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Review
Isomerization of
lactose and lactulose
production: review
Mohammed Aidera,b,* and
Damien de Halleuxc
aDepartment of Food Sciences and Technology, LavalUniversity, Quebec G1K 7P4, Canada (Department of
Food Sciences and Technology, Universite Laval,Pavillon Comtois, STA, Quebec G1K 7P4, Canada;
e-mail: mohammed.aider.1@ulaval.ca)bInstitut National des Nutraceutiques et des Aliments
Fonctionnels (INAF), Laval University, Quebec G1K7P4, Canada; e-mail: mohammed.aider.1@ulaval.cacDepartment of Food Engineering, Universite Laval,
Pavillon Comtois, Quebec G1K 7P4, Canada
Lactulose is widely used in pharmaceutical, nutraceuticals and
food industries because of its beneficial effects on human
health. Technology of lactulose production is mainly based
on the isomerization reaction of lactose in alkaline media.
However, information available on this subject is very varied.
This study is a summary of the principal techniques used for
lactulose production in order to gather maximum information
in one manuscript for a better comprehension of the
technological characteristics and specificities of lactulose
synthesis.
IntroductionSignificant part of the world population suffers from gas-
trointestinal diseases of various types. Several of these dis-
eases are caused by pathogenic bacteria which invade the
human intestine. A few days after the birth, the human in-
testine is colonized mainly by bifidobacteria which play
a very important role in the maintenance of a good health.
By changing the nutrition regime and children passage
from mother’s milk nutrition to ordinary food regime, the
pathogenic bacteria which infiltrated into the human
intestine cause diseases of various types. In order to solve
this health problem, food industry and in particular dairy
technology has developed dairy bio-products enriched
with probiotics like lactobacillus ( Lactobacillus acidophi-
lus, Lactobacillus casei, Lactobacillus bulgaricus, etc.)
and bifidobacteria ( Bifidobacteria bifidum, Bifidobacteria
longum, Bifidobacteria infantilus, Bifidobacteria adolescen-
tis) (Clark & Martin, 1994; Donkor, Nilmini, Stolic,Vasiljevic, & Shah, in press; Katz, 2006; Ninonuevo
et al., 2007; Olguin, et al., 2005; Wainwright, 2006). How-
ever, because of various reasons, this solution did not solve
the problem. These reasons could be resumed by the fol-
lowing: a great loss of bacterial cells during the production
process of different dairy products noticed by several re-
searchers, a considerable reduction of the total of bacterial
number due to storage at pH values lower than 5.5 as well
as because of the strong acid medium in the stomach (pH
y 1.5) and the negative effect of bile salts (Chou & Hou,
2000; Lankaputhra & Shah, 1995; Lian, Hsiao, & Chou,
2002). An alternative to the resolution of this problem
consists in an internal stimulation of the bifidobacteriawhich are already present in the intestine (Bouhnik
et al., 1990; Delzenne, 2003; Gibson, Beatty, Wang, &
Cummings, 1995; Mizota, Tamura, Tomita, & Okonogi,
1987). This method consists in using bifidogenic functional
food ingredients, known under general name of prebiotics
(Kaplan & Hutkins, 2000; Marteau & Boutron-Ruault,
2002; Roberfroid, 2002; Saarela, Hallamaa, Mattila-
Sandholm, & Matto, 2003; Ziemer & Gibson, 1998). These
bifidogenic ingredients stimulate the growth of bifidobacte-
ria (Tamura, Mizota, Shimamura, & Tomita, 1993). Lactu-
lose is one of these ingredients (Alander et al., 2001;
Ballongue, Schumann, & Quignon, 1997; Saarela et al.,2003). Lactulose is a synthetic disaccharide obtained by
an isomerization reaction of lactose whose milk and
lactoserum are very rich (Zokaee, Kaghazchi, Zare, &
Soleimani, 2002). The average lactose content in milk or
milk whey is approximately 4.5% (Lindmark-Mansson,
Fonden, & Pettersson, 2003). Several studies showed the
effectiveness of lactulose to stimulate the growth of bifido-
bacteria (Martin, 1996; Mizota, 1996; Sako, Matsumoto, &
Tanaka, 1999; Shin, Lee, Petska, & Ustunol, 2000;
Strohmaier, 1998). Moreover, lactulose is widely used in
pharmaceutical industry as an effective drug against differ-
ent diseases like acute and chronic constipation (Mizota,
Tamura, Tomita, & Okonogi, 1987; Tamura et al., 1993).* Corresponding author.
