Allen Paul W. Industrial Fermentations

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FOOD PRESERVATION PROCESSES 241

be neoesfifary. At this height the meat will get the benefit of the

thick smoke and still hang below the ventilator.

"Green hickory or maple wood is the best fuel for smoking. Hard

wood is preferable to soft wood. Resinous woods should never be

used, as they give an objectionable flavor to the meat. Corn cobs

may be used, but they deposit carbon on the meat, giving it a dirty

appearance.

"Allow .the meat to hang in the smokehouse twenty-four hours

before beginning to smoke. Space between the pieces of meat is neces-

sary to insure good circulation of smoke. A slow fire should be

started, so that the meat will warm up. gradually. Do not get the

house too hot. The fire can be kept going continuously until the

smoking is complete, holding the temperature as even as possible

(120 degrees F.). Thirty-six to forty-eight hours is the time required

to smoke a lot of meat, but slower and longer smoking is desirable

if the meat is to be kept any length of time. During warm weather

it is better to start the fire every other day rather than heat up the

meat too much. In the winter, however, if the fire is not kept going

the meat may cool and the smoke will not penetrate properly. As soon

as.

the meat is thoroughly smoked, open the doors and ventilator, so

that the meat can cool.

"Smoked meat, after it is hard and firm, should be wrapped in

heavy paper and put into muslin sacks."

REFERENCES QN COLD STORAGE.

Anderson, J. W., 1918. Nature, Vol. 100, pp. 465-456.

Bailey, E. M. Jour. Am. Chem. Soo. 34, pp. 1706-1730.

Barnard, H. E. Jour. Ind. and Eng. Chem., Vol. 3, No. 6, pp. 285-286.

Bureau, M., 1907. Ann. Soc. Agr. Skri. Indus. (1907), pp. 851-396.

Gardner, H. A. Jour. Ind. and Eng. Chem., Vol. 8, No. 11, pp. 997-998.

Hepburn, J. S. Jour. Frank Inst. 17, pp. 585-598.

Hilliard, C. M., et al., 1915. Science 42, pp.

770-771.

Keghel, M. De. Rev. Chum. Ind. 24, pp. 12-18.

Pennington, M. E. Jour. Bio. Chem. 4, pp. 353-393; also Vol. 16, pp. 831-368.

Preeoott, S. C. Amer. Jour. Pub. Health 2, pp. 834-839.

Ravenel, Hastings and Hammer, 1910. Jour. Inf. Diseases, Vol. 7, pp. 38-96.

Schnoeder. Orelli, O., 1911. Landw. Jahrb. Sohweiz 25. pp. 225-226.

Stevens, K, 1917. U. S. D. A. Bui. 531.

Stiles,

Jr., G. W., and Bater, C. U. S. D. A. Bur. Chem. Bui. 41£>.

FOOD DRYING.

Bailey, L. BL 1895. Cornell Univ. Agr. Exp. Sta. Bui. 100, pp. 1-40.

Bitting, A. W., 1911. U. S. Bureau of Chem. Bui. 133, pp. 1-40.

Caldwell, Joseph S., 1923. Evaporation of Fruits. U. S. D. A. Bui. 1141.

Ekroth Ckrenoe V., 1918. Amer. Jour. Pub. Health, Vol. 8, No. 3, pp. 205-207.

Fairchild. D., 1915. National Geographic Magazine, Vol. 33, No. 4, pp. 356-368.

Gould, H. P., 1917. U. S. Dept. Agr. Farmers Bui. 902.

Hushmann, Geo. C, 1916. The Raisin Industry. U. S. D. A. Bui. 349.

Lewis, C. I, and Brown, 1914. Oregon Agr. Expt. Sta. Bui. 117, pp. 1-32.

Mendel, L. B.

1916. Changes in the Food Supply and Their Relation to Nutri-

tion (Text).

242 INDUSTRIAL FERMENTATIONS

Parow, E., 1916. Z. Spiritumnd, 89, p. 439.

Perry, E. N.

1882. Dept. of Agr. of Main. Am. Rpt., 26, pp. 376-387.

Valvassori, V. Jour. Soo. Chem. Ind., Vol. 38, pp. 159.

JELLIES, JAMS, ETC.

Cloughly, 0. J. Natt. Drugg, Vol. 42, pp. 467.

