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Glass transition temperature of regular and lactose hydrolyzed milk powders
ARTICLE IN PRESS
Lebensm.-Wiss. u.-Technol. 36 (2003) 547–551
Research note
Glass transition temperature of regular and lactose hydrolyzed milk powders Emiliano Ferna! ndez, Carolina Schebor1, Jorge Chirife* Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires 1428, Argentina Received 6 August 2002; accepted 14 January 2003
Abstract The glass transition temperatures (Tg ) of lactose hydrolyzed milk powder (HMP) was determined and compared to that of regular milk powders (MP). Some physical and chemical changes (loss of flowing ability, browning development) were also evaluated during storage of the different milk powders. Sugars of HMP (glucose, galactose, lactose) influenced, but did not define, the Tg values of the product, on the contrary lactose governed the observed Tg values of MP samples. Changes in the flowing characteristics of milk powder and color development, stored at different water activity values, were much more evident in HMP than in regular milk powder. Results suggested that the Tg value is not the unique parameter governing the flowability and color development in stored milk powders. r 2003 Swiss Society of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Milk powders may contain amorphous solids, which can undergo a glassy-to-rubbery transition when stored at temperatures above the glass transition temperature (Tg ). It has been shown that increasing the moisture content can lead to the formation of interparticle liquid bridges causing the particles to stick together (Peleg & Mannheim, 1977). Further absorption of water may result in collapse, caking and crystallization of the powder and formation of a hard mass. Physical changes (stickiness, collapse, crystallization and aroma retention) have been described as temperature–time–moisturedependent phenomena (Chirife, Karel, & Flink, 1973; Tsourouflis, Flink, & Karel, 1976; To & Flink, 1978) that may cause problems in production and storage of dehydrated food products (Lazar, Brown, Smith, & Lindquist, 1956; White & Cakebread, 1966). The quality of milk powders during storage may also be decreased by fat oxidation (Shimada, Roos, & Karel, 1991) and nonenzymatic browning (Saltmarch, Vagnini-Ferrari, & Labuza, 1981). Powders containing a considerable amount of sugars are particularly susceptible to caking or flowability loss
*Corresponding author. Fax: +54-11-4-794334. 1 Member of CONICET, Argentina.
as a result of moisture sorption or exposure to elevated temperature. The reduced flow of powder particles may lead to poor rehydration and dispersibility. Milk powders with hydrolyzed lactose contain large amounts of glucose and galactose, which render a product with lower Tg than regular milk powder (Jouppila & Roos, 1994). The main objective of the present work was to determine the glass transition temperatures of milk powder with hydrolyzed lactose (HMP) as compared to regular milk powder (MP). Some physical and chemical changes (loss of flowability, browning development) were also evaluated during storage.
2. Materials and methods 2.1. Materials The following materials were employed: (1) skim milk powder (per 100 g: 1.2 g fat, 35.3 g protein, 51.2 g lactose), (2) whole milk powder (Brand A per 100 g: 28.8 g fat, 25 g protein, 36.3 g lactose), (3) whole milk powder (Brand B per 100 g: 26 g fat, 25 g protein, 36 g lactose) and (4) whole milk powder with enzymatically hydrolyzed lactose (per 100 g: 26 g fat, 25 g protein, 8 g residual lactose, 15 g glucose, 15 g galactose). The
0023-6438/03/$30.00 r 2003 Swiss Society of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0023-6438(03)00022-7
ARTICLE IN PRESS 548
! E. Fernandez et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 547–551
