Spray drying of food products 2. Prediction of insolubility index

Spray drying of food products:2.Prediction of insolubility indexJ.Straatsma *,G.Van Houwelingen,A.E.Steenbergen,P.De JongNIZO food research,PO Box 20,6710BA Ede,The NetherlandsReceived 26February 1999;accepted 14June 1999AbstractThe thermal load of food products during drying is an important factor for the ®nal powder quality.The exposure to heat can result in the formation of insoluble material,which is undesirable especially for instant powders.It is demonstrated how to carry out experiments in order to derive a kinetic model which describes the rate of formation of insoluble material during the drying of milk.It was proven possible to achieve a good ®t between the calculated values of the insolubility index obtained from an idealised model and the values obtained experimentally.This method is also suitable for other food products.The kinetic model can be used as a submodel for drying simulation models that describe the behaviour of the individual particles during spray drying.The combination of these models is an e ective tool in giving indications of how to adapt industrial dryers to reduce the insolubility of the powder and get a better product quality.Ó1999Elsevier Science Ltd.All rights reserved.Keywords:Spray drying;Milk;Food;Powder;Insolubility1.IntroductionThe insolubility index is of primary importance for the quality of instant powders.Insoluble material is formed during spray drying of concentrated milk.The actual amount depends on the temperature and moisture con-tent during the drying period.The drying history can be calculated by drying models as developed in the ®rst part of this study (Straatsma,van Houwelingen,Meulman &Steenbergen,1999).The properties of the milk (season)and the pretreatment (preheating,homogenisation,etc.)can also play a role.The mechanism by which insoluble material is formed is not yet fully understood.The cur-rent view is that the mechanism involves the unfolding of b -lactoglobulin,followed by aggregation with casein,thesame mechanism which is responsible for the fouling of apparatus when milk products are heated (Jong de,1996;Jeurnink,Verheul,Cohen Stuart &Kruif,1996).It appears that other mechanisms also play a role.As de-scribed by Ohba,Takahashi and Igarashi (1989)high-pressure atomisation causes considerable amounts of insoluble material with a relatively high fat content.Experiments carried out at NIZO food research (not published)con®rm these results.To study the formation of insoluble material,Kudo,Hols and van Mil (1990)carried out heating tests with skim milk powders with a moisture content in the range 3±7%.The tests showed that the insolubility index began to increase only after a long heat-holding period with respect to the residence time in spray dryers,which is in the range of 20±40s (see Table 1).If insoluble material is to be formed during spray drying,the material must be much more temperature sensitive than is the case with dry powder.According to Walstra and Jennes (1984),temperature sensitivity is greatest when the moisture content is between 10%and 30%.The aim of this study was to get a kinetic model de-scribing the rate of formation of insoluble material,which can be used to reduce the insolubility of dried milk powder.To get more quantitative data,additional experiments were carried out.The aim was to obtain quantitative data quickly rather than to shed light on the mechanism.Nomenclature r ISi formation rate of insoluble material,ml s À1k 0preexponential factor,ml s À1E a energy of activation,J mol À1R gas constant,J mol À1K À1T temperature,KT 0reference temperature,KM p moisture content in total mass,kg kg À1Isiinsolubility index,mlJournal of Food Engineering 42(1999)73±77/locate/jfoodeng*Corresponding author.Tel:+31-318-659511;fax:+31-318-650400;e-mail:straatsm@nizo.nl0260-8774/99/$-see front matter Ó1999Elsevier Science Ltd.All rights reserved.PII:S 0260-8774(99)00108-92.Materials and methods 2.1.Heating testsHeating tests were carried out to investigate the ki-netics of the formation of insoluble material.A sample holder (see Fig.1)was made for this purpose,consisting of two metallic cylindrical tubes.The smaller tube was placed concentrically inside the larger.The test sample (approx.28ml)was put into the gap (width 2mm)be-tween the two tubes,then the gap was sealed at the top and the bottom of the tubes.Calculations indicated that after 7s the sample in the middle of the gap undergoes a temperature change of 95%of the applied temperaturechange.The holder is therefore only suitable for heat-holding periods longer than (approx.)7s.The samples of powder and concentrated milk were prepared by mixing low-heat skim milk powder (NI-LAC)and water.The powder samples with a moisture content of 9±25%were prepared by mixing powder and water in a mortar at room temperature and then milling to a homogeneous mass in a co ee grinder.The con-centrated milk samples with a moisture content of 35%and higher were prepared by