Close, C. P., 1917. U. S. D. A. Fanners Bui., 900.

Goldthwaite, N. E. Jour. Ind. Eng. Chem., 2, pp. 457-462.

Gore, H. C. U. S. Patent,

1,284,187.

Goldthwaite, N. E. HI. Agr. Exp. Sta. Bui., No. 8.

Hausner A. Manufacturer of Preserved Foods and Sweetmeata (Text).

Trulle, A. Exp. Sta. Record, 37, pp. 15.

Wagner, E. Recipes for the Preserving of Fruits, Vegetables and Meats (Text).

FRUIT JUICES.

Bailey, E. H. S., and Long, U. 8., 1916. Ann. Food Jour., 11, pp. 624-625.

Barahrola, W. I., 1917. Schweiu. Apoth. Ztg., pp. 396-400.

Beythien, A., and Semmick, P. Z. Nahr. Genuasm 20, pp. 241-272.

Bioletti, F. T., 1914. Ref. Coll. Agr. Univ. Cal. (1914), pp.

190-191.

Bohlman,

Rolf.

Z. Nahr. Genussm., 24, pp. 516.

Cruess, W. V., and Hints, C. J. Jour. Ind. & Enn. Chem., 6, pp. 302-304.

Hartmann, B. G., and Tolman, L. M., 1918. U. S. D. A. Bui. 658.

REFERENCES ON PICKLING.

Bachmann, F. M. Jour. Ind. and Eng. Chem., 8, pp. 620-623: also Vol 7 pn

620-623.

Donald, E. Mao. Jour. Inf, Dis., 21, pp. 322-325.

Hoffmann, C, and Evans, Abie. Jour. Ind. and Eng. Chem., Vol. 3 No. 11 pp

835-838.

Hottinfcer, R. Am. Escola Polytech Soc. Paulo, 11, pp. 119-125.

Hite,

H. H., et al., 1914. W. Va. Agr. Exp. Sta., Bui. 146.

Kassowica, Alex., 1909. Z. Landw. Vera. Wes. Oestem, Vol. 12, pp. 757-810.

Kassowica, Alex. Z. Garunjraphysical, 2, pp. 78-80.

Luidet, M. L. Orig. Com. 8th Intern. Cong. Appl. Chem., 18, p. 237.

Chapter 18.

The Canning Industry.

A Frenchman, Nichols Appert, has been styled "The Father of

Canning." He invented in 1795 a process of heating foods sealed in

glass cans, which caused them to keep indefinitely. He said: "It is

obvious that this new method of preserving animal and vegetable

substances proceeds from the simple principle of applying heat in a

due degree to the several substances, after having deprived them as

much as possible of all contact with the external air. It might, on the

first view of the subject, be thought that a substance, either raw

or previously acted on by fire, and afterwards put into bottles, might,

if a vacuum were made in those bottles and they were completely

corked, be preserved equally well as with the application of heat in the

water bath. This would be an error, for all trials I have made convince

me that the absolute privation of the' contact of external air (the

internal air being rendered of no effect by the action of heat) and the

application of heat by means of the water bath, are both indispensable

to the complete preservation of alimentary substances." Dr. McBryde

says:

"While Appert had no conception of the rftle played by micro-

organisms in the spoilage of foods, his experiments dearly showed that

a vacuum alone was not sufficient for the preservation of foodstuffs,

and that the application of heat and the subsequent exclusion of the

outside air were essential to their successful preservation. We now

know that the air contains microorganisms or germs which are re-

sponsible for the fermentation and putrefaction of foods.

"The stimulus which prompted Appert's investigations was a prize

offered by the French Government for a method of preserving foods

for the use of the navy, and in 1809 the inventor was awarded the

prize of 12,000 francs. The process waB given to manufacturing

firms in France and was soon carried to England.

"At first the secret of the process was carefully guarded, but in

time the employees of the different establishments became more or

less familiar with the details of the process and in this way the method

was carried from England to America about 1815 or 1818.

"Ezra Daggett, originally in the employ of an English firm, is

said to have first brought the secret to America, and as early as 1819,

in partnership with his son-in-law, Thomas Kensett, was engaged in

the packing of hermetically sealed food in New York City. Lobster,

salmon and oysters were among the first goods packed in America,

but by 1825 fruits and vegetables' were also canned.

