NICHOLLS: VEGETABLE AND COAL TAR COLOURS IN FOODSTUFFS 585

The Detection of the Prohibited Vegetable and Coal Tar Colours in Foodstuffs. BY JOHN RALPH NICHOLLS, B.Sc., F.I.C.

THE Public Health (Preservatives, etc., in Food) Regulations, 1925, prohibit the use in foodstuffs of the vegetable colouring matter, gamboge, and five coal tar colours. The latter are specified in the Regulations as in the first three columns of the following table, the fourth column indicating the chemical name:-

9umber in Colour Index of Society of Dyers and

Colourists, 1924. Name. Synonyms. Chemical name.

7 Picric acid. Carbazotic acid. Trinitrophenol.

8 Victoria yellow. Saffron substitute; Salt of 0- and +- Dinitrocresol. dinitrocresols.

9 Manchester yellow. Naphthol yellow. Salt of dinitronaphthol. Martius yellow.

12 Aurantia.

724 Aurine.

Imperial yellow. Ammonium salt of hexanitrodiphenyl- amine.

Rosolic acid. Yellow coralline.

Mixture of trioxytri- phenylcarbinol, oxitiised aurine, ps ;ud.xosolic acid and their salts.

The object of this investigation was to provide a simple scheme for the ready detection of all the above prohibited colours, which could easily be applied for routine examinations.

Commercial samples of the colours were purchased, but it was found that the one marked “Aurantia” was a sulphonic acid colour, the makers stating that No. 12 of the Colour Index was not on the market at the present time. For the purposes of this investigation Aurantia from a specially prepared specimen of hexanitrodiphenylamine was employed.

Preliminary experiments with the specified colours showed that dyeing tests with wool were useless as a means of detection. The dye solutions were only deeply coloured when alkaline, acidification producing colourless solutions or giving yellow precipitates according to their strengths. Wool could not be dyed

(Cont iwed on $age 588.)

Publ

ishe

d on

01

Janu

ary

1927

. Dow

nloa

ded

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

27/1

0/20

14 0

4:32

:30.

View Article Online / Journal Homepage / Table of Contents for this issue

STJP

PLE

ME

NT

AR

Y

TE

STS

Test

s on

dilu

te s

oda

solu

tions

ob

tain

ed i

n s

chem

e fo

r de

tect

ion

of p

rohi

bite

d dy

es.

(1)

(2)

Mel

ting

poin

t of

ethe

real

ext

ract

fro

m

Col

our o

f w

eakl

y al

kalin

e so

lutio

n:

acid

so

h. (

from

lite

ratu

re).

Stro

ng i

n co

lour

. W

eak

in

colo

ur.

(3)

Tas

te o

f so

h.

dilu

ted

till

alm

ost

colo

urle

ss.

(4)

Stro

ng s

oda

adde

d to

alka

line

soh

.

(5)

Exc

ess

of s

alt a

dded

to

alka

line

soln

(6)

Dilu

te a

cid

adde

d to

alk

alin

e so

h. :

St

rong

.

Wea

k.

(7)

(8)

(9)

To

1 vo

l. ad

d 10

vol

s. c

once

ntra

ted

sulp

huri

c ac

id.

Alk

. so

h.

boile

d w

ith a

mm

oniu

m

sulp

hide

.

Alk

. so

h bo

iled

wit

h K

CN

.

(10)

A

cid

so

h b

oile

d w

ith S

nCh.

(1 1)

B

oile

d w

ith

zinc

dus

t and

am

mon

ia.

Re-

oxid

ised

wit

h so

dium

per

sulp

hate

.

PiC

YiC

V

icto

ria

Naf

ihth

ol

A u

rant

ia.

acid.

yello

w.

yello

w.

Trin

itrop

heno

l. D

initr

ocre

sol

Din

itron

apht

hol.

Hex

anitr

odi-

(0

- an

d +)

. fih

enyl

amin

e.

121°

-122

.5'

C.