stirring powder into water at (approx.)24°C.The heating test was carried out im-mediately after the samples were prepared.The procedure followed in order to achieve a heat-holding time t at temperature T :·preheat in a water bath for 30s at 65°C;·place in a water bath at temperature T for a time t ;·cool in a water bath for 30s at 65°C;·cool in cold water.The analyses,performed immediately after heating,were moisture content and insolubility index of non-heated material and insolubility index of heated sample.The moisture content was determined by the Karl Fisher method and the insolubility index was deter-mined by the standard IDF method (IDF,1988).The lower detection limit of this method is 0.05ml.3.Drying testsExperiments were carried out on a pilot-scale spray dryer in order to quantify the formation rate of insolu-ble material during the drying process.The diameter of this dryer (NIRO 25)is 2.25m and the total height 3.95m (2.25m for the cilindrical part and 1.70m of the conical part).The dryer has no ®nes-return,so that al-most no agglomeration of the powder particles oc-curred.The concentrated milk was prepared by dissolving low-heat skim milk powder (NILAC)to a total solids content of 40%.The concentrated milk¯owFig.1.Sample holder for heating tests.Dimensions in millimeter.Table 1Thermal stability of skim milk concentrate/powder aMoisture content (%)ISi 0(ml)T (°C)t ISi 0X 3(s)Reference 3<0.0570 1.8´105Kudo et al.(1990)7<0.0570 1.1´104Kudo et al.(1990)3<0.051001200Kudo et al.(1990)7<0.05100720Kudo et al.(1990)9<0.059520This study 12<0.0595<7This study 148.5ÀÀThis study 259.0ÀÀThis study 35<0.05ÀÀThis study 40<0.05ÀÀThis study 50<0.0585275This study 50<0.059550This studyaISi 0 insolubility index before heat treatment;t ISi 0X 3 heat-holding time needed to increase ISi to a value of 0.3ml.74J.Straatsma et al./Journal of Food Engineering 42(1999)73±77was kept very low during drying(2kg/h)with respect to the air¯ow(1100kg/h),so that the air temperature hardly decreased during the drying process.The con-centrated milk was atomised with a pneumatic nozzle (Spraying Systems No.E45)at an air pressure of1.2bar in order to obtain a large variety of droplet sizes.The temperature at atomisation was25°C.The air outlet temperature was varied from77°C to114°C.The powder produced was sieved into three size fractions with the help of two sieves with mesh widths of 45and90l m.The analyses carried out on the unsieved powder and the three size fractions were moisture con-tent by the Karl Fischer method,volume median di-ameter d v Y0X5 with a Malvern2600c particle sizer and insolubility index by the standard IDF method.4.Results4.1.Heating testsThe heating tests showed that the heat-holding period needed to form a speci®c amount of insoluble material decreases as the powder moisture content increases(see Table1).Powders that were moistened to give them a moisture content of up to12%had a low insolubility index directly after preparation(ISi<0.05).Powders that were given a higher moisture content,however,had a higher insolubility index without being subjected to heat treatment.Preparation of powders with a moisture content higher then12%by exposure to moist air in-stead of mixing with water also had high insolubility indexes(not shown).In view of the fact that powder with a moisture content of25%was not at all paste-like or sticky and even exhibited good¯ow properties,it was thought that the proteins may have gelled.Concentrated milk with a moisture content of50%exhibits fairly good thermal stability(see Table1).Samples with a moisture content of35%and40%,prepared by mixing water and powder,were di cult to homogenise and were also di cult to handle in the heating tests.However,the concentrates prepared in this way still had a low insol-ubility index(ISi<0.05).The conclusion that we can draw from these heating tests is that no insoluble material will be formed in powders with a moisture content of less than10%during the spray drying process.Concentrated milk with a moisture content of50%or more are also thermically stable within the residence time in spray dryers(20±40s). According to Walstra and Jennes(1984),temperature sensitivity is greatest when the moisture content is be-tween10%and30%.In our kinetic model we will therefore assume that formation of insoluble material occurs when the moisture content is in the range be-tween10%and30%.The heating tests described here are not very suitable for studying the intermediate range.In the®rst place the heat-holding times in a spray drier are shorter than the shortest time that can be realised(ap-prox.7s)in these heating tests,and in the second place it is not clear whether the e ects observed during sample preparation(gelling)also occur during the short drying period in a dryer.Further experiments aimed at quan-tifying the rate of formation of insoluble material were therefore carried out on a pilot-scale dryer.5.Drying testsTable2shows the results of four experiments with di erent inlet air temperatures.Only a very