248

244 INDUSTRIAL FERMENTATIONS

"In the case of meats which are 'processed

or heated in the oan,

it is ouatomary to subject the meat to a preliminary cooking or par-

boiling before placing it in the cans. This is done in order to partly

cook and at the same time to shrink the product.

"The cooking is carried on in large iron tanks, fitted with iron

tops,

usually hinged, which may be raise&or lowered. Sufficient water

is used to cover the meat, and the water is

heated by means of per-

forated steam coils in the bottom of the tank.

"When the meat has been sufficiently cooked it is removed from

the boiling water by means of large metal forks and transferred to.

the 'trimming bench/ where the gristle, surplus fat, and bone are

trimmed off. The meat then goes to the cutting machine, which cuts

it into pieces, according to the size of the cans to be filled. After

it is out to the proper siae the meat is passed down through a chute

to a lower floor to be paoked in the cans.

Filling and Capping the Cans.

"In the case of products like tongue and Vienna style sausage

where the form of the produot is to be preserved, it is necessary to fill

'stuff'

the oans by hand.

"In the case of corned beef and potted or deviled meat the cans

are filled by machinery. The stuffing machines, which usually work on

the rotary principle, consist of a series of pistons and cylinders. The

cans are placed beneath the cylinders, and the meat iB fed into the

cylinders from above and is forced down into the cans by the pistons.

"The meat is fed into the stuffing machine with scoops in whioh

the proper amount of meat for each is weighed approximately. When

the can leaves the stuffing machine, it is weighed and adjusted to

the proper weight by adding to or taking out a little of the meat.

"After the can has been weighed and the neoessary adjustment

made in the weight, the top is wiped and any projecting particles of

meat are shoved down into the can so as not to interfere with the oap.

The cap is next put in place and soldered under a rotary soldering

machine.

"The capping machines consist of a series of small revolving tables

upon which the cans are placed with the tops laid loosely in position.

The caps are then clamped down from above and are soldered either

by hand or by automatic soldering irons. When the cans leave the

oapping machine they have been completely sealed exoept for the

small vent holes in the top, through which the air within the cans is to

be later exhausted.

"The oans are next inspected for cap leaks (i.e. leaks in the solder

holding the cap), and these are repaired by hand.

Sealing the Cans Under Vacuum.

"The next step in the canning process consists in exhausting the

air from the interior of the can, and this is usually effected by means

THE CANNING INDUSTRY 246

of vacuum machines. The usual form of vacuum machine consists of

a large circular iron box with air-tight doors and a small glass window

through which the vents are sealed by means of an electric soldering

iron. The maohine is filled with cans and closed, the vacuum is then

applied, and the vents are sealed as the cans are brought beneath the

window on the movable bottom of the machine.

"From the vacuum machine the cans are run out on tables and

again inspected for leaks. Any leaks that are found are at once re-

paired by hand, the vents of these oans are then reopened, the cans

replaced in the vacuum machine, and the vents resealed under vacuum.

The oans are now ready to be processed.

Processing the Cans.

"Processing consists in heating the oans to a sufficiently high tem-

perature to insure the preservation of their contents, and constitutes

one of the most important steps in the whole canning process. Two

general methods of processing are followed, known as the 'retort

process' and the 'water process.'

"In the retort method of processing, the oans are placed in large

iron or steel boilers, known as retorts, which can be securely closed

by means of bolts. In these retorts the cans are subjected to the

action of steam under pressure and in this way high temperatures can

be secured. The length of time the cans remain in the retorts and the

temperatures employed depend upon the nature of the product and the

size of the cans.

"In the water method of processing, the cans are placed in large

open kettles or tanks rilled with water, which is raised to and

maintained at the boiling temperature by means of steam pipes.

The oans remain in the boiling water for varying lengths of

time,

depending upon the size of the can and the nature of the

product."

Along with the great growth which the canning industry has made

during the last quarter of a century there has been also a great in-

crease in the number of difficulties which have arisen. Many of these

problems of canning have been only partly solved. Within the last few

years the National Canners' Association (formed in 1907) and the

Experiment Stations have been attacking these problems, some of

which at times threatened the industry as a whole.

