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LACTOSE & D-GALACTOSE

(Rapid)

ASSAY PROCEDURE
K-LACGAR 02/21
Incorporating A Procedure For The Analysis Of “Low-
Lactose” Or “Lactose-Free” Samples Containing High Levels
Of Monosaccharides

(Improved Rapid Format)

(*115 Assays per Kit)

* The number of tests per kit can be doubled if all volumes are halved

The reagents provided in this kit are also suitable for use
with AOAC method 2006.06 – Lactose in milk.

Patented: US 7,785,771 B2 and EP1 828 407 (GB, FR, IE, DE)

© Megazyme 2021
INTRODUCTION:
Lactose, or milk sugar, is a white crystalline disaccharide. It is formed
in the mammary glands of all lactating animals and is present in their
milk. Lactose yields D-galactose and D-glucose on hydrolysis by
lactase (β-galactosidase), an enzyme found in gastric juice. People
who lack this enzyme after childhood cannot digest milk and are said
to be lactose intolerant. Common symptoms of lactose intolerance
include nausea, cramps, gas and diarrhoea, which begin about 30
minutes to 2 hours after eating or drinking foods containing lactose.
Between 30 and 50 million Americans are lactose intolerant, with
certain ethnic and racial populations being more widely affected than
others; as many as 75 percent of all African-Americans and Native
Americans and 90 percent of Asian-Americans are lactose intolerant.
The condition is least common among persons of northern European
descent.
Enzymic methods for the measurement of lactose are well known and
are generally based on the hydrolysis of lactose to D-galactose and
D-glucose with β-galactosidase, followed by determination of either
D-galactose or D-glucose. In the International Dairy Federation
Method (79B:1991) for the measurement of lactose in “dried
milk, dried ice-mixes & processed cheese”, details are given for
deproteinisation of samples, hydrolysis of lactose with β-galactosidase
and measurement of either released D-galactose or D-glucose.
The measurement of lactose as D-galactose liberated is more
generally reliable than measurement as D-glucose liberated because
preparations generally contain more free D-glucose than free
D-galactose.
Enzymic kits for the determination of D-galactose are very slow. This
is due to the low rate of natural chemical “mutarotation” between
the α- and β-anomeric forms of D-galactose. Only the β-form is
recognised by β-galactose dehydrogenase. In incubations containing
NAD+, D-galactose and β-galactose dehydrogenase, there is a very
rapid initial increase in absorbance due to the consumption of β-D-
galactose, and this is followed by a very slow approach to the end-
point. This very slow approach results from the very low rate of
chemical “mutarotation” of α-D-galactose into β-D-galactose. Using
technology developed by Megazyme (patented), a galactose mutarotase
has now been incorporated into the assay format to rapidly catalyse
this rate-limiting mutarotation step. The result is very rapid analysis
times of approx. 5 min at room temperature (Figure 1, page 10).

In addition to analysis of lactose in “normal” samples this

kit (K-LACGAR) is also suitable for the analysis of lactose in
“low-lactose” or “lactose-free” samples which contain high
levels of monosaccharides (see Procedure B, page 7).
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PRINCIPLE:
In the currently described procedure (a modification of AOAC Official
Method 984.15; lactose in milk) lactose is hydrolysed to D-galactose and
D-glucose by Aspergillus niger β-galactosidase at pH 5.0 (1).
(β-galactosidase)
(1) Lactose + H2O D-galactose + D-glucose

Interconversion of the α- and β-anomeric forms of D-galactose is catalysed

by galactose mutarotase (GalM) (2).

(GalM)
(2) α-D-Galactose β-D-galactose

The β-D-galactose is oxidised by NAD+ to D-galactonic acid in the

presence of β-galactose dehydrogenase (β-GalDH) at pH 8.6 (3).

(β-GalDH)
(3) β-D-Galactose + NAD+ D-galactonic acid + NADH + H+

The amount of NADH formed in this reaction is stoichiometric with the

amount of lactose. It is the NADH which is measured by the increase in
absorbance at 340 nm.