0924-2244/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.tifs.2007.03.005
Trends in Food Science & Technology 18 (2007) 356e364
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Taking into account all these considerations, one can
deduce the great need for a large scale production of lactu-
lose for food, nutraceuticals and pharmaceutical purposes.
The raw material for this production is largely available in
great quantity on the market (lactoserum as by-product of
the cheese industry). Annual whey production in the worldis estimated to be 72 million tons, which means that about
200,000 tons of milk proteins and 1.2 million tons of
lactose are transferred into whey annually. Even though
many uses of whey and some whey solids have been devel-
oped recently, only a little amount of the available whey
solids are utilized as ingredients in the human nutrition
and animal feed (Kosaric & Asher, 1982). Ghaly,
Ramkumar, Sadaka, and Rochon (2000) estimated that in
1998 about 137.9 million tons of whey were produced in
the world. As a particular case, in Canada, the annual
cheese production increased by 22% between 1994 and
2004. Total cheese production in Canada in 2004 was esti-mated at 0.34 million tons, which implies that over 0.27
million tons of whey was produced that year (Ferchichi,
Crabbe, Gil, Hintz, & Almadidy, 2005). Even though there
are a multitude of technological developments in the trans-
formation of milk whey to other useful products, utilisation
or disposal of whey remains one of the most significant
problem in the dairy industry (Calli & Yukselen, 2004;
Mawson, 1994).
PrebioticsPrebiotics are defined as non-digestible food ingredi-
ents that may beneficially affect the host by selectively
stimulating the growth and/or the activity of a limitednumber of bacteria in the colon. Thus, to be effective, pre-
biotics must escape digestion in the upper gastrointestinal
tract and be used by a limited number of the microorgan-
isms comprising the colonic microflora. Prebiotics are
principally oligosaccharides. They mainly stimulate the
growth of bifidobacteria, for which reason they are re-
ferred to as bifidogenic factors (Durand, 1997; Berg,
1998; Gibson & Roberfroid, 1995; Macfarlane & Cum-
mings, 1999; Roberfroid, 2000). So that a food ingredient
can be regarded as prebiotic, it must meet certain charac-
teristics which were defined gradually after the initial
work of Gibson and Roberfroid (1995). Food ingredientcan be regarded as prebiotic if it satisfies some criteria
(Gibson & Roberfroid, 1995): not digestible nor absorbed
before reaching the colon; to be a selective substrate of
one or several (preferably a low number) bacteria having
a probable or definitively established beneficial role; to be
able to modify the composition of the colic flora for better
health by supporting the growth and/or the metabolic ac-
tivity of Lactobacillus sp. or Bifidobacteria sp. (Gibson &
Roberfroid, 1995); more rarely by attenuating the viru-
lence of pathogenic bacteria like Listeria monocytogenes
(Park & Kroll, 1993).
Some researchers reported some information, where the
role of the prebiotics is not totally clear. In was postulated
that prebiotic ingestion may contribute to normalize the
gastrointestinal barrier function in burn patients (Olguin
et al., 2005). This hypothesis was based on observations
that burn injury is associated with dramatic alterations of
the intestinal microbiota and gastrointestinal permeability,
and that increasing luminal lactobacilli and bifidobacteriathrough the ingestion of prebiotics or probiotics is associ-
ated with recovery of the gastrointestinal barrier function.