composition of these commercial products is informed in the label as minimum values. It is to be noted that the composition of the powders adds up to 100 g when moisture and ash contents are taken into account. Samples (2), (3) and (4) were defatted (Soxhlet extraction of the milk samples with ethyl ether for 48 h) prior to Tg determination. Milk powders were purchased from a local store. Amorphous sugar models having a composition which resembles that of hydrolyzed milk were also prepared and consisted of (on a dry basis): 22 g lactose/100 g (from Mallinckrodt Chem. Works, St. Louis, MO), 39 g glucose/100 g (from Mallinckrodt Chem. Works, St. Louis, MO), and 39 g galactose/100 g (Anedra, Buenos Aires). Distilled water was used for all experiments. 2.2. Preparation of model systems The amorphous systems were obtained by freezedrying solutions containing 11 g/100 g of the different model systems. Aliquots of 1 mL of the solution were placed in 5 mL capacity vials, frozen 24 h at –26 C, immersed in liquid nitrogen and freeze-dried. The freezedrying process lasted 48 h. A Heto–Holten A/S, cooling trap model CT110 freeze-dryer (Heto Lab Equipment, Denmark) was used which operated at –110 C and at a chamber pressure of 4 104 mbar. The freeze-dried samples were transferred into desiccators and kept at 26 C over saturated salt solutions that provided constant relative humidities (RH) between 11% and 43% (Greenspan, 1977) or were exposed to dessicant. 2.3. Determination of total water content The water content of the humidified samples was determined (in duplicate samples) by difference in weight before and after drying in a vacuum oven at 70 C during 48 h in the presence of desiccant. 2.4. Differential scanning calorimetry (DSC) DSC was used to determine glass transition temperatures (Tg ). Glass transitions were recorded as the onset temperature of the discontinuities in the curves of heat flow versus temperature. In the low RH range (0–43%) commercial milk products show a fat melting endotherm that hinders the glass transition. For this reason, milk powders were previously defatted (Jouppila & Roos, 1994). The instrument used was a DSC 30 Mettler TA 4000 Thermal Analysis System with a TC11 TA processor and Graph Ware TA72 thermal analysis software. The instrument was calibrated using indium. All measurements were made at 10 C/min, using hermetically sealed aluminum pans (Mettler, 40 mL
capacity), and an empty pan was used as a reference. An average value of two replicate samples was reported. 2.5. Determination of water activity The water activity of the samples was determined with an Aqualab dew-point hygrometer, Decagon Devices, Inc, Pullman, Washington, USA (Schebor & Chirife, 2000). 2.6. Determination of flowability characteristics Milk powder samples were poured in 4 cm diameter plastic dishes and placed in desiccators at different relative humidities at 37 C. At different times, samples were removed and flow behavior classified in an arbitrary scale from 1 to 5, being 5 a free flowing powder (control) and 1 a hard, not flowing mass. 2.7. Determination of nonenzymatic browning Nonenzymatic browning was recordered as the color developed by milk samples stored during 1 month at 37 C at different relative humidities (eight replicates were measured). A spectrophotometer Minolta 508-d (Minolta Co. Ltd. Japan) with integrating sphere was used to measure L ; a and b values of the CIELAB chromatic space of powders during storage. A standard calibration with white and black references was performed with illuminant D65 and an observation angle of 2 . To perform the measurements, samples were placed in a circular cell of 3 cm diameter. Color differences between control (commercial conditions) and stored samples was expressed as DEab ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi DL 2 þDa 2 þDb2 (Lozano, 1979).
3. Results and discussion Tg values of regular milk powder (MP), skim milk powder (SMP), and milk powder with HMP equilibrated at different relative humidities were determined by differential scanning calorimetry. It is to be noted that Tg values corresponding to regular milk powders could not be determined by DSC due to melting peaks of fat, and for that reason these products were previously defatted, as described before. Fig. 1a illustrates the thermograms showing the glass transition temperature of MP, SMP and HMP at 11% RH; values were 61 C, 62 C, and 36 C, (onset values), respectively. Fig. 1b shows Tg values versus water content (g/kg, dry basis) for different MP samples, HMP and sugar model systems. Tg values of HMP were dramatically lower than those observed for MP. This behavior was also observed by Jouppila and Roos (1994) in freeze-dried milk powders, and they attributed this fact to the
ARTICLE IN PRESS ! E. Fernandez et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 547–551
20
0
40
60
80
100
°C
(a) 100
Tg °C
50
0
-50 0 (b)
25
50
75
100
Water content (g/Kg d.b.)