The study of the necessary heat treatment in ideal canning methods

has ocoupied considerable attention. This subject has been found

to be important not only from the standpoint of killing ordinary

spoilage organisms but because of the necessity of killing very resistant

organisms whioh are found to produce extremely poisonous toxins if

allowed to grow at all in canned goods. It is well recognized that a

few deaths due to the eating of any kind of canned goods results

temporarily in almost complete elimination of the particular kind of

oanned goods from the human diet. So this subject is being studied

246 INDUSTRIAL FERMENTATIONS

from two standpoints, that is, efficiency in food preservation and its

relation to public health.

Concerning the amount of heat treatment required by different

food in being canned, Bitting says: "Sterilization may be accom-

plished by heat below, at, or above the boiling temperature, depend-

ing upon the length of time the heat ia applied and the number of

applications made. It is not practicable to sterilize all foods in the

same way because of injury to quality or prohibitive expense. Steriliz-

ing below the boiling point is feasible only for a few products, prin-

cipally fruits, and then is advisable only when it is desired to preserve

a very fine appearance. This may be accomplished above 165 degrees

by maintaining the temperature for a longer time than when boiling,

or by repeating the operation on two or more successive days. The

object is to prevent breaking the tissue and loss of juices from the

fruits by excessive heat. This method of sterilization has been applied

experimentally and in private canning with gratifying results, but it

involved so much time and labor that it is not used commercially ex-

cept in a limited way. Sufficient work has not been done to say

definitely what products" can best be treated in this way nor what

temperatures are best suited for different foods. It has been used

chiefly with foods in glass, though equally satisfactory results are

obtained with foods in tins.

"Cooking at boiling temperature is practiced with nearly all fruits,

as the germs present are easily destroyed. Most of the fruits are

processed for from 12 to 25 minutes. The tomato is the most important

vegetable processed at boiling temperature, which is usually main-

tained for 50 minutes.

"Among the vegetables requiring a high temperature in processing

are corn, peas, beans, both green and dry, pumpkin, beets, and sweet

potatoes'. Corn is one of the difficult products to can, requiring a

temperature of from 245 degrees to 250 degrees F. for from 75 to 80

minutes, depending to a considerable extent upon how dry it is packed.

If very dry, the heat will penetrate to the center of the can very

slowly, the actual time required to raise the center to the temperature

of the bath being from 55 to 65 minutes. In a can of peas this is

accomplished in 6 or 7 minutes, the difference being due to the fact that

heat currents are set up in the liquid portion of the peas, while they

are absent in the corn. The necessity for a high temperature is

therefore dependent upon the ease with which the heat can penetrate

the product, as well as the resistance of the organisms. Some products

which were formerly processed by boiling for a long time are now

given a higher temperature for a few minutes, as the product has

a much better appearance when it is not overcooked.

"The penetration of heat in the can is dependent almost wholly

upon the ease with which convection currents are set up, occurring

most rapidly in products which permit the free circulation of water,

weak brine, or sirup between the solids, as in peas, and least rapidly

in the absence of free liquid, as in dry-packed sweet potatoes. Products

THE CANNING INDUSTRY 247

having a heavy though uniform consistency like pumpkin and squash,

require a long time for heat to penetrate to the center of the can.

Heavy tomato pulp takes a much longer time to reach the boiling point

than canned tomatoes, and soft ripe fruits, as apricots and peaches,

need more time to become sterilized than green fruit, not because

the germs are more resistant but because the heat can not penetrate

as readily as when the liquid circulates freely between the solid pieces.

Failure to recognize this principle of the movement of heat in liquid,

semi-liquid, and solid substances has caused the loss of thousands of

cases of foods. Mechanical agitation shortens the period of cooking,

especially in foods of heavy body; at the same time it places a greater

strain upon the can, with a tendency to increase the number of leakers."

Springers.

The Bittings discuss abnormal conditions in cans as follows: "A

springer is a can the contents of which are sterile but which has one

or both ends more or less convex. This condition is due to overfilling,

to generation of gas due to the action of the contents upon the con-

tainer, to packing the cans cold, or to paoking without a vacuum.

The condition may be so mild that the ends may be returned by

moderate pressure from the fingers or so severe as to cause bursting.

"A flipper is the incipient stage of a springer. The term is usually

applied to such cans as permit the ends to be returned to the normal

by pressure from the fingers or by striking the end of the can on some

firm object.

"Springers and flippers' are almost wholly confined to open top

cans,

and are most frequent in cans filled cold, as hand-packed toma-

toes.