85'4

36'

C.,

or

138'

C,

238'

C.,

with

de

com

p.

80.5

' C.

yello

w

oran

ge y

ello

w

oran

ge y

ello

w

oran

ge re

d ye

llow

ye

llow

ye

llow

or

ange

yel

low

bitt

er

-

-

-

deep

ens

no c

hang

e no

cha

nge

redd

ens

cons

ider

ably

sl

ight

ly

no c

hang

e no

cha

nge

no c

hang

e no

cha

nge

no c

hang

e ye

llow

ppt

. ye

llow

ppt

. ye

llow

ppt

no c

hang

e co

lour

less

co

lour

less

co

lour

less

colo

urle

ss

colo

urle

ss

fain

t yel

low

fa

int y

ello

w

deep

red

dish

fa

int r

eddi

sh

fain

t red

dish

de

ep b

row

n br

own

brow

n br

own

do.

deep

ens t

o de

epen

s du

ll br

own

oran

ge b

row

n

deco

lori

sed

deco

lori

sed

deco

lori

sed

yello

wis

h br

own

to

deep

redd

ish

brow

n.

deep

ens t

o de

colo

rise

d de

colo

rise

d de

colo

rise

d re

ddis

h br

own

then

gra

dual

ly

deco

lori

sed

noch

ange

no

chan

ge

noch

ange

no

chan

ge

Gum

boge

. A

urin

e.

Trih

ydro

xytr

i-

phen

ylm

etha

ne.

not m

elte

d at

75

"-80

' C.

22

0" c.

z E 0

r

yello

w

r

red

oran

ge re

d re

d v) ..

-

-

2 M

no c

hang

e sl

ight

ly tu

rbid

tr

M

out

as r

eddi

sh 2 0 z

yello

w p

pt.

0

no c

hang

e so

da s

alt t

hrow

n 0 8

yello

w p

pt.

yello

w p

pt.

r

yello

w s

oh.

colo

urle

ss

w z

whi

ch o

n bo

il-

ing

colle

cts as

brow

n dr

ops

~ 0 8

yello

w

brow

nish

no c

hang

e no

cha

nge

U

slow

ly d

is-

do.

colo

ured

noch

ange

do

.

deco

lori

sed

colo

ur s

light

ly

fade

s to

yello

w

colo

ur p

ar-

no c

hang

e ti

ally

retu

rns

Publ

ishe

d on

01

Janu

ary

1927

. Dow

nloa

ded

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

27/1

0/20

14 0

4:32

:30.

View Article Online

Boi

led

wit

h fe

rrou

s su

lpha

te a

nd a

m-

mon

ia

for

one

min

ute:

Fil

tere

d F

iltr

ate.

de

ep re

d

yello

w

yello

w

oran

ge

rose

pin

k to

or

ange

yello

w to

no

cha

nge

deep

red

yello

w o

rang

e w

hen

war

med

w

ith

acet

ic

do.

do.

noch

ange

do

.

yello

w

viol

et

brow

nish

pr

acti

call

y ye

llow

ish

brow

nish

red

fain

t yel

low

do

. de

colo

rise

d F

iltr

ate

acid

ifie

d.

Sol

n. tr

eate

d w

ith

lead

ace

tate

. no

cha

nge

no c

hang

e no

cha

nge

redd

ish

yello

w

PP

t *

do.

do.

yello

w p

pt.

do.

do.

redd

ish

ppt.

yello

w

brow

nish

no

cha

nge

yello

w

I,

,, ),

zinc

acet

ate.

1s

,,

,, ba

rium

chl

orid

e.

Tw

o or

thre

e dr

ops

alk.

sol

n. d

isso

lved

in

1

ml.

conc

. H

,S04

, co

oled

; fe

w

crys

tals

NaN

O, a

dded

and

war

med

in

wat

er b

ath

at 8

0°.to

90" C

. for

1 m

in.,

then

coo

led

in a

ir 2

to

3 m

ins.