SPECIFICITY, SENSITIVITY, LINEARITY AND PRECISION:

The assays are specific for lactose and D-galactose. The smallest
differentiating absorbance for the assay is 0.010 absorbance units. This
corresponds to 1.48 mg of lactose (or 0.74 mg of D-galactose)/L of
sample solution at the maximum sample volume of 1.00 mL. The
detection limit is 2.96 mg of lactose/L, which is derived from an
absorbance difference of 0.020 with the maximum sample volume of
1.00 mL.
The assay is linear over the range of 4 to 80 μg of D-galactose (or
8 to 160 μg of lactose) per assay. In duplicate determinations using
one sample solution, an absorbance difference of 0.005 to 0.010 may
occur. With a sample volume of 1.00 mL, this corresponds to a lactose
concentration of approx. 0.74 to 1.48 mg/L of sample solution. If the
sample is diluted during sample preparation, the result is multiplied by the
dilution factor, F. If, in sample preparation, the sample is weighed, e.g.
10 g/L, a difference of 0.02 to 0.05 g/100 g can be expected.

INTERFERENCE:
If the conversion of D-galactose has been completed within 10 min at
room temperature, it can be generally concluded that no interference has
occurred. However, this can be further checked by adding D-galactose
(approx. 40 μg in 0.1 mL) to the cuvette on completion of the reaction.
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A significant increase in the absorbance should be observed.
Interfering substances in the sample being analysed can be identified
by including an internal standard. Quantitative recovery of this
standard would be expected. Losses in sample handling and
extraction are identified by performing recovery experiments, i.e. by
adding lactose or D-galactose to the sample in the initial extraction
steps.
To confirm that lactose is completely hydrolysed by β-galactosidase,
perform the incubation for the recommended time and for twice
the recommended incubation time. The final determined values for
lactose should be the same.

Since divalent metal ions inhibit the β-galactose dehydrogenase

employed in this assay, EDTA is included in buffer 2.

SAFETY:
The general safety measures that apply to all chemical substances
should be adhered to.

For more information regarding the safe usage and handling of this
product please refer to the associated SDS that is available from the
Megazyme website.

KITS:
Kits suitable for performing 115 assays are available from Megazyme.
The kits contain the full assay method plus:
Bottle 1: Buffer (2.5 mL, pH 5.0).
Stable for > 2 years at 4°C.
Bottle 2: Buffer (25 mL, pH 8.6) plus EDTA and sodium azide
(0.02% w/v) as a preservative.
Stable for > 2 years at 4°C.
Bottle 3: NAD+.
Stable for > 5 years below -10°C.
Bottle 4: β-Galactosidase suspension (1.2 mL).
Stable for > 4 years at 4°C.
Bottle 5: β-Galactose dehydrogenase plus galactose mutarotase
suspension, 2.4 mL.
Stable for > 2 years at 4°C.
Bottle 6: D-Galactose standard solution (5 mL, 0.4 mg/mL in
0.02% w/v sodium azide).
Stable for > 2 years; store sealed at 4°C.

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PREPARATION OF REAGENT SOLUTIONS/SUSPENSIONS:
1. Dilute the contents of bottle 1 to 24 mL with distilled water.
Use this solution to dilute the entire contents of bottle 4.
Use immediately.
2. Use the contents of bottle 2 as supplied.
Stable for > 2 years at 4°C.
3. Dissolve the contents of bottle 3 in 12 mL of distilled water.
Stable for ~ 4 weeks at 4°C or stable for > 2 years below
-10°C (to avoid repetitive freeze/thaw cycles, divide into
appropriately sized aliquots and store in polypropylene tubes).
4. Transfer the contents of bottle 4 (1.2 mL) to the 24 mL of
diluted buffer in bottle 1. This is now Solution 4. Divide
into appropriately sized aliquots and store in polypropylene
containers below -10°C between use and cool during use if
possible. Stable for > 2 years below -10°C.
5. Use the contents of bottle 5 as supplied. Before opening for
for the first time, shake the bottle to remove any protein that
may have settled on the rubber stopper. Subsequently, store
the bottle in an upright position. Swirl the bottle to mix
contents before use. Stable for > 2 years at 4°C.
6. Use the contents of bottle 6 as supplied.
Stable for > 2 years; store sealed at 4°C.

EQUIPMENT (RECOMMENDED):
1. Volumetric flasks (50 mL, 100 mL and 500 mL).
2. Disposable plastic cuvettes (1 cm light path, 3.0 mL).
3. Micro-pipettors, e.g. Gilson Pipetman® (20 μL and 200 μL).
4. Positive displacement pipettor, e.g. Eppendorf Multipette®
- with 5.0 mL Combitip® (to dispense 0.2 mL aliquots of diluted
β-galactosidase, buffer 1 and 0.1 mL aliquots of NAD+ solution).
- with 25 mL Combitip® (to dispense 2.0 mL aliquots of
distilled water).
5. Analytical balance.
6. Spectrophotometer set at 340 nm.
7. Vortex mixer (e.g. IKA® Yellowline Test Tube Shaker TTS2).
8. Stop clock.
9. Whatman No. 1 (9 cm) filter papers.