This postulate was based on the observation that regular
intake of Lactobacillus spp. decreased the gastric perme-
ability alterations. In relation to burn injury, a decrease
of the intestinal anaerobic microbiota, including bifidobac-
teria, has been observed in rats, while at the same time aer-
obic bacteria and fungi increase. This resulted in an
imbalance of the aerobic/anaerobic ratio and in a decrease
of colonization resistance in these animals. These changes
were associated with increased bacterial translocation and
endotoxinemia, histological lesions of the mucosa. Similaralterations have been observed in burn patients. Supple-
mentation of burn rats with a bifidobacteria preparation
reduced the imbalance of the aerobic/aerobic ratio, the
endotoxinemia and the mucosal lesions; the same prepara-
tion with bifidobacteria decreased gastrointestinal symp-
tomatology and diarrhea in humans who suffered burns
(Chen, Zhang, & Xiao, 1998; Gotteland, Cruchet, &
Verbeke, 2001; Olguin et al., 2005). Stimulation of endog-
enous lactobacilli or bifidobacteria by prebiotics may also
exert a protective effect against gastrointestinal mucosa alter-
ations. Lactosucrose, for example, has been shown to protect
against indomethacin-induced gastric ulcerations in rats
(Honda et al., 1999). Although a number of studies havebeen carried out in animal models, data are scarce in
humans. In the study reported by Olguin et al. (2005),
oligofructose, whose administration is known to dose-
dependently increase fecal bifidobacteria in humans, was
used. This prebiotic did not improve the gastrointestinal
barrier function alterations. A possible explanation for
this lack of effect is the use of high doses of antibiotics
in all these patients, which may interfere with lactoba-
cilli and bifidobacteria growth even after stimulation
by the administrated prebiotic (oligofructose). In the
case of probiotics, this may be overcome by the contin-
uous administration of these exogenous, live bacteriawhich may compensate for the mortality induced by anti-
biotics; prebiotics, however, act by stimulating the growth
of endogenous bacteria, and this is probably decreased
when these microorganisms are affected by antibiotics.
The results obtained in the above mentioned studies
may be interpreted as suggesting that prebiotics probably
are not the best option for subjects on high doses of an-
tibiotics, and that administration of probiotics or symbi-
otic, a mixture of pre and probiotics may be a better
choice for these patients (Olguin et al., 2005). However,
even if the used prebiotic showed some negative aspects,
we can not generalise this conclusion to all the prebiotics,
including lactulose. The use of antibiotics would be the
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cause of the negative effect reported in the study of Ol-
guin et al. (2005).
Lactulose productionTheoretical aspect of lactulose production
Theoretically, lactulose (Fig. 1) can be obtained start-ing from lactose (Kochetkov & Bochkov, 1967) by re-
grouping the glucose residue to the fructose molecule
with a passage form an aldose form to ketosis one.
The mechanism of this transformation can be achieved
by various manners. The first consists of the reaction
of Lobry de Bruynevan Ekenstein which is summarized
by the formation of the enolic intermediate shape of
lactose and epilactose in alkaline media with the transfor-
mation of the glucose of the lactose molecule into
fructose and gives as result molecule of lactulose
(Fig. 2). The second way consists of a reaction of lactose
with ammonia or amines. In this case, the formed lacto-sylamine undergoes a regrouping (rearrangement) of
Amadori (Fig. 3) towards the lactulosylamine (Hodge,
1955) and after hydrolysis of the complex lactulose could
be obtained. Currently, in practice, the first way of syn-
thesis is the most used with various catalysts. The energy
of activation for synthesis of lactulose by using lactose as
raw material differs according to the type of the catalyst
(European Patent No 0320670, 1990; European Patent No
0339749, 1991; U.S. Patent No 5034064, 1991).
To transform lactose into lactulose, acceptors of protons
are essential. This could be carried out by using various re-
agents which give an alkaline medium after dissolution
(European Patent No 0339749, 1991; U.S. Patent No
3814174, 1971; U.S. Patent No 5034064, 1991; U.S. Patent
No 4536221, 1985; U.S. Patent No 3514327, 1970; U.S.
Patent No 3546206, 1970). The great number of reagents
used shows well that the ideal catalyst was not found yet
for the isomerization of lactose to lactulose. This catalyst
must answer some important criteria, among which are
enumerated by the following:
It must guarantee a maximum level of isomerization
with a minimum of reaction by-products;
To be environmentally safe and not toxic;
The cost of the catalyst must be as possible low and to
be available in great quantity;
It must be easy to remove from the medium by tradi-
tional demineralization tools;
To give repetitive results of isomerization.
However, in practice, the catalysts used for the isomeri-
zation of lactose to lactulose present positive and negative
aspects. Systematic analysis of the most used catalysts for
lactulose production could be divided into three principal
groups. They are strong acids, strong bases and amphoteric
catalysts represented mainly by hydroxyls, sulphites and
borates.
Principal methods as well as the reactions which control
the process of isomerization of lactose into lactulose will be
treated in what follows.
