Crystallization in amorphous lactose-maltodextrin mixtures
Naritchaya Potesa, Yrjö H. Roosb
a
School of Food and Nutritional Sciences, University College Cork, Cork, Ireland ([email protected])
b School of Food and Nutritional Sciences, University College Cork, Cork, Ireland ([email protected])
ABSTRACT
Solutions of maltodextrins (MD) with the dextrose equivalent (D.E.) 9-12 and D.E. 23-27 were mixed with
lactose solutions at different ratios. Freeze-dried amorphous solids of lactose and lactose-MD were prepared
from solutions containing 20% total solids. The ratios of lactose and MD were 90:10, 80:20, and 70:30
(w/w). Crystallization rate of amorphous lactose was followed from changes in water sorption properties.
Freeze-dried amorphous materials were stored over saturated salt solutions over the range 11.4 to 76.1%
relative humidity (RH) in vacuum desiccators at room temperature (20±2°C) for 25 days. Changes in weight
were followed at intervals. The glass transition temperatures (Tg) of the amorphous materials were measured
using differential scanning calorimetry (DSC) following storage over P2O5 and over saturated salt solutions
(11.4-44.1% RH) in vacuum desiccators.
At low RH (11.4-44.1% RH) all materials showed typical sorption behavior of food systems up to a RHdependent steady state water content. Lactose crystallization was affected by MD at all ratios at high RH
(54.5-76.1% RH) and the rate of crystallization decreased with increasing MD concentration. The amorphous
lactose-MD mixtures (70:30) inhibited crystallization up to 54.5% RH. The Tg of the amorphous mixtures
increased with increasing MD concentration. The mixtures of lactose-MD D.E. 9-12 (70:30) showed the
highest Tg. The lactose-MD D.E. 23-27 (90:10) showed similar Tg to amorphous lactose being the lowest Tg
for the mixtures. The amorphous lactose-MD D.E. 9-12 (90:10) showed similar Tg to the amorphous lactoseMD D.E. 23-27 (70:30) but the inhibition of crystallization of lactose was different. MD D.E. 23-27 was a
better inhibitor of crystallization of lactose than MD D.E. 9-12. Thus the crystallization of lactose was not
dependent only on Tg but it was also affected by the molecular weight of the system.
Keywords: crystallization; glass transition temperature; lactose; maltodextrin; water sorption
INTRODUCTION
Crystallization is an important factor affecting physical changes of foods containing amorphous sugars that
may occur during storage. It results from an increase in molecular mobility (decrease of viscosity) above
glass transition and forms the highly ordered crystalline equilibrium state leading to loss of water sorbed by
the amorphous material [1]. The rate of crystallization of amorphous sugars is governed by water content and
temperature of storage above the glass transition, T-Tg [2]. An increased water content leads to increased
molecular mobility, which accelerates crystallization of amorphous sugars by decreasing the Tg and water
plasticization [2, 3].
Previous studies showed that crystallization of amorphous sugars was delayed by mixing sugars with high
molecular weight carbohydrate components, e.g., starch [2, 4], MD [5, 6, 7] and corn syrup solids D.E. 20
and 42 [8]. An addition of high molecular weight components generally causes an increase in viscosity, and
the average molecular weight, and a decrease in molecular mobility of amorphous systems [9, 10]. They may
also increase the Tg value. Below the Tg, molecular mobility of amorphous materials is restricted to
vibrations and rotations due to decreased free volume and packing of molecules. However, the effect of
addition of high molecular weight components to decrease the rate of crystallization is not entirely a result of
the increased Tg of the amorphous system [2, 8] depending on the type of sugars. Roos and Karel [3]
suggested that the rate of crystallization of lactose at varying water contents and temperatures was controlled
by the Tg. Crystallization of amorphous sugars can be reduced or delayed by increasing viscosity, decreasing
diffusion, and reduced molecular mobility above the Tg [2, 11], including effects of the presence of various
other molecular species and impurities. Mazzobre et al. [12] reported that the addition of a second sugar
component, such as trehalose, to an amorphous lactose system delayed crystallization, without affecting to
the Tg value. They explained this to have resulted from lattice interference or steric hindrance effects of the
second component and effects on nucleation or crystal growth. It should, however, be noted that the Tg values
of lactose and trehalose are similar and the sugars may be mixed in any ratio with no significant effects on
the Tg of the blend.
The objective of this study was to determine the effects of D.E. and glass transition of MD with lactose on
lactose crystallization behavior in amorphous mixtures. This study is useful for understanding effects of MD
on the crystallization of lactose in food ingredients as the D.E. of MD may have an important role in
preventing sugar crystallization.