Fig. 1. DSC thermograms from milk samples: milk powder (MP), skim milk powder (SMP) and milk powder with hydrolyzed lactose (MPH), equilibrated at 11% RH (at 26 C), showing the glass transition (a). Effect of moisture content on glass transition temperature (onset values) of milk powder: MP A (’), MP B (m); milk powder with hydrolyzed lactose: MPH (K), lactose (J); lactose:glucose:galactose (22:39:39): (&) equilibrated different RH at 26 C (b).
presence of glucose and galactose in the hydrolyzed milk samples. It is known that monosaccharides (glucose and galactose) have much lower glass transition temperatures than the disaccharide lactose (Roos, 1993). It is to be noted that Tg values corresponding to regular milk powders were similar to those obtained for lactose alone; similar results were obtained by Jouppila and Roos (1994). Burin, Buera, Hough, and Chirife (2002); Shimada et al. (1991), Karmas, Buera, and Karel (1992) and Labrousse, Roos, and Karel (1992) reported that the Tg values of food model systems based on lactose were similar to that of lactose which was their main component. It was suggested that either lactose governed the Tg of the system (Jouppila & Roos, 1994) or lactose and other components may exist in different phases, as immiscible compounds, and DSC method could only detect the carbohydrate-rich phase. Jouppila and Roos (1994) reported that the Tg values of freezedried skim milk powders with hydrolyzed lactose were higher than those of anhydrous galactose and glucose,
549
and suggested that incomplete hydrolysis of lactose would render a system with a high Tg component that should increase the Tg of a carbohydrate mixture. However, our model system (lactose:glucose:galactose) had the same sugar composition that hydrolyzed milk; and we observed that HMP had higher Tg values than those of the sugar model. This may indicate that the presence of proteins and other milk components may affect differently the glass transition behavior of lactose and the monosaccharides (glucose+galactose). Table 1 shows the flow behavior of commercial spraydried MP and HMP exposed to different relative humidities during 4 weeks at 37 C. As mentioned before, the samples were rated from 5 (free-flowing) to 1 (hard-not flowing mass). Important flowability changes for HMP samples started at aw : 0.22 (e.g. 3 in the scale adopted), and increased for higher aw values, the samples being almost a hard mass at aw : 0.43. For MP samples, noticeable changes were only observed at aw : 0.43. Non enzymatic browning development was also recorded in spray-dried MP and HMP samples after 4 weeks storage at 37 C (Fig. 2). HMP samples showed higher color development than MP samples, and it increased drastically at T Tg > 0: It is to be noted that the molar concentration of reducing sugars is doubled in lactose hydrolysis. Also, the higher color development in HMP samples could be due to the greater reactivity of glucose and galactose as compared to lactose (Buera, Chirife, & Resnik, 1990). Schebor, Buera, Karel, and Chirife (1999) observed noticeable browning development
Table 1 Effect of water activity and time of storage at 37 C on flowability of MP and MPH aw
Time
0.11
Week Week Week Week
0.22
MP
MPH
1 2 3 4
5 5 5 5
5 5 5 5
Week Week Week Week
1 2 3 4
5 5 5 5
4 4 3 3
0.33
Week Week Week Week
1 2 3 4
5 4 4 4
3 3 3 3
0.43
Week Week Week Week
1 2 3 4
4 3 3 3
3 2 1 1
1—Hard mass, not flowing; 5—free flowing.
