Influence of heating conditions in continuous-flow microwave or tubular heat exchange systems on the vitamin B1 and B2 content of milk

Lait 80 (2000) 601–608
601
© INRA, EDP Sciences
Original article
Influence of heating conditions in continuous-flow
microwave or tubular heat exchange systems
on the vitamin B and B content of milk
1
2
Isabel SIERRA, Concepción VIDAL-VALVERDE*
Instituto de Fermentaciones Industriales (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain
(Received 17 January 2000; accepted 10 May 2000)
Abstract — The effect of continuous-flow microwave heating of milk on the stability of vitamins B1
and B was determined by ion-pair reverse-phase high-performance liquid chromatography. Results
2
were compared with those obtained using a conventional process having the same heating, holding
and cooling phases. When milk was heated in a continuous microwave heating system, at 90 °C
without a holding phase, no vitamin B and vitamin B losses were observed. However, when the hold-
1
2
ing time was raised to 30 s or 60 s, while the content of vitamin B was not modified, the content of
2
vitamin B was lowered (3% and 5%, respectively). Analogous results were obtained when the milk
1
was submitted to a similar heating process using a conventional system. These results indicate that
continuous-flow microwave heating of milk at high temperature does not offer any advantage with
respect to the vitamin B and B retention compared with a conventional heating process having the
1
2
same heating, holding and cooling times.
vitamin B / vitamin B / milk / microwave heating
1
2
Résumé — Étude de l’effet des différentes conditions de chauffage dans un micro-ondes à flux
continu et dans des systèmes tubulaires d’échange de chaleur sur la teneur en vitamines B et
1
B du lait. L’effet du chauffage dans un micro-ondes à flux continu sur la stabilité des vitamines B
2
1
et B du lait a été déterminé par HPLC en phase reverse. Il a été comparé aux résultats obtenus par
2
la méthode conventionnelle avec les mêmes phases de chauffage, maintien à température et refroi-
dissement. Le chauffage du lait dans un micro-ondes à flux continu à une température de 90 °C et sans
maintien à température ne conduit pas à des pertes en vitamines B et B . Par contre, si le temps de
1
2
maintien à température est de 30 ou 60 s, la teneur en vitamine B est maintenue tandis que celle en
2
vitamine B est diminuée (3 % et 5 % respectivement). Des résultats similaires ont été obtenus en uti-
1
lisant la méthode conventionnelle. Cette étude met en évidence que le chauffage du lait dans un
micro-ondes à flux continu n’offre aucun avantage sur la conservation des vitamines B et B
1
2
* Correspondence and reprints
Tel.: (34) 91 5622900; fax: (34) 91 5644853; e-mail: ificv12 @ifi.csic.es

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602
I. Sierra, C. Vidal-Valverde
en comparaison à la méthode conventionnelle utilisant les mêmes phases de chauffage, maintien à tem-
pérature et refroidissement.
vitamine B / vitamine B / lait / chauffage par micro-ondes
1
2
1. INTRODUCTION
significantly to the dairy intake of most of
them [13, 14, 20]. However, due to the fact
The technique of heating using microwaves
that vitamins are very sensitive nutrients,
offers a number of advantages over con-
the most serious threat to the nutritional
ventional methods of processing foods. This
value of milk by processing is the destruc-
is mainly due to the rapid temperature rise
tion of these nutrients [15]. Like every heat
and to the ability of microwaves to pene-
treatment, microwave energy can influence
trate the product and heat the bulk of the
the vitamin content of milk. A detailed
food [4]. However, although microwaves
review about the effect of heating milk in
could be used in any unit process involving
domestic microwave ovens on several vita-
the application of heat, in practice other
mins can be found in Sieber et al. [17]. More
forms of heating may be more suited for
scarce is the information found in the liter-
technical or economic reasons. Advantages
ature about the effect of continuous-flow
and disadvantages must be carefully con-
microwave processing of milk on its vita-
sidered to ensure a successful application
min content. In a previous paper [18] con-
of microwaves [6].