MATERIALS & METHODS
Preparation of amorphous lactose and lactose-maltodextrin mixtures
Amorphous materials were prepared by freeze-drying 20% (w/w solids) solutions of α-lactose (monohydrate,
Sigma Chemical Co., St. Louise, MO., U.S.A.) and mixtures of lactose-MD D.E. 9-12 and 23-27 (Maltrin
M100 and Maltrin M250; Grain Processing Corp., IOWA, U.S.A.) in the ratios 90:10, 80:20, and 70:30 (w/w
solutions). The solutions were frozen (5 mL in a preweighed vial, semi-closed with a septum) at -20°C for 24
h, followed by at -80°C for 3 h, and freeze-dried for 60 h at p<0.1 mbar (Lyovac GT2, Amsco Finn-Aqua
Gmbh, Hürth, Germany) to obtain amorphous materials. All vials were closed inside the freeze dryer under at
p<0.1 mbar. Freeze-dried materials in the sealed vials were stored over P2O5 in vacuum desiccators [13] at
room temperature (20±2°C) to protect samples from any water uptake.
Water sorption
Triplicate vials of each freeze-dried material were stored for 25 days over saturated solutions of LiCl,
CH3COOK, MgCl2, K2CO3, Mg(NO3)2, NaNO2, and NaCl (Sigma Chemical Co., St. Louise, MO., U.S.A.) at
11.4, 23.1, 33.2, 44.1, 54.5, 65.6, and 76.1% RH, respectively, in vacuum desiccators at room temperature.
The vials with samples were weighed at 0, 1, 3, 5, 7, 10, 24, 48, 72, 120 h, and then every 48 h during storage
to record water contents. The weights of samples were monitored for up to 25 days. All vials, when removed
from desiccators for weighing, were closed with septums to minimize water exchange with the environment.
The crystallization of amorphous lactose was followed from the loss of sorbed water. Water contents of the
materials were measured as a function of time, and the average weight of triplicate samples was used in
calculation.
Differential scanning calorimetry (DSC)
For anhydrous samples, the freeze-dried materials were transferred to preweighed DSC aluminium pans (40
µL, Mettler Toledo Schwerzenbach, Switzerland) and hermetically sealed before weighing and analysis using
a punctured pan. For varying water contents, the freeze-dried materials were transferred to preweighed DSC
aluminium pans and weighed rapidly. These unsealed pans were rehumidified for 72 h over a series of
saturated salt solutions over the range from 11.4 to 44.1% RH in vacuum desiccators at room temperature.
After equilibration the pans were hermetically sealed and reweighed. An empty punctured pan was used as a
reference. The DSC instrument was calibrated for temperature and heat flow as reported by Roos and Karel
(1991) [2]. The lids of DSC aluminium pans of anhydrous samples were punctured to allow evaporation of
any residual water during the measurement. Freeze-dried materials with varying water contents were scanned
in DSC using hermetically sealed pans. Triplicate samples of each material were analysed. Samples were
scanned from ∼30°C below to over the Tg region at 5°C/min and then cooled at 10°C/min to initial
temperature. The second heating scan was run to well above the Tg. The onset Tg values were analyzed and
recorded using STARe thermal analysis software, version 8.10 (Mettler Toledo Schwerzenbach,
Switzerland). Triplicate samples of each material were analyzed and average values of onset Tg were
calculated. The Gordon and Taylor equation (E.q. 1) was fitted to the average values of onset Tg.
Tg =
w1Tg 1 + kw2Tg 2
w1 + kw2
(1)
RESULTS & DISCUSSION
Water Sorption
At low RH (11.4-44.1 %) all materials showed typical sorption behavior of food systems up to a RHdependent steady state water content. No crystallization of lactose and lactose-MD mixtures occurred at low
RH after 25 days. Lactose crystallization was affected by MD at all ratios at high RH (54.5-76.1% RH) and
the rate of crystallization decreased with increasing MD concentration. At 54.5% RH, crystallization of
lactose and lactose-MD D.E. 23-27 (90:10, 80:20) was complete with loss of sorbed water at 120, 264, and
456 h, respectively. Amorphous lactose-MD D.E. 9-12 (90:10, 80:20) showed high variation at time of
complete loss of sorbed water. The lactose-MD mixtures (70:30) did not show crystallization within 25 days.
Therefore, the amorphous lactose-MD mixtures (70:30) showed prevented crystallization up to 54.5% RH.