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550
20 T= 37°C
∆E
15
-24°C
MP MPH
10 -10°C 5 50°C 36°C 41°C 6°C 32°C 0
0.11
0.22
0.33
6°C
0.43
aw Fig. 2. Nonenzymatic browning development in spray-dried MP and MPH samples stored at 37 C during 4 weeks at different RH values. Tg value for each sample is indicated over each bar.
in glassy skim milk powder stored at 60 C, showing that Tg does not completely account for the development of browning reactions. It is likely that in dried systems browning may have been facilitated because of the proximity of reactant molecules (sugars and amino compounds) brought about by their relatively high concentration in the dried state. Samples corresponding to commercial spray-dried MP (aw of the product in the original package=0.20) and HMP (aw of the product in the package=0.15) were placed in hermetically sealed petri dishes and stored during 1 month at 37 C. Stickiness and caking are believed to be mainly governed by T Tg (Aguilera et al., 1995; Peleg, 1993). The Tg value of HMP at the aw corresponding to the commercial product is near 37 C, and the physical aspect of this sample was not modified after 1-month storage. However, stickiness is a temperature–moisture–time-dependent phenomenon, and 1 month was probably not enough to observe changes. The physical aspect of regular MP was not modified during 1-month storage at 37 C, and this may be in part attributed to its higher Tg : The sugars (monosaccharides+lactose) present in HMP samples influenced but could not define completely the Tg values of the product, while apparently lactose governed Tg values of regular MP samples. If HMP samples were stored exposed to ambient conditions (open package), visible changes in the flowability and color development could be evident, even at low ambient relative humidity values (e.g. 22%). Although HMP samples showed physical and chemical changes more important than MP samples, these changes could not be deduced from their Tg values only. HMP samples, having a (T Tg ) as high as 50 C, showed visible flowability loss, but collapse and caking were not observed. This behavior can be explained by the presence of nonsugar components (proteins, fat), which affect the physical state of the dried matrix. Whereas nonenzymatic browning was favored by the
higher molecular mobility (i.e. lower Tg values in HMP samples), the presence of highly reactive monosaccharides also enhanced the development of this reaction. We may conclude that in complex food systems, such as milk powder, the Tg values of their main carbohydrates do not account completely for the observed behavior regarding the flowing characteristics and the development of chemical reactions as nonenzymatic browning. The presence of other components in these food systems (proteins, fat) may also play an important role in the physical and the chemical stability.
Acknowledgements The authors acknowledge finantial support from the ! Cient!ıfica y TecnolAgencia Nacional de Promocion ! ogica (Proyecto 06251, pre! stamo BID 1201/OC-AR).
References Buera, M. P., Chirife, J., & Resnik, S. (1990). Nonenzymatic nonoxidative browning in hydrolyzed shelf-stable concentrated cheese whey. Journal of Food Science, 55, 697. Burin, L., Buera, M. P., Hough, G., & Chirife, J. (2002). Thermal resistance of b-galactosidase in dehydrated dairy model systems as affected by physical and chemical changes. Food Chemistry, 76, 423–430. Chirife, J., Karel, M., & Flink, J. (1973). Studies on mechanisms of retention of volatile in freeze-dried food models: The system PVPn-propanol. Journal of Food Science, 38, 671–674. Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards, 81(A), 89–96. Jouppila, K., & Roos, Y. (1994). Glass transitions and crystallization in milk powders. Journal of Dairy Science, 77, 2907–2915. Karmas, R., Buera, M. P., & Karel, M. (1992). Effect of glass transition on rates of non-enzymatic browning in food systems. Journal of Agricultural and Food Chemistry, 40, 873–879. Labrousse, S., Roos, Y., & Karel, M. (1992). Collapse and crystallization in amorphous matrices with encapsulated compounds. Sciences des Aliments, 12, 575–769. Lazar, M., Brown, A., Smith, G., & Lindquist, F. (1956). Food Technology, 10, 129–134. ! y Ciencia, 39, Lozano, R. (1979). Diferencias de color. Investigacion 8–14. Peleg, M., & Mannheim, C. (1977). The mechanism of caking of onion powder. Journal of Food Processing and Preservation, 1, 3–11. Roos, Y. (1993). Melting and glass transition of low molecular weight carbohydrates. Carbohydrate Research, 238, 39–48. Saltmarch, M., Vagnini-Ferrari, M., & Labuza, T. (1981). Theoretical basis and application of kinetics to browning in spray-dried whey food systems. Progresses in Food Nutrition and Science, 5, 331. Schebor, C., & Chirife, J. (2000). A survey of water activity and pH values in fresh pasta packed under modified atmosphere elaborated in Argentina and Uruguay. Journal of Food Protection, 63, 965. Schebor, C., Buera, M. P., Karel, M., & Chirife, J. (1999). Color formation due to non-enzymatic browning in amorphous, glassy, anhydrous, model systems. Food Chemistry, 65, 427–432.
