The plant acids, the plant pigments, and the cellulose are constituents of the vegetable less often considered than the other food nutrients. Knowledge of the common reactions of these constituents renders one able to serve fruits and vegetables that are attractive in appearance, color, and texture.

Plant acids. Plants contain organic acids. According to chemical classification the acids may belong to different groups, but for consideration in cooking they may be divided into two classes, the volatile and nonvolatile ones. Volatile acids are the ones that volatilize and pass from the liquid as vapor. The odor of acetic acid or vinegar during cooking is well known. The following acids of the CnH1nO2 series are volatile, the first ones being more volatile than the last ones:

Formic acid H. COOH

Acetic acid CH3.COOH

Propionic acid CH3 . CH1 . COOH

Butyric acid CH3 . CH1 . CH1 . COOH

Valeric acid CH3.CH1.CH1CH1COOH

Caproic acid CH3 . CH1 . CH1 . CH1 . CH1 . COOH

Formic and acetic acids have been obtained from plants during distillation. Onslow states that propionic acid has only rarely been found in plants.

The amount of volatile acids found in plants varies; it also varies in the same plant. Koch found that cooking 125 grams of spinach for 1 hour and collecting the distillate required 18 cc. of N/10 NaOH to neutralize the volatile acids. Another experiment in which the same weight of spinach was used and cooked for the same length of time required only 6 cc. of N /10 NaOH.

Non-volatile acids. Not all the acids found in fruits and vegetables are volatile. The following are some of the more common non-volatile ones found in foods:

Dicarboxylic acids

Oxalic acid COOH. COOH

Malonic acid COOH . CH1 . COOH

Succinic acid COOH . CH1 . CH1 . COOH

Glutaric acid COOH . (CH1)3 . COOH

Adipic acid COOH . (CH1)4 . COOH

Hydroxydicarboxylic acids

M alic acid COOH . CHOH . CH1 . COOH

Tartaric acid COOH . CHOH . CHOH . COOH

Hydroxytricarboxylic acids

Acids in vegetables

Acids in vegetables. The acids are found in the fruits, leaves, stem, and root stocks. The acid may occur in the free form, but is often combined as a salt or an ester.

Because of oxaluria the oxalic acid content of vegetables is of interest. Onslow states that oxalic acid "occurs very frequently and widely distributed in plants, usually as the calcium salt, and apparently less frequently as the sodium and potassium salts. It has rarely been detected as the free acid. The calcium salt is precipitated on adding calcium acetate to a solution of the acid. Calcium oxalate is insoluble in acetic acid, but soluble in dilute mineral acids." It is found in the rumex or dock family, of which different docks and sour grass are used as food. Sorrel and rhubarb contain a rather high percentage. Rider has reported the most extensive determinations of oxalic acid in leafy vegetables. She has found that the amount in spinach ranges from 0.486 to 0.692 per cent. Beet greens give an equivalent or slightly higher amount than spinach, and New Zealand spinach contains a still higher percentage. No oxalic acid was found in dandelion greens, kale, turnip greens, and mustard greens. McLaughlin has reported the oxalic acid content of eight samples of New Zealand spinach as ranging from 0.49 to 0.53 per cent, anhydrous. As these figures indicate, the amount in different samples of the same vegetable may vary. Rider mentions this fact in connection with the samples she analyzed. Nelson has reported 0.31 per cent oxalic acid in spinach.

Succinic acid is found in many plants, and glutaric and adipic have been isolated from the sugar beet.

Malic acid is found as the free acid and as the salts of malic acid in many plants, and particularly in apples and pears.

Citric acid is found in tomatoes and it is found in smaller quantities in other foods. Blunt reports small amounts in cabbage, asparagus, and string beans.

Nelson has reported that spinach contains both citric and malic acids. The ratio of citric to malic acid in broccoli was 3:2. The leaves and buds contained approximately the same proportions. Small amounts of oxalic and succinic are also present. The acid content of lettuce was as follows: oxalic, 1-malic, and citric, 0.011, 0.065, and 0.048 per cent respectively.