Lactose isomerization by hydroxyls Lactulose was obtained for the first time by Montgomery
and Hudson (1930) following the heating of a solution
made up of a mixture of lactose and lime at a temperature
of 35S during several days. In order to obtain crystalline
lactulose, several stages of purification were used. Reagents
such as sulphuric acid, calcium carbonate, ethanol, metha-
nol, ether, activated carbon and bromine were used. There-
after, a new method was developed (Matvievsky, 1979;
Russian Patent No 1392104, 1988; U.S. Patent No.
3272705, 1966; Yakovleva, 1963) in which calcium hy-
droxide was used as catalyst of the isomerization reaction
of lactose to lactulose. In this method, 60% lactose solutionwas combined with 0.1% of calcium hydroxide under a
temperature of 100e102 C and reaction time of 15 min.
Thereafter, final solution was demineralised by combina-
tion of electrolysis and ion exchange resins. However,
this method was rather tiresome. In the work reported in
Patent of Germany No 222468033 (1976), the use of vari-
ous catalysts such as (Ca(OH)2, NaOH, CaO, Na2CO3,
KOH, K 2HPO4, Ba (OH)2) was reported (Dalev & Tsoneva,
1982; Lodigin, 1999; Patent of Germany No 297999, 1992;
Pereligin, Podgornov, & Sitnikov, 1999; Russian Patent No
Fig. 1. Schematic representation of lactulose molecule.
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2101358, 1998; Russian Patent No 97120603/13, 1999;
Stomberg & Semchenko, 1999; U.S. Patent No 4264763,
1978). In this study, effect of these catalysts on galactose
rate as by-product of disaccharides (lactose and/or lactu-
lose) decomposition (degradation) was also reported. Whilebeing based on results reported in Patent of Germany No
222468033 (1976), the following conclusions could be
mentioned:
Independently on the quantity of NaOH used, there ex-
ists a linear dependence between galactose rate pro-
duced as reaction by-product and lactulose rate which
represents rate the reaction isomerization rate. This rela-
tion seems to increase considerable at isomerization
rates higher than 21.4%;
Galactose/lactulose ratio remains constant for a constant
ratio NaOH/lactose independently on isomerization time
and temperature.
Basing on results reported in Patent of Germany No
222468033 (1976), an industrial application was carried
out (France Patent No 2147925, 1973; Patent of Germany
No 222468033, 1976). In the process used for the industrial
production, lactulose was purified and demineralised by an-
ion and cation exchange resins. The end product had a con-
centration of the lactulose of about 42e48% of total dry
matter. However, even if this method was effective, it re-
mains that it needed rather important material and energy
expenditure. Another method was developed in Russian
patent No 7374626 (1980). In this method, 15e20% of lac-
tose solution was mixed with 0.35e
0.45% of the calcium
hydroxide to reach pH 11. The mixture was thermostated
at 68e72 S during 15e20 min. The final solution pH
was 8.8e9.0. Thereafter, solution was neutralized with cit-
ric acid up to pH 5.5e6.5. Citric acid quantity added in
a form of saturated solution was 0.115e0.125%. The pH
decrease was carried out to avoid an autocatalytic degrada-tion of lactulose and to facilitate partial demineralization of
final solution by means of calcium citrate formation (cal-
cium complexation) removed by centrifugation.
An additional operation of demineralization with of ions
exchange resins was necessary.
Effect of lactose concentration . According to fundamen-
tal laws of chemical kinetics, the lactose isomerization pro-
cess into lactulose depends on lactose concentration in the
feed solution. Systematic analysis of data on lactulose pro-
duction showed significant variability of this parameter and
it varies in the range of 5e60%. Moreover, the choice of
lactose optimal concentration in the feed solution dependsmainly on type of the catalyst. Using hydroxides (sodium,
potassium or calcium) as catalysts and in order to determine
the optimal lactose concentration giving maximum lactu-
lose reaction rate with minimum coloration of the final
solution, experiments with lactose concentrations varied
between 5% and 30% were carried out (Ryabtsova,
1992). Lactose used in this work was of food grade but
not refined. It was purified from proteins to avoid reaction
between lactose and protein amine groups. Isomerization
reaction was carried out by using sodium hydroxide as cat-
alyst with an initial solution pH of 11.0 0.2 under 70
2
S and reaction time of 20 2 min. Data reported byRyabtsova (1992) showed that lactose concentration in
the range of 5e30% did not have any significant effect
on the reaction isomerization rate. However, these same
data showed that at lactose initial concentration higher
than 20%, reaction by-products rate was higher then
when lower lactose concentration were used. Same results
were reported with calcium hydroxide as catalyst. More-
over, to avoid lactose crystallization after cooling the
solution, it is important to use concentration in the range
of 20e25%.