Alk

. so

h. m

ixed

wit

h eq

ual

vol.

conc

. H

,SO

,, a

litt

le s

olid

K

MnO

, ad

ded

and

boile

d fo

r 1

min

. F

ew c

ryst

als

of re

soqc

inol

add

ed a

nd b

oile

d ti

ll w

ater

ev

apor

ated

an

d fu

min

g st

arts

. P

oure

d in

to s

oda

soh

.

To

one

vol.

alk.

so

h.

add

1 v

ol.

alco

hol,

1 v

ol.

ethe

r an

d on

e vo

l. 30

% s

oda

and

shak

e:

Upp

er l

ayer

. L

ower

laye

r.

To

one

vol.

alk.

sol

n. a

dd 2

vol

s. 3

0%

soda

, m

ix a

nd s

tand

1 m

in.

Add

1

vol.

alco

hol,

1 vo

l. et

her a

nd s

hake

: U

pper

lay

er.

Low

er la

yer.

Mak

e ac

id a

nd e

xtra

ct w

ith

petr

oleu

m

spir

it.

Pet

role

um s

piri

t lay

er.

do.

do.

yello

w

0 0 F 2 0 0

F

0

d w cn z

-

stro

nggr

een -

-

fluo

resc

ence

yello

w

colo

urle

ss

yello

w

colo

urle

ss

pink

co

lour

less

ye

llow

ye

llow

co

lour

less

co

lour

less

ye

llow

c o

1 our

1 e s s

colo

urle

ss

yello

w

yello

w

yello

w

colo

urle

ss

colo

urle

ss

colo

urle

ss

colo

urle

ss

yello

w

red

to p

ink

pink

co

lour

less

colo

urle

ss

colo

urle

ss

colo

urle

ss

colo

urle

ss

colo

urle

ss

yello

w (

fain

tly

(ppt

. not

ye

llow

sus-

so

lubl

e)

pens

ion)

no

cha

nge

no c

hang

e gr

eeni

sh

brow

n to

gre

enis

h bl

ack

no c

hang

e co

lour

bec

omes

col

ourl

ess,

then

ye

llow

, and

ye

llow

whe

n on

add

ing

alka

line

N/ 10

soda

tu

rns

gree

n,

then

blu

e,

then

vio

let

no c

hang

e no

cha

nge

Add

few

dro

ps a

lcoh

olic

FeC

l, so

h. t

o

petr

oleu

m s

piri

t la

yer

(No.

20)

, sha

ke

and

allo

w t

o s

tand

.

To

one

ml.

soln

. add

dil

. ac

id t

ill

just

ac

id,

then

dil

. so

da t

ill

just

alk

alin

e.

Add

5

mls

. al

coho

l an

d 2

drop

s sa

tura

ted

brom

ine

wat

er.

The

n ad

d N/10 so

da,

drop

by

drop

.

no c

hang

e

no c

hang

e no

cha

nge

no c

hang

e

CJI 00 4

Publ

ishe

d on

01

Janu

ary

1927

. Dow

nloa

ded

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

27/1

0/20

14 0

4:32

:30.

View Article Online

588 NICHOLLS : THE DETECTION OF THE PROHIBITED VEGETABLE

from alkaline solutions, and wool boiled in the acid solution showed no appreciable colour when spotted with alkali. The boiling of the acid solution resulted, in some cases, in the complete loss of the colouring matter through volatilisation with the steam.

From a consideration of the chemical nature of the prohibited coal tar colours, it was apparent that acidification should give in each case a substance of highly acidic character, which would probably be soluble in an immiscible solvent. This was found to be the case, and methylated ether proved to be an excellent solvent, provided the solution was not too strongly acid. With an acidity greater than that stated in the table, some, or all, of the colour may not be removed by the ether. It is not advisable to wash the ethereal extract with water, as picric acid is partially washed out. From the ethereal solution all except one of the dyes can be re-extracted with dilute alkali. The exception is aurantia, which remains in solution in the methylated ether. If, however, an equal volume of petroleum spirit is added to the methylated ether solution, dilute alkali will now extract aurantia. The colouring matter of gamboge is also of an acidic nature and can be extracted from an acid solution by means of ether, and alkali will remove it from the latter solution.