ARTICLE IN PRESS ! E. Fernandez et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 547–551 Shimada, Y., Roos, Y., & Karel, M. (1991). Oxidation of methyl linoleate encapsulated in amorphous lactose-based food model. Journal of Agriculture and Food Chemistry, 39, 637. To, E., & Flink, J. (1978). ‘‘Collapse’’, a structural transition in freezedried carbohydrates III Prerequisite of recrystallization. Journal of Food Technology, 13, 586–594.
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Tsourouflis, S., Flink, J., & Karel, M. (1976). Loss of structure in freeze-dried carbohydrates solutions: Effect of temperature, moisture content and composition. Journal of the Science of Food and Agriculture, 27, 509–519. White, G., & Cakebread, S. (1966). The glassy state in certain sugarcontaining food products. Journal of Food Technology, 1, 73.
Lebensm.-Wiss. u.-Technol. 36 (2003) 547–551
Research note
Glass transition temperature of regular and lactose hydrolyzed milk powders Emiliano Ferna! ndez, Carolina Schebor1, Jorge Chirife* Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires 1428, Argentina Received 6 August 2002; accepted 14 January 2003
Abstract The glass transition temperatures (Tg ) of lactose hydrolyzed milk powder (HMP) was determined and compared to that of regular milk powders (MP). Some physical and chemical changes (loss of flowing ability, browning development) were also evaluated during storage of the different milk powders. Sugars of HMP (glucose, galactose, lactose) influenced, but did not define, the Tg values of the product, on the contrary lactose governed the observed Tg values of MP samples. Changes in the flowing characteristics of milk powder and color development, stored at different water activity values, were much more evident in HMP than in regular milk powder. Results suggested that the Tg value is not the unique parameter governing the flowability and color development in stored milk powders. r 2003 Swiss Society of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Milk powders may contain amorphous solids, which can undergo a glassy-to-rubbery transition when stored at temperatures above the glass transition temperature (Tg ). It has been shown that increasing the moisture content can lead to the formation of interparticle liquid bridges causing the particles to stick together (Peleg & Mannheim, 1977). Further absorption of water may result in collapse, caking and crystallization of the powder and formation of a hard mass. Physical changes (stickiness, collapse, crystallization and aroma retention) have been described as temperature–time–moisturedependent phenomena (Chirife, Karel, & Flink, 1973; Tsourouflis, Flink, & Karel, 1976; To & Flink, 1978) that may cause problems in production and storage of dehydrated food products (Lazar, Brown, Smith, & Lindquist, 1956; White & Cakebread, 1966). The quality of milk powders during storage may also be decreased by fat oxidation (Shimada, Roos, & Karel, 1991) and nonenzymatic browning (Saltmarch, Vagnini-Ferrari, & Labuza, 1981). Powders containing a considerable amount of sugars are particularly susceptible to caking or flowability loss
*Corresponding author. Fax: +54-11-4-794334. 1 Member of CONICET, Argentina.
as a result of moisture sorption or exposure to elevated temperature. The reduced flow of powder particles may lead to poor rehydration and dispersibility. Milk powders with hydrolyzed lactose contain large amounts of glucose and galactose, which render a product with lower Tg than regular milk powder (Jouppila & Roos, 1994). The main objective of the present work was to determine the glass transition temperatures of milk powder with hydrolyzed lactose (HMP) as compared to regular milk powder (MP). Some physical and chemical changes (loss of flowability, browning development) were also evaluated during storage.