In one other study (Montilla, del Castillo, Sanz, &
Olano, 2005); concentrates of whole whey and ultrafiltrates
were used to produce lactulose. Whole milk whey and milk
Fig. 2. Representation of the Lobry de Bruynevan Ekenstein transformation.
Fig. 3. Schematization of the Amadori rearrangement.
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whey ultrafiltrate concentrated up to 5.2 and 7.6 fold were
used. Results indicated that the amount of produced lactu-
lose increased by increasing the concentration factor of
the raw material. Indeed, using simple milk, 0.388 and
1.17 g/100 ml were obtained after 30 and 120 min, respec-
tively, of treatment. By concentrating the whey with a con-centration factor up to 5.2 fold, the amounts of produced
lactulose were 3.41 and 4.80 g/100 ml after 30 and 120
min, respectively. Further concentration of the initial
milk whey up to a concentration factor of 7.6 fold
permitted to obtain lactulose concentrations of 5.33 and
7.15 g/100 ml after 30 and 120 min of treatment (Montilla
et al., 2005).
Effect of lactose and catalyst concentrations on solution pH . In the case of lactose isomerization into lactulose,
solution pH characterizes concentration of proton acceptorsand it plays a very important role and could be regarded as
one of the more influencing factors on the isomerization re-
action of lactose into lactulose. For this reason, combined
effect of lactose concentration and alkaline catalyst on pH
of the medium was studied by several authors. Also, this
aimed to determine optimal amount of the catalyst to main-
tain solution pH in favourable interval for a maximum
isomerization with a minimal rate of the reaction by-
products. Experimental results showed that there is no sig-
nificant difference between pH values of model solutions
composed by lactose of high purity compared with refined
lactose solutions. Moreover, analysis of experimental data
showed that independently on lactose concentration, curveof solution pH evolution shows logarithmic behavior. Dur-
ing first reaction stage, solution pH abruptly increases and
it was reported that to increase pH of lactose solution from
5.5 0.5 up to 9.0 0.2, NaOH concentration needed is
the same one independently on lactose concentration. In
[100], NaOH concentration needed to increase pH from
5.5 0.5 up to 9.0 0.2 is 0.004 0.001 M. At the second
stage of pH curve evolution, increase of pH was very low.
During period, lactose concentration had significant effect
on pH increase. It is more difficult to increase the pH of
the concentrated lactose solutions. For example, to increase
pH up to 11.0e
11.5 of lactose solution of concentration of 0.015e0.03 M it was necessary to add 0.03e0.05 M of
NaOH and for lactose solution of a concentration of 0.06
M, NaOH concentration of 0.06e0.07 M was needed. In
dilute media, no significant difference was reported on
NaOH concentration needed to increase pH from 5.5
0.5 up to 11.5 0.1 between raw and refined lactose.
But, for lactose concentration above 0.06 M, to reach the
same pH (11.5 0.1), it is necessary to use more NaOH
in the case of raw lactose then with refined lactose. This
difference is caused by the presence of minerals and nitro-
genized compounds with high buffer capacity. Similar re-
sults as those obtained with NaOH were reported when
Ca(OH)2 was used as catalyst of the isomerization reaction
of lactose into lactulose. To reach pH value of 11.0 0.2,
concentrations of 0.02e0.04 and 0.05.0.06 M of Ca(OH)2were added to 0.015e0.03 and 0.06N lactose solutions, re-
spectively (Ryabtsova, 1992). As general conclusion on re-
lationship between lactose solution concentration and
catalyst concentration, it could be resumed as follows: atpH 11.0 for each 1 M NaOH it needs 10 M of model solu-
tion lactose independently on concentration; 3e7 M NaOH
in the case of concentrated lactose solution from milk whey
dependently on the cheese process and finally 1e2 M NaOH
for lactose solution made from raw lactose of high quality.
So, we can see that by increasing lactose solution buffer
capacity, lactose/catalyst ratio decreases. In general, it is
important to know purity of lactose for a good choice of
an optimal catalyst concentration because differences be-
tween results obtained with model lactose solutions and
real solutions are significant (Ryabtsova, 1992). It was
also reported that during the isomerization of lactose tolactulose with sodium hydroxide, a high level of degrada-
tion occurred and to decrease the amount of the formed
reaction by-products, it is better to use the lower ratios of
sodium hydroxide to lactose (about 0.5% w/w). In this
case the maximum conversion of lactose to lactulose was
about 20% and total by-product is about 5e7% (Zokaee
et al., 2002).