Practically all animal and vegetable colouring matters naturally present in foodstuffs give no colour to the alkaline solution when treated in the above manner. In addition, most dyes likely to be used to colour foodstuffs are insoluble in ether. Among dyes which might be extracted are simple nitroso dyes, certain acidic non- sulphonated azo dyes, oxazones, simple anthraquinones and phthaleins ; but these are rarely used in foodstuffs. The extraction method is therefore of advantage in showing the absence of prohibited dyes very quickly.

Numerous tests were carried out on dilute alkaline solutions of the various colours a t concentrations such as might be obtained in practice. The most useful of these tests are given later, and from them a scheme was drawn up for routine work. The scheme enables each dye to be detected, and some of the other tests can be used as confirmation. Tests 18 and 19 used in the general scheme are a little unusual. Test 18 depends upon the salting-out effect of strong soda upon the sodium salt of the dye. Test 19 depends upon a similar effect after a pre- liminary treatment for 1 minute with strong soda, whereby the dyes with three or more nitro-groups are partly decomposed and behave differently .

SCHEME FOR THE DETECTION OF PROHIBITED COLOURS.

Make an ammoniacal extract of the foodstuffs by any suitable means, filtering, if necessary, and using as large a quantity of material as convenient. 'To the ammoniacal extract add 1 drop of methyl orange, neutralise with acid, then make acid to the extent of about N/100 to N/50. Extract once or twice with methy- lated ether, transferring the ether to a separating funnel. Extract the ether with successive quantities of about 5 to 10 ml. of approximately N/100 sodium

Publ

ishe

d on

01

Janu

ary

1927

. Dow

nloa

ded

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

27/1

0/20

14 0

4:32

:30.

View Article Online

AND COAL TAR COLOURS IN FOODSTUFFS 589

Ethereal solution is colourless and soda solution is pink if aurine is present. Confirm by tests 1 1 and 22.

hydroxide solution until no more colour is removed. Add an equal quantity of petroleum spirit to the methylated ether and again extract with dilute sodium hydroxide solution.

Absence of colour in alkaline layers indicates absence of prohibited dyes.

Ethereal solution is yellow and soda solution colourless if others are present.

Extract from methylated ether is coloured yellow by picric acid; yellow to orange by Victoria yellow, Manchester yellow and gamboge; red by aurine.

Victoria yellow.

Manchester yellow.

~~~

(B). To 1 ml. of solution add 2 ml. of 30 per cent. sodium hydroxide solution, Add 1 ml. methylated ether and 1 ml. mix and leave for 1 minute.

alcohol and shake.

Apply test 12.

Ex tract from petroleum spirit and

methylated ether is

orange-red if aurantia is present.

Apply tests 10and 19.

Apply tests 16 and 17.

Ethereal solution is colourless and soda solution is (a) yellow if picric acid is present, (b ) pink if aurin is present. A yellow colour in the soda layer may not be seen if both are present. In that event apply the confirmatory tests for picric acid, viz. tests 3, 9, 11, and the for- mation of picrates with naphtha- lene, quinine, etc.

(C). To original solution add acid and

Ether coloured yellow indicates gamboge. Confirm by tests 21 and 13, 14, 15 (if necessary con- centrating the solution).

--__________- - Ethereal solution is yellow and soda solution colourless if others are present.

rtract with petroleum spirit.

The scheme is put forward as a preliminary attempt to detect the prohibited dyes. It has, however, been in use in this laboratory for nine months, and no interfering colours have yet been extracted.

The author desires to thank the Government Chemist for permission to publish this work.

It is possible that in certain cases it may fail.

GOVERNMENT LABORATORY, LONDON.

Publ

ishe

d on

01

Janu

ary

1927

. Dow

nloa

ded

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

27/1

0/20

14 0

4:32

:30.

View Article Online