tinuous-flow microwave heating of milk at
The heating process using microwaves
85 °C was compared favourably with a con-
has been described as offering great poten-
ventional heating process using a labora-
tial benefits to the dairy industry. Processes
tory scale plate heat exchanger at 80 °C,
where microwaves can be applied include
because it produced lower vitamin B losses
1
tempering, pasteurisation, sterilisation, cook-
(0 and 4% loss, respectively). Taking into
ing and drying [22, 26]. The introduction
account that the conventional system had
of alternative methods for heating milk has
higher time requirements to achieve the pre-
underlined the importance of evaluating the
scribed heating temperature, these results
effects produced as a result of these new
were attributed to the shorter residence time
processes, in order to establish the condi-
for the milk in the microwave system as
tions that provide the desired degree of
well as the lack of hot surfaces contacting
safety with a minimum loss in product qual-
the milk. However, as has been proved in
ity. In this sense, microwave heating of milk
the case of other milk nutrients, there is no
has been studied primarily to determine inac-
evidence of less destruction of vitamin B1
tivation of milkborne pathogens [1–3, 5]
taking place during the come-up time when
and to evaluate its suitability for use in pas-
milk is heated at higher temperatures whit a
teurisation [7–10, 12, 24, 25]. Thus, in a
microwave system as compared with a con-
recent study, the use of a continuous
ventional unit having the same residence
microwave system, including a holding
time.
phase to maintain the time and temperature
In the case of other milk vitamins, i.e.
conditions achieved in the microwave oven,
vitamin B , the stability of this vitamin in
proved to be an effective system for pas-
2
buffered solutions heated with microwaves
teurising milk [10].
and conventional systems has been demon-
It is well known that milk is a very rich
strated previously [21]. In milk, vitamin B2
source of vitamins and it contributes
losses after microwave heating using a batch

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Different heating conditions on milk vitamins
603
process have no been observed [11, 17]
2.2. Microwave heating system
however, as in the case of vitamin B , there
1
are no studies about the effect of continu-
A continuous-flow microwave heating
ous-flow microwave heating of milk as com-
system was set up using a 2450 MHz MDS-
pared with a conventional unit having the
2000 oven (CEM Corporation, Buckingham,
same residence time.
UK), set to operate at 100% power. A coil
Teflon tubing with an internal diameter of
The objective of the present work was to
0.5 cm and 200 cm in length (39.26 cm3 of
study the effect of continuous-flow
volume) was introduced into the oven cavity
microwave heating of milk at high temper-
through two holes of 0.7 cm diameter drilled
ature on vitamin B and vitamin B , and to
1
2
in the lower left side of the oven. Inlet and
compare it with a conventional heating pro-
outlet temperatures were continuously mon-
cess under similar conditions and with the
itored using Digiten D 2000 digital ther-
batch microwave heating [11, 23].
mometers positioned just outside the cavity.
The milk, initially at 20 °C, was pumped
through the system using a Variable Speed
2. MATERIALS AND METHODS
Tubing Pump (Millipore, Bedford, Mass.,
USA), at a flow rate of 95.2 mL.min–1,
which provided an outlet temperature of
2.1. Milk samples
90 °C (Experiment A, Tab. I). The milk
leaving the oven was either cooled imme-
Raw cow’s whole milk was obtained
diately or pumped through insulated Teflon
from a local farm. Milk was kept refrigerated
tubing of variable lengths (42.8 cm and
at 5 °C until it was processed.
84.17 cm) with an internal diameter of
Table I. Heating conditions used in the continuous-flow microwave and conventional heating
systems.
Tableau I. Conditions de chauffage appliquées dans le micro-ondes à flux continu et dans le système
d’échange de chaleur conventionnel.