Particularities of lactose isomerization by sodium and calcium hydroxide . Isomerization of lactose into lactu-
lose with NaOH as catalyst of the reaction was carried
out by using raw lactose solution of high quality at a con-centration of 20%. Initial solution pH was fixed at 11.0
0.2. Lactose solution was thermostated in batch mode at
70 C. Samples were analyzed for optical density, pH evo-
lution and isomerization rate which was represented by
lactose/lactulose ratio. Experimental data reported by
Ryabtsova (1992) showed that at the first stage of the isom-
erization reaction, there was a considerable growth of lactu-
lose rate with a light change of solution color which
characterizes formation of reaction by-products. The sec-
ond reaction stage was characterized by a stability followed
by a decrease of the isomerization rate. This was more in-
tense when pure lactose solutions were used. During thisstage, the decrease pH was significant and followed by an
intensification of the solution color. The pH decrease was
also intensified by the formation of reaction by-products
with an acid character. Decrease of solution pH is an indi-
cation of the decease of proton acceptors concentration
which in its turn diminishes the probability of intramolecu-
lar regrouping of lactose and its isomerization into
lactulose. Moreover, because of pH decrease, lactulose deg-
radation resulting in galactose and fructose formation could
be accelerated. Another phenomenon is also possible fol-
lowing pH decrease which is the reversible isomerization
of the lactose into lactulose through an enolic form. Same
results were reported with calcium hydroxide as catalyst
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of lactose isomerization reaction. However, calcium
hydroxide intensification of solution mixing is important
to avoid precipitation of the catalyst. In general, it appears
from systematic data analysis that maintenance of high
alkalinity of the medium is an essential condition for an
optimal isomerization rate of lactose into lactulose.
Formation of isomerization reaction by-products . Ana-
lysis of data on the variation of optical density and pH dur-
ing degradation of sugars in alkaline media showed
existence of three stages, which are closely related to
the stages of lactulose formation during lactose isomeriza-
tion. The first stage corresponds to a minimal degradation
and this period corresponds to the highest level of lactu-
lose formation. The second stage corresponds to the
growth of sugars degradation by-products. This stage is
followed by pH decrease and lactulose formation. Finally,a third stage which is characterized by a stabilization of
solution color growth even at relatively high reaction tem-
perature (72 2 C). During this third stage, pH decrease
could reach neutrality. However, even if measurement of
solution optical density could not give sufficient informa-
tion about rate and nature of the formed dyes following
lactose and/or lactulose degradation, it remains that this
information could be used for better planning of lactulose
production process, especially with regard to stages of re-
sidual lactose crystallization and lactulose refining which
is the end product of the process (Rudenko, 1999; Ru-
denko & Bobrovnik, 1999). In order to better understand
the process of color growth (dye formation) during isom-erization of lactose into lactulose, 20% lactose solution
was studied with sodium and calcium hydroxide as cata-
lyst under temperature of 72 2 C. Results reported
by Ryabtsova (1992) on optical density measurements
showed that in the region of visible spectrum, growth of
the absorbance was linear and that the maximum was re-
ported to a wavelength of 360 nm. By increasing the re-
action time, highly significant increase of the mixture
optical density was recorded. This could be explained
by the fact that in the zone of visible spectrum, different
functional groups of dyes products have identical absor-
bance spectra which differ only by intensity as reportedin Sapronov (1975). By increasing reaction time with so-
dium hydroxide as catalyst, a spectrum of absorbance to
490 nm was shifted and this could correspond to a change
of the ratio between reaction by-products. The same data
were reported using calcium hydroxide as catalyst but
with a longer reaction time compared to NaOH. Optical
density measurements of 0.5% lactose mixed with lactu-
lose solution in UV zone showed maximal absorbance at
270e280 nm, which is a zone of the absorbance of lac-
tose. Once the mixed solution of lactose/lactulose was
thermostated under temperature of 72 2 C, the shape
of the initial spectrum had changed significantly and other
spectra of absorbance appeared. By increasing reaction
time, maximum of absorbance was moved towards 260e
270 nm, which corresponds to the zone of absorbance
of lactose/lactulose degradation by-products in alkaline
media. Study of UV spectrum during lactose isomeriza-
tion into lactulose could be used as base to confirm that
the growth color in solution under reaction could becaused by degradation of reducing sugars when NaOH
was used as catalysts (Parrish, Hicks, & Doner, 1980).