The fruit has more or less air or gases incorporated in the inter-

stices of the tissues, and this is retained in the can if sealed cold or

given only a light exhaust. The subsequent rise in temperature will

cause expansion, which produces the convex end on the can. Springers

due to this cause tend to become more tense when subjected to

moderate heat, as in an incubator, and to disappear on being sub-

jected to a low temperature, as in a refrigerator. It has been a

frequent occurrence that foods packed in the North, especially in the

fall,

and shipped South to a warmer climate have developed the

trouble, and when returned to the factory, are found in perfect condi-

tion. Likewise the same goods which are perfect in appearance in

the winter present the appearance of springers the following summer.

If the materials packed were inert toward 'the can, the springers

would remain practically quiescent subject only to temperature varia-

tions.

"The effect of the retention of the air in the unexhausted or very

poorly exhausted can, is to increase the activity of the attack by

the content upon the metal of the container, and thus liberate hydrogen

gas,

which further increases the pressure and causes springers of

more marked character which will not return to normal by a moderate

248 INDUSTRIAL FERMENTATIONS

reduction of temperature or by pressure. It was assumed for a time

that because there is always head space in a soldered can the springer

was in reality due to overfilling and that the condition could be cor-

rected by leaving head space. Head space, unless the air be exhausted,

only aggravates the condition. In the case of the soldered can, the

sealing heats the air above the food to a higher degree than is done

by exhausting, causing it to be driven out through the vent, which

materially corrects the condition.

"Springers are also oaused by the action of the food material upon

the container, a condition independent of the fill or of the exhaust,

though slack filling and poor exhausting increase the rate and the

amount of activity. (One cubic centimeter of hydrogen is liberated

by each 4.8 mg. of tin,or 2.2 mg. of iron when dissolved in acid at

ordinary temperature.)

"In the case of springers there will be no vacuum, the gas given

off is hydrogen which will burn, and is emitted with one sudden spurt;

the contents are normal and fit for food unless they have been sub-

jected to prolonged heating, as those which have become.stack-burned.

The cans may show a marked galvanized-like effect or etching, de-

pending upon the kind of product."

Swells and Flat Sours.

A. W. Bitting and K. G. Bitting have the following to say by way

of describing what is meant by swells and flat sours: "Defects from

laok of sterilization are designated in the trade as swells' or puffs and

flat

sours,

though this classification is not striotly correct. A food

may not be sterile and spoilage may be very pronounced without the

production of gas or the development of a sour taste.

"A swell is a can having one or both ends or some part of the body

distended by pressure generated by gas-produoing organisms.

"A flat sour is a can in which the contents have been changed,

usually with an increase in acidity, but without the production of gas

or of only a very small quantity. The exterior of the oan presents

a normal appearance.

"The flat sour gets its name from the fact that the can shows

no distention and the food develops a very sour taBte due to the for-

mation of acid. This condition is not recognized nearly gro early as the

swell, due to the fact that there is no external evidence of trouble

to attract attention. Furthermore, the breaking down of tissue and

the formation of acid is a slower process than that of the gas formation.

Flat sours are rarely recognized under two weeks and frequently not

until after a month. If the trouble be due to thermophilic forms

then storage of only partially cooled cans in a warm warehouse will

favor the development.

"Flat sourer are generally associated with delays in handling foods

through the factory, as cut peas or jerked corn permitted to stand in

the yard or under the shed over night and to become more or less

THE CANNING INDUSTRY 249

heated. It is possible to have corn and peas sour so slightly before

packing as to be unnoticed, but later the heat of the process develops

a distinctly acid taste. This is not a real flat sour, as the number

of organisms are relatively small and are killed during processing,

and a flat or an acid taste is present from the start. In flat sours

the number of organisms is always high though they may not retain

their vitality for a very long time. Lack of oooling and stacking oans

while hot are the chief causes of flat sours. The cans which might be

sterile if cooled may not be sterile if not cooled

{

and this is true par-

tioularly if the process is close to the minimum line. This corresponds

with making media in a laboratory, where experience has shown that

tubes or flasks left in the sterilizer and which cool slowly are prone

to spoil, while those taken out and cooled remain perfect. A method

of testing for flat sours without opening the cans is to submerge them

in hot water until the ends spring out and then lay them on a table

to cool. The ends of the normal oans will collapse in a short time

while the defective will not collapse or will do so more slowly.