Experiment Flow
Tia
trb
thc
Tod
(mL.min–1)
(°C)
(s)
(s)
(°C)
Microwave heating
A 95.2
20
24.6
0
90
B 95.2
20
24.6
30
90
C 95.2
20
24.6
60
90
Conventional heating
D 95.2
20
24.6
0
90
E 95.2
20
24.6
30
90
F 95.2
20
24.6
60
90
a
Ti: inlet milk temperature; b tr: residence time; c th: holding time; d To: outlet milk temperature. Mean values of
the three experiments for each heating system (coefficient of variation ? 1.0%).
a Ti : température du lait à l’entrée ; b tr: temps de séjour ; c th : temps de maintien à température ; d To : tempé-
rature du lait à la sortie. Valeurs moyennes de 3 expériences pour chaque système de chauffage (coefficient de varia-
tion ? 1,0 %).

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604
I. Sierra, C. Vidal-Valverde
0.8 cm (47.6 cm3 and 84.17 cm3 volumes),
2.4.2. Preparation of standards
in order to provide either 30 s or 60 s of
holding time at the flow rate used (Experi-
Individual standard stock solutions of
ments B and C, Tab. I). For rapid cooling
vitamin B (thiamin) (Sigma Chemical Co.,
1
of milk a 0.4 cm × 200 cm coil of Teflon
St. Louis, USA) and vitamin B (riboflavin)
2
tubing immersed in an ice/water bath was
(Merck, Darmstadt, Germany) were pre-
used.
pared by dissolving appropriate amounts of
each one in HCl 0.01 mol.L–1. These stock
solutions were stable if stored at –20 °C
2.3. Conventional heating system
without light for at least 3 months. Four
stock solutions of thiamin and riboflavin
The conventional heating system (tubular
were made by suitable dilutions in HCl
heat exchanger) was set up exactly in the
0.01 mol.L–1. The overall concentration
same manner except for the fact that the heat-
ranges were 6.20–49.00 ng.mL–1 for thi-
ing section was replaced by a coil of stainless
amin and 40–320 ng.mL–1 for riboflavin.
steel tubing, with an internal diameter of
0.45 cm and length of 246 cm (39.26 cm3
of volume), and a wall thickness of
2.4.3. Thiamin HPLC analysis
0.93 mm, immersed in a temperature-con-
trolled water bath. The temperature of the
Analysis of thiamin by HPLC was car-
water bath was adjusted to provide the same
ried out according to Sierra et al. [19].
outlet temperatures as the microwave oven
Apparatus: A Waters Associates Chro-
under the same flow rate conditions,
matograph (Waters Associates, Milford,
thus ensuring the same heating rate (Exper-
CT), equipped with both a model 510 and
iments D–F, Tab. I).
M 45 HPLC pumps, a Rheodine sample
All of the experiments (A–F) were
injector, a mBondapak C
column (300 ×
18
repeated 3 times with the same raw milk
3.9 mm i.d. and 10 mm particle size), a
sample.
C /Porasil B Bondapak guard-column
18
(20 × 3.9 mm i.d.) and a Waters 470 scan-
ning fluorescence detector set at 360 nm
2.4. Analytical determination
(excitation) and 435 nm (emission) wave
2.4.1. Extraction procedure
length were employed. The detector signal
was recorded on a Maxima 820 Chro-
An acid and enzymatic extraction proce-
matography Workstation (Waters Associ-
dure for vitamins B and B was carried out
ates).
1
2
according to Sierra et al. [19]. Milk (10 mL)
Chromatographic conditions: The mobile
was hydrolysed with 0.3 mol.L–1 HCl
phase methanol/water/acetic acid (31/68.5/
(30 mL) in an autoclave at 121 °C for 20 min.
0.5), containing 5 mmol.L–1 sodium hex-
After cooling to ambient temperature and
anesulphonate (Sigma) was pumped at a
pH adjustment to 5–5.5 with 4 mol.L–1
flow rate 1.5 mL.min–1. The column tem-
sodium acetate, 5 mL of 20% aqueous solu-
perature was 35 °C and the injection vol-
tion Taka-Diastase from Aspergillus oryzae
ume were 50 mL.