2. Materials and methods 2.1. Materials The following materials were employed: (1) skim milk powder (per 100 g: 1.2 g fat, 35.3 g protein, 51.2 g lactose), (2) whole milk powder (Brand A per 100 g: 28.8 g fat, 25 g protein, 36.3 g lactose), (3) whole milk powder (Brand B per 100 g: 26 g fat, 25 g protein, 36 g lactose) and (4) whole milk powder with enzymatically hydrolyzed lactose (per 100 g: 26 g fat, 25 g protein, 8 g residual lactose, 15 g glucose, 15 g galactose). The
0023-6438/03/$30.00 r 2003 Swiss Society of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0023-6438(03)00022-7
ARTICLE IN PRESS 548
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composition of these commercial products is informed in the label as minimum values. It is to be noted that the composition of the powders adds up to 100 g when moisture and ash contents are taken into account. Samples (2), (3) and (4) were defatted (Soxhlet extraction of the milk samples with ethyl ether for 48 h) prior to Tg determination. Milk powders were purchased from a local store. Amorphous sugar models having a composition which resembles that of hydrolyzed milk were also prepared and consisted of (on a dry basis): 22 g lactose/100 g (from Mallinckrodt Chem. Works, St. Louis, MO), 39 g glucose/100 g (from Mallinckrodt Chem. Works, St. Louis, MO), and 39 g galactose/100 g (Anedra, Buenos Aires). Distilled water was used for all experiments. 2.2. Preparation of model systems The amorphous systems were obtained by freezedrying solutions containing 11 g/100 g of the different model systems. Aliquots of 1 mL of the solution were placed in 5 mL capacity vials, frozen 24 h at –26 C, immersed in liquid nitrogen and freeze-dried. The freezedrying process lasted 48 h. A Heto–Holten A/S, cooling trap model CT110 freeze-dryer (Heto Lab Equipment, Denmark) was used which operated at –110 C and at a chamber pressure of 4 104 mbar. The freeze-dried samples were transferred into desiccators and kept at 26 C over saturated salt solutions that provided constant relative humidities (RH) between 11% and 43% (Greenspan, 1977) or were exposed to dessicant. 2.3. Determination of total water content The water content of the humidified samples was determined (in duplicate samples) by difference in weight before and after drying in a vacuum oven at 70 C during 48 h in the presence of desiccant. 2.4. Differential scanning calorimetry (DSC) DSC was used to determine glass transition temperatures (Tg ). Glass transitions were recorded as the onset temperature of the discontinuities in the curves of heat flow versus temperature. In the low RH range (0–43%) commercial milk products show a fat melting endotherm that hinders the glass transition. For this reason, milk powders were previously defatted (Jouppila & Roos, 1994). The instrument used was a DSC 30 Mettler TA 4000 Thermal Analysis System with a TC11 TA processor and Graph Ware TA72 thermal analysis software. The instrument was calibrated using indium. All measurements were made at 10 C/min, using hermetically sealed aluminum pans (Mettler, 40 mL
capacity), and an empty pan was used as a reference. An average value of two replicate samples was reported. 2.5. Determination of water activity The water activity of the samples was determined with an Aqualab dew-point hygrometer, Decagon Devices, Inc, Pullman, Washington, USA (Schebor & Chirife, 2000). 2.6. Determination of flowability characteristics Milk powder samples were poured in 4 cm diameter plastic dishes and placed in desiccators at different relative humidities at 37 C. At different times, samples were removed and flow behavior classified in an arbitrary scale from 1 to 5, being 5 a free flowing powder (control) and 1 a hard, not flowing mass. 2.7. Determination of nonenzymatic browning Nonenzymatic browning was recordered as the color developed by milk samples stored during 1 month at 37 C at different relative humidities (eight replicates were measured). A spectrophotometer Minolta 508-d (Minolta Co. Ltd. Japan) with integrating sphere was used to measure L ; a and b values of the CIELAB chromatic space of powders during storage. A standard calibration with white and black references was performed with illuminant D65 and an observation angle of 2 . To perform the measurements, samples were placed in a circular cell of 3 cm diameter. Color differences between control (commercial conditions) and stored samples was expressed as DEab ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi DL 2 þDa 2 þDb2 (Lozano, 1979).