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PROCEDURE A: (Standard Assay Procedure)
Wavelength: 340 nm
Cuvette: 1 cm light path (glass or plastic)
Temperature: ~ 25°C
Final volume: 2.72 mL
Sample solution: 4-80 μg of D-galactose (or 8-160 μg of lactose)
per cuvette (in 0.20-1.00 mL sample volume)
Read against air (without a cuvette in the light path) or against water

Lactose Galactose
Pipette into cuvettes
Blank Sample Blank Sample

sample solution - 0.20 mL - 0.20 mL

solution 4 (β-Galactosidase) 0.20 mL 0.20 mL - -

Ensure that all of the solutions are delivered to the bottom of the cuvette. Mix the
contents by gentle swirling, cap the cuvettes and incubate them for 10 min at ~ 25°C.
Add:

distilled water (at ~ 25°C) 2.20 mL 2.00 mL 2.40 mL 2.20 mL

solution 2 (buffer) 0.20 mL 0.20 mL 0.20 mL 0.20 mL
solution 3 (NAD+) 0.10 mL 0.10 mL 0.10 mL 0.10 mL

Mix*, read the absorbances of the solutions (A1) after approx. 3 min
and start the reactions by addition of:
suspension 5 (β-GalDH/GalM) 0.02 mL 0.02 mL 0.02 mL 0.02 mL

Mix*, read the absorbance of the solutions (A2) at the end of the reaction (< 5 min). If
the reaction has not stopped after 6 min, continue to read the absorbances at 1 min
intervals until the absorbances remain the same over 1 min**.

* for example with a plastic spatula or by gentle inversion after closing

the cuvette with a cuvette cap or Parafilm®.
** if the absorbance A2 increases constantly, extrapolate the
absorbance to the time of addition of suspension 5 (β-GalDH/GalM).

CALCULATION:
Determine the absorbance differences (A2-A1) for blanks and samples.
Subtract the absorbance difference of the blank from the absorbance
difference of the corresponding sample, thereby obtaining the change in
absorbance (ΔA) resulting from the sample, as follows:
Determination of D-galactose:
ΔAD-galactose = (A2-A1)galactose sample - (A2-A1)galactose blank

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Determination of lactose + D-galactose:
ΔAlactose + D-galactose = (A2-A1)lactose sample - (A2-A1)lactose blank

Determination of lactose:
ΔAlactose = ΔAlactose + D-galactose - ΔAD-galactose

The values of ΔAD-galactose and ΔAlactose + D-galactose should as a rule

be at least 0.100 absorbance units to achieve sufficiently accurate
results. However, with milk, the D-galactose content is very low.
The concentration of D-galactose and lactose can be calculated as follows:
c = V x MW x ΔA [g/L]
ε x d x v
where:
V = final volume [mL]
MW = molecular weight of the substance assayed [g/mol]
ε = extinction coefficient of NADH at 340 nm
= 6300 [l x mol-1 x cm-1]
d = light path [cm]
v = sample volume [mL]

It follows for D-galactose:

c = 2.72 x 180.16 x ΔAD-galactose [g/L]
6300 x 1 x 0.2
= 0.3889 x ΔAD-galactose [g/L]
for lactose:
c = 2.72 x 342.3 x ΔAlactose [g/L]
6300 x 1 x 0.2
= 0.7389 x ΔAlactose [g/L]

If the sample has been diluted during preparation, the result must
be multiplied by the dilution factor, F. When analysing solid and
semi-solid samples which are weighed out for sample preparation,
the content (g/100 g) D-galactose or lactose is calculated from the
amount weighed as follows:
= c [g/L sample solution] x 100 [g/100 g]
weightsample [g/L sample solution]

NOTE: These calculations can be simplified by using the Megazyme

Mega-Calc™, downloadable from where the product appears on the
Megazyme website (www.megazyme.com).