(Serva Feinbiochemica GmbH & Co., Hei-
delberg, Germany) was added and the sam-
Post column derivatization: An additional
ple was incubated at 45 °C for 3 h. The sam-
model M 45 pump was employed to pump
ple solution was filtered through No. 40
derivatization reagent (0.001 mol.L–1 potas-
Whatman filter paper and filled up with dis-
sium hexacyanoferrate (III) in 0.25 mol.L–1
tilled water to 100 mL. An aliquot was fil-
NaOH) into the eluent stream leaving
tered through a 0.22 mm pore size nylon fil-
the column through a T-junction piece, at
ter membrane and analyzed by HPLC.
flow rate 0.7 mL.min–1. A stainless-steel

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Different heating conditions on milk vitamins
605
reaction coil (3.0 m × 0.5 mm i.d.) was used.
2.5. Statistical analysis
The derivatization reagent was prepared
every day from 0.03 mol.L–1 potassium hex-
Data obtained from the chemical analysis
acyanoferrate (III) aqueous stock solution,
of the samples were subjected to multifactor
1 h before starting analysis, and was used
analysis of variance (ANOVA) by a statis-
within the next 6 h. Peak identification was
tical program (Statgraphics Graphics Sys-
based on the comparison of retention time of
tem 5.0 Computer Software).
standards, as well as by spiking peaks. Cal-
ibration curves were obtained by plotting
peak height versus concentration with stan-
3. RESULTS AND DISCUSSION
dard solutions subjected to the extraction
procedures described above. The correla-
Table II collects the content of vitamins
tion coefficients obtained were superior to
B and B in raw milk and milk heated using
1
2
0.990.
the continuous-flow microwave system
and the tubular heat exchanger (Experi-
2.4.4. Riboflavin HPLC analysis
ments A–D).
The vitamin B content of the control
1
Analysis of riboflavin by HPLC was car-
milk was 0.28 mg.L–1, which is in agree-
ried out according to Sierra et al. [19].
ment with previously published data for raw
Apparatus: A Waters Associates chro-
milk (0.3 to 0.6 mg.kg–1) [13, 20]. When
matograph, equipped with a model 510 HPLC
control milk was heated with the continu-
pump, a Rheodine sample injector, an ODS2
ous-flow microwave system at 90 °C with-
Spherisorb column (300 × 3.9 mm i.d. and
out a holding phase (Experiment A), the
10 mm particle size), a C /Porasil B Bon-
vitamin B content was not significantly
18
1
dapak guard-column (23 × 3.9 mm i.d.) and
modified (P ? 0.05). However, the same
a Waters 470 scanning fluorescence detec-
heat treatment but applying holding times
tor set at 445 nm (excitation) and 520 nm
of 30 s and 60 s (Experiments B and C) led
(emission) wave length were employed. The
to significant vitamin B losses, which
1
detector signal was recorded on a Maxima
increased with increased holding time (i.e.
820 Chromatography Workstation (Waters
97% and 95% retention, respectively).
associates).
When the milk was subjected to analo-
Chromatographic conditions: A mobile
gous treatment using the conventional tubu-
phase methanol/water/acetic acid (31/
lar heat exchanger (90 °C for 0 s, 30 s and
68.5/0.5), containing 5 mmol.L–1 sodi-
60 s), the vitamin B retention values were
1
umhexanesulphonate (Sigma) at a flow rate
similar to those reported above, so there
1 mL.min–1 was used. The column temper-
were no significant differences (P ? 0.05)
ature was 35 °C and the injection volume
between the vitamin B content of milk
1
was 50 mL. Peak identification was based on
heated either with the continuous-flow
the comparison of retention times, as well as
microwave unit or with the tubular heat
by spiking with standards.
