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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: [email protected])bInstitut National des Nutraceutiques et des Aliments

Fonctionnels (INAF), Laval University, Quebec G1K7P4, Canada; e-mail: [email protected] 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

mailto:[email protected]





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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

357M. Aider, D.de Halleux / Trends in Food Science & Technology 18 (2007) 356 e364

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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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