CANNED AND BOTTLED VEGETABLES
No class of foods on the market has less need for antiseptics than
canned goods, yet their use is rather common. Products thus treated are
easier canned and are not so apt to spoil. The chemicals used as
preservatives are sulfurous acid, and the sulfites, salicylic acid and
saccharin, benzoic acid, and sometimes formaldehyde. Sulfurous acid is
used to bleach such foods as canned corn. Saccharin possesses some
antiseptic properties, but its main use is as a sweetener. Alum is used
to make pickles hard and crisp.
Some canned or bottled goods, as tomato-catsup, is colored with
cochineal or coal-tar dyes. Green pickles, beans, peas, and such
vegetables are colored by copper salts or are cooked in copper vessels,
with the addition of acetic acid, hence the beautiful green color.
Turmeric is sometimes used to color mixed pickles.
The heavy metals as lead, zinc, and tin are generally present in canned
goods, the amount varying with the corrosive power of the vegetable.
When there is a year of scarcity in corn, peas, beans, and such
vegetables, the dried product is often soaked and canned. Some of this
goods is sold for the regular green vegetable, while some may be
properly marked “Soaked Goods.”
PRESERVATIVES
It is best to make a systematic examination for the different
preservatives. The sample may be prepared by mixing 50 grams of the
pulped material with sufficient water in a 250 cc. graduated flask. Add
phosphoric acid till distinctly acid in reaction. Fill to the mark with
water. Place in a distilling flask, and distil in a linseed oil or a
paraffin bath till 30 cc. have been collected. Save this distillate for
the following tests.
FORMALDEHYDE
To 5 cc. of the above distillate in a test tube, add 2 or 3 drops of a 1
per cent aqueous solution of phenol and mix well. Incline the tube and
carefully pour down the side 5 cc. of concentrated commercial sulfuric
acid so that the two liquids do not mix. If formaldehyde is present
there will be a crimson zone at the plane of union of the solutions.
This coloration takes place when the formaldehyde is present in the
proportion of 1 part in 100,000 parts. When there is a greater quantity
of formaldehyde present a white turbidity or a light-colored precipitate
forms above the coloring.
_Phenylhydrazine Hydrochloric Test._--Dissolve 2 grams of
phenylhydrazine hydrochlorid and 3 grams of sodium acetate in 20 cc. of
water. Add 2 to 4 drops of this reagent and the same number of drops of
sulfuric acid to 1 or 2 cc. of the above distillate, to be examined in a
test tube. A green coloration is produced when formaldehyde is present.
_Hydrochloric Acid Test._--Add 5 cc. of the distillate to be tested to
about 5 cc. of milk known to be pure, and about 10 cc. of concentrated
hydrochloric acid (sp. gr. 1.2) which contains 1 cc. of a 10 per cent
ferric chlorid solution to each 500 cc. of the acid. Heat slowly to 80°
or 90° C. over the free flame, agitating it at the same time to break up
the curd. A violet coloration indicates formaldehyde.
SULFUROUS ACID AND THE SULFITES
Free sulfurous acid is not largely used as a food preservative, though
its salts are quite commonly employed.
_Detection._--Mix 150 grams of the finely ground sample with enough
water to make a thin paste. Acidify with phosphoric acid and distil till
25 cc. have been collected. (The delivery tube of the condenser should
dip below the surface of a little water.) Treat the distillate with a
few drops of bromine water and boil for a short time. If a precipitate
forms on the addition of barium chlorid the presence of sulfurous acid
is indicated.
SALICYLIC ACID
Acidify 50 cc. of the sample with sulfuric acid, and shake vigorously
with 50 cc. of a mixture of equal parts of ether and petroleum spirit.
When the liquids have separated, draw off as much as possible of the
solvent and filter. If an emulsion forms use a centrifugal machine, and
evaporate with a small flame. If needle-shaped crystals form, salicylic
acid is present. Add a few drops of water and a drop of very dilute
ferric chlorid solution in such a way that the solutions will come
together slowly. The presence of salicylic acid gives a purple or violet
color.
SACCHARIN
This is used quite extensively as a sweetening agent in canned sweet
corn, and other similar products.
Macerate about 20 grams of the sample after mixing with 30 to 40 cc. of
water and strain through muslin. Acidify with 1 or 2 cc. of sulfuric
acid (1 to 3) and extract with ether. (If an emulsion forms, use a
centrifugal machine.) Separate the ether layer and let the ether
evaporate spontaneously and use the residue in the following tests:
Take up a part of the residue with water and taste. If it is very sweet
saccharin is present. Confirm by the following:
_Schmidt’s Test._--Add about 1 gram of sodium hydroxide to another part
of the residue, and heat in an air-oven or oil bath, for half an hour at
about 250° C., to convert the saccharin into salicylic acid. After it
has cooled, acidify with sulfuric acid, extract and test for salicylic
acid with 2 or 3 drops of ferric chlorid solution, letting the solutions
come together slowly. A purple or violet coloration proves the presence
of salicylic acid, which in turn indicates the presence of saccharin.
This test cannot be used if salicylic acid was used as a preservative in
the original product. A test for the acid should first be made.
_Bornstein’s Test._--Heat the remainder of the above ether residue with
resorcin and a very little sulfuric acid till it begins to swell. (It is
best to do this heating in a test-tube.) Let cool till the action stops,
heat again and repeat the operation several times. After cooling the
last time, dilute with water and add sodium hydrate till neutral. If
saccharin is present, there will be a red-green fluorescence.
BENZOIC ACID
Acidify 50 cc. of the sample with sulfuric acid and shake vigorously
with 50 cc. of a mixture of equal parts of ether and petroleum spirit.
Let the liquids separate, then draw off as much as possible of the
solvent and filter. (Use a centrifugal machine if an emulsion forms.)