exchanger (Experiments D, E and F com-
pared to experiments A, B and C, respec-
The peak heights of riboflavin in sample
tively).
extracts were measured and compared with
standards. Calibration curves were obtained
There is not much data in the literature
by plotting peak height versus concentration
on the extent of vitamin B losses during
1
with standard solutions subjected to the
continuous-flow microwave processing of
extraction procedures described above. The
milk. In a previous paper [18] we have
correlation coefficients obtained were
reported bigger losses of vitamin B when
1
always superior to 0.990.
milk was heated using a conventional plate

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606
I. Sierra, C. Vidal-Valverde
Table II. Vitamins B and B content in milk heated under similar conditions in a continuous-flow
1
2
microwave and a conventional heating system.
Tableau II. Teneur en vitamines B et B du lait chauffé dans le micro-ondes à flux continu et dans
1
2
le système conventionnel.
Milk sample
Holding time
Vitamin B *
Vitamin B *
1
2
(s)
(mg.L–1)
(mg.L–1)
Raw
.... 0.279
± 0.002a
1.679 ± 0.009a
Microwave heating
A 0
0.277
± 0.001a
1.678 ± 0.011a
B 30
0.271
± 0.001b
1.672 ± 0.021a
C 60
0.266
± 0.002c
1.655 ± 0.008a
Conventional heating
D 0
0.276
± 0.003a
1.682 ± 0.061a
E 30
0.270
± 0.002b
1.675 ± 0.017a
F 60
0.265
± 0.001c
1.688 ± 0.009a
* Values are the mean of nine determinations ± standard deviation. The same superscripts in the same column for
each vitamin indicate no significant differences (P ? 0.05).
* Les valeurs proviennent de la moyenne de 9 déterminations ± écart-type. Dans chaque colonne, les mêmes
lettres en exposant indiquent qu’il n’y a pas de différence significative (p ? 0,05) entre les traitements.
heat exchanger at 80 °C compared to a con-
the prescribed temperature appears to be the
tinuous-flow microwave unit at 85 °C (0
main cause of the vitamin losses observed
and 4% losses, respectively). These results
when milk was heated using a conventional
were attributed to the shorter residence time
plate heat exchanger [18].
needed to achieve the prescribed temperature
The vitamin B content of control milk
when milk was heated in the microwave
2
was 1.68 mg.L–1 (Tab. II), which is within
system (come-up time equal to 16.5 s), in
the range of 1.4 to 2.3 mg.kg–1 reported in
comparison with the 70.99 s needed with
the literature for raw milk [13, 20]. Micro-
the plate heat exchanger. Besides this, due to
wave or conventional heating to 90 °C for
the fact that heating of milk in the conven-
0 s, 30 s and 60 s did not significantly mod-
tional system takes place through heat-trans-
ify (P ? 0.05) the content of this vitamin.
fer surfaces, the bigger vitamin B losses
1
This is confirmation of the previously
were also attributed to milk overheating.
reported thermal stability of this vitamin
Results obtained in the present work indi-
[16, 17] and indicates that continuous-flow
cate no significant differences in the vita-
microwave heating of milk does not cause
min B content of milk heated either with
any additional destructive effect. In this way,
1
the tubular heat exchanger or with the contin-
van Zante and Johnson [21] found no sig-
uous-flow microwave oven (Experiments A
nificant differences between buffered solu-
and D). Because the come-up time was sim-
tions of vitamin B heated with microwaves
2
ilar in both cases, these results indicate that
or conventional systems.
the hot surfaces of the tubular exchanger in
The literature on the effect of microwave
contact with the milk do not produce vitamin
heat treatment of milk on vitamins is incon-
B losses. The higher time needed to achieve
1
sistent and, generally, values cannot be

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Different heating conditions on milk vitamins
607
compared. This is mainly due to the fact
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To access this journal online:
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