3. Results and discussion Tg values of regular milk powder (MP), skim milk powder (SMP), and milk powder with HMP equilibrated at different relative humidities were determined by differential scanning calorimetry. It is to be noted that Tg values corresponding to regular milk powders could not be determined by DSC due to melting peaks of fat, and for that reason these products were previously defatted, as described before. Fig. 1a illustrates the thermograms showing the glass transition temperature of MP, SMP and HMP at 11% RH; values were 61 C, 62 C, and 36 C, (onset values), respectively. Fig. 1b shows Tg values versus water content (g/kg, dry basis) for different MP samples, HMP and sugar model systems. Tg values of HMP were dramatically lower than those observed for MP. This behavior was also observed by Jouppila and Roos (1994) in freeze-dried milk powders, and they attributed this fact to the
ARTICLE IN PRESS ! E. Fernandez et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 547–551
20
0
40
60
80
100
°C
(a) 100
Tg °C
50
0
-50 0 (b)
25
50
75
100
Water content (g/Kg d.b.)
Fig. 1. DSC thermograms from milk samples: milk powder (MP), skim milk powder (SMP) and milk powder with hydrolyzed lactose (MPH), equilibrated at 11% RH (at 26 C), showing the glass transition (a). Effect of moisture content on glass transition temperature (onset values) of milk powder: MP A (’), MP B (m); milk powder with hydrolyzed lactose: MPH (K), lactose (J); lactose:glucose:galactose (22:39:39): (&) equilibrated different RH at 26 C (b).
presence of glucose and galactose in the hydrolyzed milk samples. It is known that monosaccharides (glucose and galactose) have much lower glass transition temperatures than the disaccharide lactose (Roos, 1993). It is to be noted that Tg values corresponding to regular milk powders were similar to those obtained for lactose alone; similar results were obtained by Jouppila and Roos (1994). Burin, Buera, Hough, and Chirife (2002); Shimada et al. (1991), Karmas, Buera, and Karel (1992) and Labrousse, Roos, and Karel (1992) reported that the Tg values of food model systems based on lactose were similar to that of lactose which was their main component. It was suggested that either lactose governed the Tg of the system (Jouppila & Roos, 1994) or lactose and other components may exist in different phases, as immiscible compounds, and DSC method could only detect the carbohydrate-rich phase. Jouppila and Roos (1994) reported that the Tg values of freezedried skim milk powders with hydrolyzed lactose were higher than those of anhydrous galactose and glucose,
549
and suggested that incomplete hydrolysis of lactose would render a system with a high Tg component that should increase the Tg of a carbohydrate mixture. However, our model system (lactose:glucose:galactose) had the same sugar composition that hydrolyzed milk; and we observed that HMP had higher Tg values than those of the sugar model. This may indicate that the presence of proteins and other milk components may affect differently the glass transition behavior of lactose and the monosaccharides (glucose+galactose). Table 1 shows the flow behavior of commercial spraydried MP and HMP exposed to different relative humidities during 4 weeks at 37 C. As mentioned before, the samples were rated from 5 (free-flowing) to 1 (hard-not flowing mass). Important flowability changes for HMP samples started at aw : 0.22 (e.g. 3 in the scale adopted), and increased for higher aw values, the samples being almost a hard mass at aw : 0.43. For MP samples, noticeable changes were only observed at aw : 0.43. Non enzymatic browning development was also recorded in spray-dried MP and HMP samples after 4 weeks storage at 37 C (Fig. 2). HMP samples showed higher color development than MP samples, and it increased drastically at T Tg > 0: It is to be noted that the molar concentration of reducing sugars is doubled in lactose hydrolysis. Also, the higher color development in HMP samples could be due to the greater reactivity of glucose and galactose as compared to lactose (Buera, Chirife, & Resnik, 1990). Schebor, Buera, Karel, and Chirife (1999) observed noticeable browning development