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PROCEDURE B: (For “low-lactose” or “lactose-free” samples
containing high levels of monosaccharides)
Step 1:
Add 1 mL of milk (or homogenised sample) to 4 mL of water, mix then
add 1 mL of 10 mg/mL sodium borohydride (dissolved in 50 mM NaOH
and less than 5 h old). Incubate this solution in a sealed plastic container
at 40°C for 30 min then neutralise by the addition of 2.5 mL of 0.2 M
acetic acid. Filter through Whatman No. 1 filter paper or centrifuge in a
microfuge at 13,000 x g and use the filtrate or supernatant directly in the
assay or with an appropriate dilution in distilled water (if required). The
filtrate will be hazy but this is stable in the assay and contributes very little
to the absorbance. Typically use a sample volume of 0.2 mL. in Step 2.
Step 2:
Wavelength: 340 nm
Cuvette: 1 cm light path (glass or plastic)
Temperature: ~ 25°C
Final volume: 2.72 mL
Sample solution: 4-80 μg of D-galactose (or 8-160 μg of lactose)
per cuvette (in 0.20-1.00 mL sample volume)
Read against air (without a cuvette in the light path) or against water
Lactose Galactose
Pipette into cuvettes
Blank Sample Blank Sample

sample solution - 0.20 mL - 0.20 mL

solution 4 (β-Galactosidase) 0.20 mL 0.20 mL - -

Ensure that all of the solutions are delivered to the bottom of the cuvette. Mix the
contents by gentle swirling, cap the cuvettes and incubate them for 2 h at ~ 25°C.
Add:

distilled water (at ~ 25°C) 2.20 mL 2.00 mL 2.40 mL 2.20 mL

solution 2 (buffer) 0.20 mL 0.20 mL 0.20 mL 0.20 mL
solution 3 (NAD+) 0.10 mL 0.10 mL 0.10 mL 0.10 mL

Mix*, read the absorbances of the solutions (A1) after approx. 3 min
and start the reactions by addition of:
suspension 5 (β-GalDH/GalM) 0.02 mL 0.02 mL 0.02 mL 0.02 mL
Mix*, read the absorbance of the solutions (A2) at the end of the reaction (< 5 min). If
the reaction has not stopped after 6 min, continue to read the absorbances at 1 min
intervals until the absorbances remain the same over 1 min**.

* for example with a plastic spatula or by gentle inversion after closing

the cuvette with a cuvette cap or Parafilm®.
** if the absorbance A2 increases constantly, extrapolate the absorbance
to the time of addition of suspension 5 (β-GalDH/GalM).
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CALCULATION: (Procedure B)
The calculation of results for Procedure B is the same as given for
Procedure A (see page 5 and 6). However, the dilution factor of 8.5
from Step 1 of Procedure B must be accounted for, plus any further
dilution of the sample prior to Step 2.

SAMPLE PREPARATION:
1. Sample dilution.
The amount of D-galactose present in the cuvette (i.e. in the 0.2 mL of
sample being analysed) should range between 4 and 80 μg (i.e. lactose
in the sample solution used should range between approx. 8 and
160 μg). The sample solution must therefore be diluted sufficiently to
yield a lactose concentration between 0.04 and 0.80 g/L.

Dilution Table
Estimated concentration
Dilution with water Dilution factor (F)
of lactose (g/L)
< 0.8 No dilution required 1
0.8-8.0 1 + 9 10
8.0-80 1 + 99 100
> 80 1 + 999 1000
If the value of ΔAD-galactose or ΔAlactose + D-galactose is too low (e.g.
< 0.100), weigh out more sample or dilute less strongly. Alternatively,
the sample volume to be pipetted into the cuvette can be increased up
to 1.00 mL making sure that the sum of the sample, distilled water and
solution 4 components in the reaction is 2.40 mL, and using the new
sample volume in the equation.