A. W. Bitting says: "A proper Birup is a necessity in the packing

of most fruits, and has become as much an essential of the grade as the

size and quality of the pieceB. The Birup may vary from very light

to very heavy, or between 10 degrees and 60 degrees on the Balling

scale. By common consent the sirups are generally made to be 10, 20,

30,

40, 50 or 60 degrees Balling."

Condition of Fruit to Be Canned.

In the canning of fruit Bitting says: "The first essential is that

the fruits be harvested when in prime condition, handled with oare

to prevent injury or bruising and conveyed with speed from the tree

or vine to the factory. For canning purposes it is not necessary, and

may not be desirable, that all fruits be as far advanced or as soft as

for eating, but they should be ripe, with the flavor characteristic of

the ripe fruit. They should not be so far advanced that they will not

withstand the ordinary cooking necessary for sterilization without

breaking to pieces. The*prime condition for canning is that state of

maturity in which the flavor and other characteristic qualities have

been developed to the maximum and may be retained during

sterilization.

"Bruised or damaged fruit can not be made attractive, and its

use involves heavy waste. The proper handling of the fruit is there-

fore very important. Apricots, peaches, pears, etc., should be handled

in shallow boxes which will not hold more than a bushel and will not

admit of more than three or four layers of fruit. The top should be

protected with cleats, so that one box can be set upon another without

touching the fruit, thus insuring some ventilation. The small fruits-^-

Btrawberries

, raspberries, blackberries, and loganberries—are handled

almost exclusively in chests, which are illustrated in detail in Plate II.

The California packers have developed this part of the business to a

250 INDUSTRIAL FERMENTATIONS

higher degree of perfection than those in any other section of the

country. The conical basket used in handling tomatoes in the East

should be abolished. The depth is

too great and the shape such that

the weight of superimpoeed fruit wedges the lower layers tightly

together, causing crushing, rotting, and excessive waste. The baskets

are weak, do not stack without bruising or cutting the fruit, and easily

become disarranged or broken in shipping.

"Rapid transfer of the fruit to the factory after it has been picked

is very essential. Deterioration in flavor and weight begins early;

conditions favor the growth of organisms, and bacteria, yeast, and

mold may develop wherever the fruits press together or the skins are

broken. Delicate fruits, such as berries, if picked in the morning

should be at the factory in the afternoon, or if picked in the evening

should be delivered in the morning. Fruits with hard skins will last

much longer, but the rule with all should be quick action. One of

the disadvantages of a factory located in the city is the delay in

receiving fruit promptly; dependence upon the surplus of the fresh

fruit market is hazardous.

"One source of trouble and a cause of spoilage of much fruit is

contamination from sour and moldy boxes. When a box is used several

times it becomes permanently infected and a cause of infection by

spoilage organisms. This can be controlled without much difficulty

by having a tight room in which the worst boxes are placed after they

are emptied, with steam turned on to saturate the atmosphere and

sulphur burned or sulphurous acid gas liberated'to act as a disinfectant.

This does not require a large place, much time, or expense."

Spoilage may be a rapid or a slow process. Bitting says: "Spoilage

may result from insufficient processing, defective containers, or the

use of unfit material. These losses are generally classed under the

heads of swells, flat sours, and leaks. Formerly losses were heavy at

many factories, but these are becoming lesB each year, owing to a

better knowledge of what is necessary in material, handling and im-

proved appliances. More attention is paid to testing for bacteria, and

greater care is taken in obtaining accurate thermometers and gauges',

automatic temperature-regulating devices and time recorders, so that

little is left to the judgment of the processor or helper.

"Spoilage due to insufficient processing is generally divided into

two classes—swells and flat sours. In the former there is

generation

of gas, causing the ends of the can to become distended; in the latter

the content of the can is sour, but there is nothing in the appearance

of the can to enable the customer to determine the condition until the

can is opened. Swells are generally due to the underprocessing good

material, while flat sours most often result from giving the regular

process to material which has been allowed to stand for some time,

such as peas remaining in a load over night or corn left in a car or in

a pile until it begins to heat. The raw material may show no evidence

of fermentation on superficial examination, but this condition fre-

quently exists under the conditions just cited. Swells are therefore