small partTable2Insolubility index from experiments on pilot-scale dryer aT ai/T ao(°C)94/77104/86114/96134/114 Moisture content(%)Powder 6.80 6.85 5.36 4.02Fraction1 6.71 6.77 5.41 4.84Fraction2 6.65 6.68 5.33 4.39Fraction37.217.18 5.80 4.31d v Y0X5 (l m)Powder40574558Fraction121222227Fraction269686768Fraction3144156139143ISi(ml)Powder<0.05<0.050.35 2.80Fraction1<0.05<0.050.050.55Fraction2<0.05<0.050.20 2.50Fraction3<0.050.150.80 4.80a T ai inlet air temperature;T ao outlet air temperature;d v Y0X5 volume median diameter;ISi insolubility index;fraction1:d<45l m;fraction 2:45<d<90l m;fraction3:d>90l m.J.Straatsma et al./Journal of Food Engineering42(1999)73±7775(approx.3°C)of the temperature di erence of17±20°C between the inlet and outlet air is attributable to heat loss due to water evaporation.The largest part is caused by loss of heat to the environment.Because di erences in the temperature of air within the dryer are small and mixing of the air in this dryer is fairly good,we can assume that the temperature prevailing in the dryer is everywhere identical to the outlet temperature.The re-sults show clearly that the insolubility index increases with the particle diameter.This agrees with theoretical predictions:the larger particles dry more slowly owing to the di usion limitations and need more time to pass through the most temperature-sensitive range of mois-ture content.6.Kinetic modelBecause it is di cult to obtain quantitative data re-lating to the e ect of the moisture content on the rate of formation of insoluble material it was decided to use a strongly idealised kinetic model.The assumptions are:·insoluble material is formed during spray drying only when the powder moisture content is between10% and30%;·in this range the rate of formation is independent of the moisture content;·the rate of formation exhibits zero-order kinetic be-haviour with temperature dependency according to Arrhenius;·the maximum insolubility index is10.The rate of formation(r Isi)is therefore described by the equation:r ISi 0 1 when M p<0.10or M p>0.30or ISi>10andr ISi k0ÁexpÀi aÁ1À12when0.10<M p<0.30and ISi<10.The drying courses of the three size fractions during the four experiments were simulated with the DrySim model,as described in the®rst part of this study (Straatsma et al.,1999).Because the air temperature in the dryer is constant during the tests,the distribution of the air and the drying droplets during the residence time is irrelevant.This simulation results in a table of tem-perature and moisture content at di erent locations within the drying particles as a function of time.Based on this table the local insolubility indexes were calcu-lated with the kinetic model described and integrated to obtain the insolubility index of the whole particles.The kinetic constants k0and E a were adapted until a good agreement between experiments and model was achieved.Fig.2shows that the experimentally deter-mined ISi values agreed well with the calculated values if the kinetic constants are taken to be k0 0.054ml/s, E a 2.7´105J/mol and T0 348K.The standard error of estimate of Ln(Isi)is0.44.The high value of E a is typical for heat-sensitive reactions such as denaturation of proteins.7.ConclusionExperimenting on a pilot-scale dryer is a useful method to derive a kinetic model which describes the rate of formation of insoluble material during the drying of concentrated milk.It was proven possible to achieve a good®t between the calculated values of the insolubility index obtained from an idealised model and the values obtained experimentally.This method is also suitable for other heat-sensitive food products.ReferencesJeurnink,T.J.M.,Verheul,M.,CohenStuart,M.,&Kruif,C.G.de.(1996).Deposition of heated whey proteins on a chromium oxide surface.Colloids and surfaces B:Biointerfaces,6,291±307. Jong,P.de.(1996).Modelling an optimisation of thermal treatments in the dairy industry.ISNB90-9009034-7.Thesis University of Technology,Delft.IDF standard129A(1988).Dried milk and dried milk products: Determination of insolubility index.International Dairy Federation, Brussels.Kudo,N.,Hols,G.,&van Mil,P.J.J.M.(1990).The insolubility index of moist skim milk powder:In¯uence of the temperatureof Fig.2.ISi versus particle diameter.Experimental results(symbols) and model calculation(lines).Temperatures:86°C(1,d);96°C(2,n) and114°C(3,m).Model constants:k0 0.054ml/s;E a 2.7´105 J/mol;T0 348K.76J.Straatsma et al./Journal of Food Engineering42(1999)73±77the secondary drying herlands Milk and Dairy Journal,44, 89±98.Ohba,T.,Takahashi,K.,Igarashi,S.(1989).Mechanism of insoluble material formation in spray-dried whole milk powder.Reports of Re-search Laboratory(vol.88,pp.91±108),Snow Brand milk products.Straatsma,J.,van Houwelingen,G.,Meulman,A.P.,&Steenbergen,A.E.(1999).Spray drying of food products:1.Simultion models.Journal of Food Engineering,42,67±72.Walstra,P.,&Jenness,R.(1984).In Dairy Chemistry and Physics.New York:Wiley.J.Straatsma et al./Journal of Food Engineering42(1999)73±7777。