2. Sample clarification.
a. Solutions:
Carrez I solution. Dissolve 3.60 g of potassium hexacyanoferrate (II)
{K4[Fe(CN)6].3H2O} (Sigma cat. no. P9387) in 100 mL of distilled water.
Store at room temperature.
Carrez II solution. Dissolve 7.20 g of zinc sulphate (ZnSO4.7H2O)
(Sigma cat. no. Z4750) in 100 mL of distilled water. Store at room
temperature.
Sodium hydroxide (NaOH, 100 mM). Dissolve 4 g of NaOH in
1 L of distilled water. Store at room temperature.
b. Procedure:
Pipette 1 mL of the liquid sample into a 100 mL volumetric flask which
contains approx. 60 mL of distilled water, or weigh sufficient quantity
of the sample into a 100 mL volumetric flask and add 60 mL of distilled
water. Carefully add 5 mL of Carrez I solution, 5 mL of Carrez II
solution and 10 mL of NaOH solution (100 mM). Mix after each
addition. Fill the volumetric flask to the mark, mix and filter.
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3. General considerations.
(a) Liquid samples: clear, slightly coloured and approximately
neutral, liquid samples can be used directly in the assay.
(b) Acidic samples: if > 0.1 mL of an acidic sample is to be used
undiluted (such as wine or fruit juice), the pH of the solution should
be increased to approx. 8.6 using 2 M NaOH, and the solution
incubated at room temperature for 30 min.
(c) Carbon dioxide: samples containing a significant amount of
carbon dioxide, such as beer, should be degassed by increasing the
pH to approx. 8.6 with 2 M NaOH and gentle stirring, or by stirring
with a glass rod.
(d) Coloured samples: an additional sample blank, i.e. sample with
no β-GalDH/GalM may be necessary in the case of coloured samples.
(e) Strongly coloured samples: if used undiluted, strongly
coloured samples should be treated by the addition of 0.2 g of
polyvinylpolypyrrolidone (PVPP)/10 mL of sample. Shake the tube
vigorously for 5 min and then filter through Whatman No. 1 filter
paper.
(f) Solid samples: homogenise or crush solid samples in distilled
water and filter if necessary.
(g) Samples containing fat: extract such samples with hot
water at a temperature above the melting point of the fat, e.g. in
a 100 mL volumetric flask at 60°C. Adjust to room temperature
and fill the volumetric flask to the mark with water. Store on ice
or in a refrigerator for 15-30 min to allow separation of the fat
and then filter. Discard the first few mL of filtrate and use the
clear supernatant (which may be slightly opalescent) for assay.
Alternatively, clarify with Carrez reagents.
(h) Samples containing protein: deproteinise samples containing
protein by adding an equal volume of ice-cold 1 M perchloric acid
with mixing. Centrifuge at 1,500 g for 10 min and neutralise the
supernatant with 1 M KOH. Alternatively, clarify with Carrez
reagents.

SAMPLE PREPARATION EXAMPLES:

(a) Determination of lactose in milk, cream or yogurt.
Accurately weigh approx. 1 g of milk, cream, yogurt or condensed
milk into a 100 mL volumetric flask, add approx. 60 mL of distilled
water, mix and store at 50°C for 15 min with occasional swirling.
Add 2 mL of Carrez I solution and mix. Add 2 mL of Carrez II
solution and mix. Add 4 mL of 100 mM NaOH solution and mix
vigorously. Dilute to volume with distilled water and mix thoroughly.
9
Filter an aliquot of the solution through Whatman No. 1 filter paper.
Discard the first few mL of filtrate. Use the clear filtrate (sample
solution) in the assay. Typically, for milk, cream and yogurt, no dilution
is required and a sample volume of 0.2 mL is satisfactory; for condensed
milk, a dilution of 1:3 and sample volume of 0.2 mL are satisfactory.
(b) Determination of lactose in cheese and chocolate.
Add 10 g of grated cheese or 0.5 g of grated chocolate to a 200 mL
beaker. Add approx. 60 mL of distilled water and a stirrer bar, and
mix on a magnetic stirrer at 50°C for approx. 15 min. Add 2 mL of
Carrez I solution and mix. Add 2 mL of Carrez II solution and
mix. Add 4 mL of 100 mM NaOH solution and mix vigorously.
Quantitatively transfer the solution to a 100 mL volumetric flask and
dilute to volume with distilled water. Mix thoroughly and filter an
aliquot of the solution through Whatman No. 1 filter paper. Discard
the first few mL of filtrate. Use the clear filtrate (sample solution) in
the assay. Typically, for chocolate and most cheeses, no dilution is required
and a sample volume of 0.2 mL is satisfactory; however for red cheddar
cheese, a dilution of 1:10 and sample volume of 0.2 mL are satisfactory.

REFERENCE:
Beutler, H. O. (1988). Lactose and D-Galactose. “Methods of
Enzymatic Analysis” (Bergmeyer, H. U., ed.), 3rd ed., Vol. VI, pp. 104-
112, VCH Publishers (UK) Ltd, Cambridge, UK.






 

 
 



 



    




Figure 1. Effect of galactose mutarotase on the rate of oxidation of

D-galactose by β-D-galactose dehydrogenase. Incubations were
performed with 20 μL of β-D-galactose dehydrogenase at 100, 200
and 400 U/mL and with β-D-galactose dehydrogenase (200 U/mL)
plus galactose mutarotase, as indicated.
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Telephone: (353.1) 286 1220
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Internet: www.megazyme.com
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WITHOUT GUARANTEE
The information contained in this booklet is, to the best of our knowledge, true and accurate, but
since the conditions of use are beyond our control, no warranty is given or is implied in respect of
any recommendation or suggestions which may be made or that any use will not infringe any patents.

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