded and interstitial air contents of dried casein were the minimum and consequently, the bulk density was maximum and amongst the three products. 5.3 The porosity of the buffalo skim milk powder was found to be 72.86%, whey protein concentrate powder as 69.55% and dried casein as 61.80%. 5.4 The flowability was measured in terms of angle of repose (α) and was found to be o for buffalo skim milk powder, o for whey protein concentrate powder and o for dried casein. The powder is more free flowing if angle of repose is smaller. No difference was found in the flowability of buffalo skim milk powder and cow skim milk powder 5.5 Wetting time of buffalo skim milk powder was found to be 36 s at 20 C. It was found to be 42 s for whey protein concentrate powder and 12 s for dried casein. Higher the wetting time lower was the wettability. Wettability was minimum in WPC powder followed by skim milk powder and dried casein. 5.6 Dispersibility is the rapid penetration of water in a mass of powder. The dispersibility of skim milk powder was 89.76%. The dispersibility of whey protein concentrate powder was and dried casein was 68.05%. 5.7 The insolubility index is a measure of volume of insoluble sediments and the ability of a powder to dissolve in water. The insolubility index of fresh dried casein was 6.36 ml followed by whey protein concentrate powder with 1.63 ml. The insolubility index of fresh skim milk powder from buffalo milk was observed to be 1.23 ml. 167

3 5.8 The moisture adsorption behavior of skim milk powder, dried acid casein and whey protein concentrate powder is manifested in the form of sigmoid shaped curves reflecting a Type II isotherm, which is typical to the most of the foods. No discontinuity was observed in the graphical data. 5.9 The experimental data points of isotherm showed clear temperature dependence. An increase in temperature resulted in decrease in equilibrium moisture content at a given water activity of buffalo skim milk powder, dried acid casein and whey protein concentrate powder. The analysis of variance, however, revealed that the effect of temperature on moisture adsorption and desorption isotherms of all theses products was statistically not significant (P>0.05) over the temperature range o C The experimental sorption data for buffalo skim milk powder below 0.33 a w showed a reverse trend where an increase in equilibrium moisture content was observed with the increase in temperature. This phenomenon is known as inversion point. However, in the present study, since the observed difference in equilibrium moisture content at different temperatures was small and statistically not significant, the above trend could not be attributed to the inversion point All the three products skim milk powder, dried acid casein and whey protein concentrate powder showed similar shape of adsorption isotherm nearly up to 0.3 water activity. At water activities higher than 0.3 the skim milk power sorbed more water followed by whey protein concentrate powder and dried acid casein respectively. Above water activities 0.8 the isotherm of whey protein concentrate powder showed a trend similar to isotherm of skim milk powder. 168

4 5.12 The experimental data on moisture sorption was fitted to seven different models and their estimated parameters and root mean square percent error were evaluated. The G.A.B. models best described the experimental adsorption data for buffalo skim milk powder, dried acid casein and whey protein concentrate powder throughout the entire range of water activity The adsorption monolayer moisture content of buffalo skim milk powder decreased from at 25 o C to g/100g solids at 45 o C and the desorption monolayer decreased from 6.20 at 25 o C to 5.21 g/100g solids at 45 o C. The adsorbed monolayer moisture content of dried casein decreased from at 25 o C to at 45 o C, whereas, the corresponding decrease in desorption monolayer was from to g/100g solids. Similarly, the adsorbed monolayer in whey protein concentrate powder decreased from at 25 o C to at 45 o C, whereas, the corresponding decrease in desorption monolayer was from to g/100g solids. In all the products, monolayer moisture was higher in desorption than the adsorption and deceased with increase in temperature. The calculated values of monolayer from BET isotherm equation have been found to be lower than the corresponding values obtained by using GAB equation Equations were developed to correlate the GAB parameters with temperature by incorporating temperature term into sorption equations to predict the variation in water activity at any given temperature. 169

5 5.15 More sorption hysteresis was exhibited by skim milk powder, dried acid casein and whey protein concentrate powder at all the temperatures with a closure point of hysteresis loop varying between 0.2 to 0.25 water activities and the upper closure point varying between 0.80 to 0.9 water activities. The distribution of hysteresis loop relative to water activity showed a marked change at various water activities. There was no hysteresis effect in the monolayer moisture content region and the hysteresis occurred predominantly in the water activity range of 0.45 to 0.8 a w for skim milk powder, 0.3 and 0.6 a w for dried casein and 0.2 and 0.85 a w for why protein concentrate powder at 25 o C. Paired t-test revealed that the adsorption and desorption differ significantly at all the three temperatures for each of the three products The magnitude of hysteresis loop in terms of relative hysteresis units at room temperature (25 o C), was maximum in whey protein concentrate powder (2.03) followed by dried acid casein (1.65) and skim milk powder (1.39). By increasing the isotherm temperature from 25 to 45 o C, the total hysteresis reduced to 1.67, 1.22 and 1.08 units in the products respectively The maximum value for hysteresis amplitude ratio was for whey protein concentrate powder followed by dried casein 0.192, and skim milk powder at 25 o C The highest hysteresis energy was found in the same order of whey protein concentrate powder having maximum energy (343.7 J at 25 o C, J at 35 o C and J at 45 o C) followed by dried casein (307 J at 25 o C to 236 J at 35 o C at and J at 45 o C) and skim milk powder (277 J at 25 o C, J at 35 o C 170