Water sorption of amorphous freeze-dried lactose and lactose-MD mixtures at 65.6 and 76.1% RH at room
temperature for 25 days is shown in Figure 1. In this study the crystallization of lactose occurred most
rapidly at 65.6 and 76.1% RH. The final water contents of all amorphous freeze-dried materials increased
with increasing MD concentration. The mixtures of lactose-MD D.E. 23-27 lost sorbed water after reaching a
maximum water uptake less rapidly than lactose-MD D.E. 9-12. The MD D.E. 23-27 gave a stronger
inhibition of crystallization of lactose than MD D.E. 9-12. The appearance of lactose and lactose-MD D.E.
23-27 showed less collapse than was observed for lactose-MD D.E. 9-12 at 65.6 and 76.1% RH at the same
storage time.
Figure 1. Water sorption kinetics for freeze-dried lactose and lactose-MD mixtures at 65.6% and 76.1% RH
at room temperature.
Glass transition
The Tg values for anhydrous and humidified freeze-dried lactose and lactose-MD mixtures are shown in
Figure 2. The Tg of anhydrous lactose was 105°C in agreement with Haque and Roos [14], and the Tg was
slightly higher with increasing MD D.E. 9 concentrations in mixtures, but MD D.E. 23-27 had less effect to
values of Tg of mixtures. Anhydrous lactose-MD D.E. 9-12 showed slightly higher Tg (110-115°C) than
anhydrous lactose-MD D.E. 23-27 mixes (108-110°C). At 11.4% RH, the mixtures of lactose-MD D.E. 9-12
(53-57°C) and 23-27 (55-56°C) showed similar Tg to lactose (52°C). At 23.1 to 33.2% RH, the MD D.E. 912 and 23-27 at all ratios did not show effect on the Tg of mixtures. At 44.1% RH, the amorphous lactose had
the lowest value for Tg (13°C) in agreement with Haque and Roos [14], and the Tg values of amorphous
lactose-MD D.E. 9-12 (22°C) and 23-27 (20°C) mixtures at ratio 90:10 were slightly higher than at other
ratios. The lactose-MD D.E. 9-12 and 23-27 mixes at all ratios had similar water contents to lactose, but pure
MD D.E. 9-12 showed a higher water content than pure MD D.E. 23-27 at 11.4 to 44.1% RH as MD with a
lower D.E. contains a higher number of large molecules enhancing water sorption at low RH. This result
showed that the addition of MD D.E. 9-12 and 23-27 at ratios 90:10, 80:20, and 70:30 had minor effects on
the Tg values of the systems, which was in accordance with Roos and Karel [9], who reported that sucroseMD mixes with less than 50% (w/w) MD showed no significant increase of the Tg.
Figure 2. Glass transition temperature (Tg), water content and water activity (aw) for the freeze-dried lactose and lactoseMD systems. Experimental data are shown for lactose (+), lactose-MD (90:10) (‘), lactose-MD (80:20) (…), lactoseMD (70:30) (U), and MD (c). The solid, dashed, and long dashed dotted lines correspond to the Tg predicted by the
Gordon-Taylor equation. Data for Tg of anhydrous MD9 are from Roos and Karel [9] and data for MD D.E. 9-12 (MD 9)
and 23-27 (MD23) are our unpublished data.
The mixtures of amorphous lactose-MD D.E. 9-12 showed similar Tg to the amorphous lactose-MD D.E. 2327, but the inhibition of crystallization of lactose above the Tg was different. The crystallization of lactose
was not dependent only on the Tg of the systems. This result agreed with Gabarra and Hartel [8], who
reported that the best inhibitors of crystallization of sucrose were not the materials with the highest Tg values.
Mazzobre et al. [12] found that lactose crystallization was inhibited in freeze-dried lactose-trehalose systems
with respective ratios of 80:20 and 70:30 of solids, but the Tg values did not differ from that of lactose. An
addition of a second component may delay crystallization of lactose, as it disturbs nucleation and crystal
growth by hindering diffusion of lactose molecules. The crystallization inhibition of lactose was affected by
the glass transition of the mixes and the number average molecular weight of the system.
CONCLUSION
We have shown that the crystallization of lactose was delayed with the addition and increasing concentration
of MD. The D.E. of MD has an important role in preventing lactose crystallization. A lower molecular size
(high D.E.) of MD was a better inhibitor of crystallization of lactose. A similar Tg of amorphous lactose-MD
systems showed different inhibition effects on lactose crystallization. These results could prove that the
number of average molecular weight of an amorphous system has an effect on the crystallization rather than
the Tg values of the system alone.
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