Separate the extract into 2 parts and evaporate each to dryness over a
small flame and make the following tests:
_Ferric Chlorid Test._--Dissolve one of these residues in ammonia, and
evaporate to dryness on a water-bath. Take up the residue with warm
water, filter, and collect the filtrate in a small test tube. Add a drop
of ferric chlorid solution, and if benzoic acid is present a
characteristic flesh or brownish colored precipitate of ferric benzoate
forms. Sometimes in such products as sweet pickles, a basic ferric
acetate precipitate comes down and the following test had better be
applied.
_Peter’s Method._--Take about 0.1 gram of the second part of the above
ether residue, place in a large test tube (about 50 cc.) and dissolve in
5 to 8 cc. of concentrated sulfuric acid. Add from 0.5 to 0.8 gram of
barium peroxide, a little at a time. Shake each time and cool in water
if necessary. This should produce a permanent froth on the sulfuric
acid. Let stand 25 or 30 minutes, then fill the tube three fourths full
of water, shake and cool rapidly to the temperature of the room, and
filter off the barium sulfate. Extract with chloroform or ether. Remove
the extract and test it for salicylic acid with dilute ferric chlorid.
(See first test under salicylic acid.) In this method salicylic acid
must first be proven absent.
_Mohler’s Test._--Treat the remainder of the second part of the above
ether residue with 2 or 3 cc. of concentrated sulfuric acid. Heat till
white fumes appear. Add a few crystals of potassium nitrate and when
cool dilute with water. Add an excess of ammonia, then a drop or two of
ammonium sulfid. If a red color appears immediately on the surface, it
shows the presence of benzoic acid.
COLORING MATTER IN CATSUPS AND TOMATOES
COCHINEAL
_Girard and Dupre Test._--Shake well a portion of the sample with water
and filter, acidify with hydrochloric acid, then extract with amyl
alcohol, and if cochineal is present the extract will be colored yellow
or orange, the particular shade depending on the amount of cochineal
present. Remove the amyl alcohol and wash with water until it is
neutral. To half of this, add a very dilute solution of uranium acetate,
drop by drop, and shaking well after the addition of each drop.
Cochineal, if present, will produce a characteristic emerald-green
color.
Confirm by adding a drop or two of ammonia to the second half of the
amyl alcohol extract and a violet coloration will be produced if
cochineal is present.
COAL-TAR COLORING MATTER
_Sostegni and Carpentieri Test._--Free from grease a piece of woolen
cloth (nun’s veiling will do) by boiling first in very dilute caustic
soda solution and then in water. Acidify a portion of the sample with 2
to 4 cc. of 10 per cent solution of hydrochloric acid and filter. Strips
of the cleansed cloth are boiled in this filtrate for 5 or 10 minutes,
then removed, washed in water and boiled with very dilute hydrochloric
acid solution. Wash out the acid and dissolve the color from the cloth
by boiling in a solution of ammonium hydroxid (1 to 50). (The time
required will depend upon the dye present.) Remove the cloth from the
solution and acidify the latter with hydrochloric acid and another piece
of the cleansed cloth is immersed and again boiled. This second dyeing
fixes only coal-tar colors on the cloth, hence, no fear of mistaking
them for the natural color of the vegetable.
IN GREEN PICKLES, BEANS, PEAS, ETC.
COPPER SALTS
Burn 20 grams of the sample to an ash and wet the ash with concentrated
nitric acid, dilute with water and boil. Add ammonia till strongly
alkaline and filter. If the filtrate is blue, copper is present.
Confirm by acidifying the filtrate with acetic acid and adding potassium
ferrocyanid. A red or brownish precipitate or coloration proves the
presence of copper. The test for other heavy metals may be made by the
general method given under meats.
IN MIXED PICKLES
TURMERIC
Shake with alcohol to extract the color. Soak a piece of filter paper in
the extract and dry in an air oven at 100° C. Wet the filter paper with
a weak solution of boric acid to which a very little hydrochloric acid
has been added. If turmeric is present, a cherry-red color will appear
when the filter paper is dry.
“SOAKED” VEGETABLES
PEAS, BEANS, AND CORN
There is really no chemical test for this class of foods. Certain
helpful directions given in Bul. 65, p. 54, of the Bureau of Chem., will
assist in identifying such goods. All or nearly all of the green color
of peas and beans is destroyed by the process of “soaking.” They have
the appearance of the well-matured product, and are firm and mealy with
well-formed cotyledons. The process of soaking starts the growth of the
caulicle of the pea. The kernel of corn is plump and hard and does not
have the milky consistency of the immatured product. The characteristic
succulence of the green pea, bean, and corn is absent in the soaked
product.
ALUM IN PICKLES
This is sometimes added to the pickling solution to produce hardness and
crispness.
Burn to ash a sample of the pickles, and, if they are free from copper,
fuse in a platinum dish with sodium carbonate. Extract with boiling
water, and after filtering add ammonium chlorid solution. If alum is
present, a flocculent precipitate will form.
EXAMINATION OF THE CAN OR BOX IN WHICH VEGETABLES ARE SEALED
Generally when the ends of a can are convex, instead of plane or
concave, it is spoiled. In the souring of canned sweet corn, it is
exceptional that the ends are forced outward.
Strike the can and the spoiled cans will give a dull sound while the
good ones will give a distinct tone. Some practice will be necessary to
use this test.
One can judge of the amount of tin dissolved by the corrosion of the
inside of the can.
Reject cans that show much rust around the cap on the inside of the
head.
If more than one hole is found soldered in the cap, reject the can. Cans
of salmon are the only exception that has come to the author’s notice. A
second hole, in general, indicates that decomposition had set in and the
can had been punctured and resealed.