Table 1 Effect of water activity and time of storage at 37 C on flowability of MP and MPH aw
Time
0.11
Week Week Week Week
0.22
MP
MPH
1 2 3 4
5 5 5 5
5 5 5 5
Week Week Week Week
1 2 3 4
5 5 5 5
4 4 3 3
0.33
Week Week Week Week
1 2 3 4
5 4 4 4
3 3 3 3
0.43
Week Week Week Week
1 2 3 4
4 3 3 3
3 2 1 1
1—Hard mass, not flowing; 5—free flowing.
ARTICLE IN PRESS ! E. Fernandez et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 547–551
550
20 T= 37°C
∆E
15
-24°C
MP MPH
10 -10°C 5 50°C 36°C 41°C 6°C 32°C 0
0.11
0.22
0.33
6°C
0.43
aw Fig. 2. Nonenzymatic browning development in spray-dried MP and MPH samples stored at 37 C during 4 weeks at different RH values. Tg value for each sample is indicated over each bar.
in glassy skim milk powder stored at 60 C, showing that Tg does not completely account for the development of browning reactions. It is likely that in dried systems browning may have been facilitated because of the proximity of reactant molecules (sugars and amino compounds) brought about by their relatively high concentration in the dried state. Samples corresponding to commercial spray-dried MP (aw of the product in the original package=0.20) and HMP (aw of the product in the package=0.15) were placed in hermetically sealed petri dishes and stored during 1 month at 37 C. Stickiness and caking are believed to be mainly governed by T Tg (Aguilera et al., 1995; Peleg, 1993). The Tg value of HMP at the aw corresponding to the commercial product is near 37 C, and the physical aspect of this sample was not modified after 1-month storage. However, stickiness is a temperature–moisture–time-dependent phenomenon, and 1 month was probably not enough to observe changes. The physical aspect of regular MP was not modified during 1-month storage at 37 C, and this may be in part attributed to its higher Tg : The sugars (monosaccharides+lactose) present in HMP samples influenced but could not define completely the Tg values of the product, while apparently lactose governed Tg values of regular MP samples. If HMP samples were stored exposed to ambient conditions (open package), visible changes in the flowability and color development could be evident, even at low ambient relative humidity values (e.g. 22%). Although HMP samples showed physical and chemical changes more important than MP samples, these changes could not be deduced from their Tg values only. HMP samples, having a (T Tg ) as high as 50 C, showed visible flowability loss, but collapse and caking were not observed. This behavior can be explained by the presence of nonsugar components (proteins, fat), which affect the physical state of the dried matrix. Whereas nonenzymatic browning was favored by the
higher molecular mobility (i.e. lower Tg values in HMP samples), the presence of highly reactive monosaccharides also enhanced the development of this reaction. We may conclude that in complex food systems, such as milk powder, the Tg values of their main carbohydrates do not account completely for the observed behavior regarding the flowing characteristics and the development of chemical reactions as nonenzymatic browning. The presence of other components in these food systems (proteins, fat) may also play an important role in the physical and the chemical stability.
Acknowledgements The authors acknowledge finantial support from the ! Cient!ıfica y TecnolAgencia Nacional de Promocion ! ogica (Proyecto 06251, pre! stamo BID 1201/OC-AR).
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