Isomerization of lactose by sulphites and phosphates Sulphites and phosphates have the characteristic to pre-
vent oxidation of disaccharides and for this reason their use
as catalyst of lactose isomerization reaction into lactulose
allows the use of high temperatures and high lactose con-
centrations. According to data reported in Patent of Austria
No 288595 (1971), for lactulose production, lactose
solutions of 60e
65% were used and temperature of 80e100 S. Under these conditions, sulphites were added
at a rate of 0.05e0.05 M per kg of lactose monohydrate.
Thereafter, the mixture (lactose solution with catalyst)
was thermostated at this temperature until obtaining a con-
stant value of the optical rotation of the solution. Then, the
solution was cooled followed by crystallization of part
of residual lactose. After crystallization, the solution of
lactose/lactulose was treated by ion exchange resins for
purification from sulphites (catalyst) and organic acids.
Following this operation, another part of lactose was crys-
tallized by cooling. To accelerate the crystallization process
of lactose, it was recommended to add a sowing in the form
of fine lactose crystals for a maximum crystallization yield(Polyansky & Shestov, 1995). The end product was syrup
with 54.5% of dry matter, 38.7% of the lactulose. Galactose
and lactose contents in the final product were about 8.2%
and 3.8%, respectively. Another method for lactulose
production was reported in Patent of Great Brittan No
2031430 (1980). According to the method described by
the authors, 2.1e8.6% of phosphates was added to a satu-
rated lactose solution. The temperature of the reaction
was 104 S during 20e240 min, dependently of the ratio
lactose/catalyst. In this case, maximum isomerization rate
reported was 20%. At the end of reaction time, part of lac-
tose was crystallized by cooling and was removed from themedium by filtration. The remainder solution was diluted
up to 15% and treated by anion and cation exchange resins
for purification from organic acids formed as reaction by-
products and phosphate (catalyst). After this operation,
the solution was concentrated another time and part of lac-
tose was removed by crystallization. Other methods were
also reported (U.S. Patent No 4536221, 1985). The catalyst
used in these cases was a mixture of sodium hydroxide and
sodium sulphite at a concentration of 0.3e1% with 60%
lactose solution under a temperature of 75e80 S during
15 min. The isomerization rate reported reached 30%. At
the other hand, using 0.7% of sodium sulphite as catalyst,
isomerization rate reached 40%. In the patent described
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in U.S. Patent No 4536221 (1985), the use of magnesium
hydroxide mixed with sodium hydrosulphide in lactose so-
lution of 60e70% concentration was reported. The catalyst
concentration in this case was 0.05e0.2% and the temper-
ature used was 90e100 S.
Isomerization of lactose by aluminates and borates Using amphoteric electrolytes such aluminium hydrox-
ide or boric acid, total isomerization yield of lactose into
lactulose could reach 70e80% (European Patent
0320670, 1990; Mendicino, 1960; U.S. Patent 4273922,
1981). In these cases, the catalyst must be added at a rate
of 0.5e4 M per mole of lactose as reported in the US patent
No 4957564. In U.S. Patent No 3546206 (1970), it was re-
ported that catalyst (aluminate) was added to lactose solu-
tion. Thereafter, the mixture was heated and thermostated
a certain time and then the catalyst was removed fromthe medium using crystallization by cooling. The pH was
readjusted with HCl or aluminium hydroxide, dependently
on the case. At the end of the process, it was recommended
to isolate lactulose using methanol. Carrobi and Innocenti
(1990) proposed using of membrane to remove catalyst af-
ter isomerization. Following this work, patent was depos-
ited (U.S. Patent No 4957564, 1990). In these patents, it
was reported that 25e50% lactose solution was used. The
catalyst (sodium aluminate) was added in the form of
35e45% concentration solution. The ratio aluminate/lac-
tose was 0.3/1 up to 1/1, dependently on lactose concentra-
tion. Isomerization was carried out under a temperature of
50e
70 C during 30e
60 min. At the end of the reactiontime, solution was neutralized with 3e4 N sulphuric acid
to keep pH in the range of 4.5e8.0. Aluminium hydroxide
suspension was formed and then removed from the medium
by centrifugation followed by membrane treatment. Other
authors reported the use of sodium tetraborate, sodium hy-
droxide or triethylamine mixed with boric acid as catalyst
(Hicks, Raupp, & Smith, 1984; Mizota et al ., 1987). Crys-
tallization, pasteurization and purification operations by ion
exchange resins were necessary.