6 and J at 45 o C.) The effect of temperature was thus found to reduce hysteresis energy Isosteric heat of adsorption was found to decrease with increase in moisture content and the trend seemed to become asymptotic as the moisture content of above 20 % dry basis was approached. The isosteric heat of adsorption for skim milk powder ranged from kjmol -1 at 3% (d.b.) to 44.6 kjmol -1 at 30 % (d.b.) moisture content. Correspondingly the heat of desorption ranged from to 44.8 kjmol -1. The range of above values for dried casein were to kjmol -1 for adsorption and to kjmol -1 for desorption for the same range of moisture contents. The corresponding values for isosteric heat of whey protein concentrate powder were to kjmol -1 for adsorption and to kjmol -1 for desorption. The isosteric heat for both adsorption and desorption were highest in skim milk powder followed by dried casein and whey protein concentrate powder The Gibb s free energy of adsorbent increased with increase in moisture content. The rate of increase, however, was slow at higher moisture levels. At the moisture content of 3% (d.b.) the maximum value of G for adsorption was noted dried casein as 5312 J / mol, followed by 4680 J / mol for skim milk powder and 2378 J / mol in whey protein concentrate powder. Similar trend was noticed in desorption process with maximum value of G in dried casein as 5756 J / mol, followed by 4875 J / mol for skim milk powder and 4618 J / mol in whey protein concentrate powder The entropy data displays a strong dependence on moisture content. The adsorption data gives lower values than those for desorption. In dried casein 171

7 and whey protein powder the difference in adsorption and desorption entropy ended at 20 % (d.b.) moisture content and both adsorption and desorption curves coincided with each other. In skim milk powder the entropy decreased with increase in moisture content, but the decrease was not smooth. This difference in adsorption and desorption entropy also persisted to 30% (d.b.) moisture content In skim milk powder the bulk density increased from 0.35 g /cm 3 at water activity (EMC 3.781% d.b.) to 0.52 g /cm 3 at water activity (EMC 17.97% d.b.). The corresponding increase in dried casein for the above water activity range was 0.65 g /cm 3 (EMC 3.50 % d.b.) to 0.79 g /cm 3 (EMC 12.01% d.b.) and for whey protein concentrate powder was 0.33 g /cm 3 (EMC 2.11 % d.b.) to 0.54 g /cm 3 (EMC % d.b.) The flowability of all the products decreased with increase in water activity. For buffalo skim milk powder, the angle of repose increased from o at water activity to o at water activity. The corresponding increase in angle of repose for dried casein was o to o and for whey protein concentrate powder was from 40.3 o to o The density of sorbed water at 25 o C was 1.97 g /cc in skim milk powder, 1.88 in dried casein and 1.91 in whey protein concentrate powder. The numbers of adsorbed monolayers were 3.97, 4.30 and 2.61 respectively in skim milk powder, dried casein and whey protein concentrate powder. Per cent bound water in skim milk powder was and that in dried casein was and in whey protein concentrate powder was Surface area of sorption was highest in dried casein m 2 /g followed by skim milk powder and 172

8 whey protein concentrate powder had the lowest surface area of sorption as m 2 /g Temperature of sorption had profound influence on properties of bound water and surface area of adsorbent. In skim milk powder during adsorption, the surface area of adsorbent reduced from to m 2 /g as the temperature rose from 25 to 45 o C. The corresponding decrease in dried casein was from to m 2 /g and in whey protein concentrate powder from to m 2 /g. Similarly, per cent bound water decreased from to in skim milk powder, to in dried casein and to in whey protein concentrate powder. The decrease in density of bound water was from 1.97 to 1.58, 1.88 to and 1.95 to 1.79 g /cc for skim milk powder, dried casein and whey protein concentrate powder respectively as the temperature was increased from 25 to 45 o C. Suggestions for future work The engineering properties of skim milk powder and two of the protein derivatives of buffalo skim milk, viz., dried acid casein and whey protein concentrate powder have been successfully quantified in this study. Emphasis should be laid on future research to explore the possibilities of utilizing the results on moisture sorption studies in research and development applications for developing the shelf life simulation models and packaging requirements for these products. The characterized engineering properties data on skim milk powder, dried acid casein and whey protein concentrate be utilized in exploiting the competitive advantages of buffalo milk for value addition and developments of new milk or milk-cereal based products with special attributes. 173