Tomatoes, which have the highest amount of acid of our common vegetables, contain citric acid. Blunt reports 0.42 per cent.

The acids found in fruits. Conflicting reports appear in the literature regarding the particular acid in different foods. Bigelow and Dunbar suggest that this is due to the inaccurate methods of analysis often used and to the difficulty in separating the different acids from one another. Nelson has reported the approximate percentages of the various acids in fresh fruits, the average total acidity being calculated as citric. His results are given in Table 15.

Nelson states that the most common acids in fruits, citric and malic, may occur in different proportions or one alone may be present. 'The total acidity of most fruits varies with the variety and the degree of ripeness. The relative proportions of the various acids may also vary with the degree of ripeness, variety, and climatic and soil conditions."

Rhubarb contains some oxalic acid; cranberries and plums some benzoic. Other acids sometimes found in small quantities are succinic, lactic, isocitric, and acetic. Nelson reports acetic acid in figs, isocitric in blackberries, and tartaric in grapes. Bigelow and Dunbar have reported that both citric and malic acids are found in gooseberry, the total acid for different varieties varying from 1.72 to 2.63 per cent.

Table 15 The Acids of Fruits (Nelson)

Fruit

In-ves-tiga-tor

Cit-ric

%

Mal-ic

%

Tar-tar-ic

%

Ben-zoic

%

Oxal-ic

%

Suc-cin-ic

%

Lac-tic

%

Iso-cit-ric

%

Total acidity

(titratable) %

Apple (Winesap)

N

trace

0.50

..........

................

............

............

...........

..........

0.51

Apricot (dried)

N

0.35

0.81

..........

...........

trace

..........

.........

........

1.16

Banana........

P

..........

0.24

..........

...........

.........

..........

.........

........

0.26

Blackberry.....

N

trace

0.16

..........

...........

trace

trace

.........

0.92

1.08

Blueberry..........

N

1.56

0.10

..........

...........

..........

..........

.........

.........

1.66

Cherry........

F

0.01

1.25

..........

...........

trace

0.07

0.13

.........

1.36

Cranberry.....

N

1.82

0.46

..........

0.07

..........

..........

.........

.........

2.35

Currant.......

F

2.30

0.05

..........

..........

trace

trace

.........

.........

2.35

Fig...........

N

0.34

trace

..........

..........

..........

..........

.........

.........

0.35

Grape.........

N

..........

0.65

0.43

..........

..........

...........

.........

.........

1.08

Grapefruit.....

H

1.46

.........

..........

..........

..........

...........

.........

.........

1.46

Lemon........

N

3.84

trace

..........

...........

..........

...........

.........

.........

3.88

Loganberry. . . .

N

2.02

0.08

...........

...........

..........

...........

.........

.........

2.10

Orange........

N

0.98

trace

...........

...........

..........

...........

.........

.........

0.99

Peach.........

N

0.37

0.37

...........

...........

.........

...........

.........

.........

0.74

Pear..........

N

0.24

0.12

...........

...........

........

.........

.........

.........

0.36

Pineapple......

N

0.84

0.12

...........

...........

..........

...........

.........

.........

0.96

Pomegranate.....

N

1.25

..........

...........

...........

..........

........

.......

......

1.25

Quince........

N

..........

0.68

...........

...........

..........

........

.......

......

0.68

Raspberry, blk.

N

1.06

.........

...........

...........

..........

........

.......

.......

1.06

Raspberry, red

N

1.30

0.04

...........

...........

........

........

.......

.......

1.34

Rhubarb.......

N

0.41

1.77

...........

..........

0.12

.......

......

.......

2.30

Strawberry.....

N

0.91

0.10

...........

..........

.......

......

........

1.01

Tamarind......

F

trace

0.50

7.76

..........

trace

trace

.......

.......

8.00

Tomato.......

N

0.30

0.20

...........

...........

.......

.......

.......

......

0.50

F refers to Fransen: N to Nelson: H to Hartman: and P to Pratt.