Isomerization of lactose by alkaline-substituted sepiolites De la Fuente, Juarez, de Rafael, Villamiel, and Olano
(1999) reported that strong base catalysts, prepared by
substituting a part of the Mg2þ located at the borders of
the channels of sepiolite with alkaline ions (Liþ, Naþ, K þ
and Csþ), were investigated as catalysts for the isomeriza-
tion of lactose to lactulose and epilactose. The activities ex-
hibited by alkaline-exchanged sepiolites were significantly
higher than that of natural sepiolite. The influence of
temperature, time of the reaction and catalyst loading
were also evaluated. A 20% conversion was obtained at
90 C at a catalyst loading of 15 g/l. At the other hand,
Villamiel, Corzo, Foda, Montes, and Olano (2002) reported
that alkaline-substituted (Naþ, K þ) sepiolites were used as
catalysts for the formation of lactulose in milk permeate.
Besides lactose and lactulose, other carbohydrates, such
as galactose and epilactose, produced in side reactions,
were determined. The effect of different washing cycles
of sepiolite on the isomerization of lactose and the ex-change of cations with the permeate was also investigated.
In general, the activity of the sodium sepiolite was higher
than that of potassium form. Twenty per cent of lactulose
formation (1000 mg/100 ml), with respect to the initial lac-
tose, was obtained after 150 min of reaction, using sodium
sepiolite washed during 10 cycles. Under these conditions,
25% of lactose degradation was detected, whereas small
amounts of epilactose and galactose were formed. The ex-
change of Naþ between sepiolite and permeate decreased
considerably with the number of washing cycles. The pres-
ent work shows an appropriate method for obtaining lactu-
lose in milk permeate with acceptable yields and withoutcomplicated purification steps.
Lactulose production by ion exchange resins Production of lactulose in a form of concentrated solu-
tions or powder for use as additive in functional food is
more and more of topicality. This type of product must
be at the same time safe and easy to produce, whether for
environment or human consumption. Based on this concept,
several works were carried out during the 10 last years in
order to find effective technologies for lactose isomeriza-
tion into lactulose without use of reactive agents. As it
was mentioned above, lactulose production requires useof chemicals (hydroxide, borates, aluminates, etc.) and
purification procedures by ion exchange resins.
In the Russian Patent No 2101358 (1998), anion ex-
change resins were used to intensify process isomerization
of lactose into lactulose by exploiting OH ions exchange
between solution in reaction and resins and to simplify lac-
tulose production process by using the same resins for de-
mineralization of the end product. The advantages of this
process are
There is no need to add catalyst for isomerization
process;
Demineralization stage is not used; No additional operation to purify the end product from
dyes;
He process is more profitable in comparison with tradi-
tional methods;
Lactulose produces by this technology could be used in
functional food for children, and in specialized food into
which bifidobacteria are introduced (Hramtsov et al.,
2004).
Concluding remarksLactulose production is a complex process, which is af-
fected by several operation and technological conditions.
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Most of the changes incurred to the lactulose by heating are
disadvantageous to quality of the final product. However,
degradations can be minimized by appropriate design of
the isomerization process, type of catalyst used and
lactose quality. Designing process of lactose isomeri-
zation into lactulose must be done in a comprehensiveway considering pre-isomerization, isomerization and post-
isomerization processes. Isomerization of lactose into
lactulose must be preceded by adequately chosen raw
material yields with expected quality and optimal lactose/
catalyst ratio. That quality can be maintained during lactu-
lose isolation by application of appropriate post-processing
parameters. Because of complex influence on the product
and many technological variables, which can be controlled
during processing, isomerization is a versatile way to treat-
lactose, which is an abundant product of cheese processing.
A thorough knowledge of pros and cons of the isomerization
process is needed in order to design optimal technologicalprocess for lactose isomerization into lactulose with a mini-
mum of by-products and to obtain final product of desired
quality for a variety of use.
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