wines, Brandies, *
and Cordials from Citrus Fruits HARRY W. VON LOESECKE, H. H. MOTTERN, AND GEORGE N.PULLEY Citrus products Station, u.s.Department of Agriculture, Winter Haven, Fla.
Orange and grapefruit wines of pleasant aroma and taste are prepared by adding corn or cane sugar to the juice, inoculating with pure cultures of wine yeast, and fermenting under carefully controlled conditions. Fortified orange and grapefruit wines are made by'adding orange or grapefruit spirits to the respective wines to increase the alcoholic content to 18 or 22 per cent by volume. Bayng th wine for about 60 days at -4O to 55q" C. (125" to 130" F.) gives a p oduct with a sherry-like flavor and color. Orange and grapefruit spirits and brandy are obtained by distilling fermented orange and grapefruit juices. The brandy is subsequently aged in plain oak barrels. Citrus cordials are made by the addition of citrus oils and sugar sirup to citrus spirits.
P
CCORDISG to the Food and Drug Administration (?) and the Alcohol Tax Unit, Internal Revenue Bureau (8), "wine is the product made by the normal alcoholic fermentation of sound ripe grapes." However, at present it is permissible to designate as wine the products obtained by fermenting citrus juices, with proper qualifications as to the fruit of origin; such wines are taxed according to their alcoholic content in the same manner as wine from grapes ( I ) . The scientific literature contains only a few references to citrus wines. Probably the earliest work was that by Cruess (a) who prepared orange wines containing between 4.25 and
a
4.50 per cent alcohol by volume. 3lcNair (6) cited a fenanalyses of such wines, and more recently preliminary work relative to the possibilities of alcoholic citrus beverages was reported by theBureau of Chemistry and Soils (Q,IO),-and by Hill (4), and Joslyn and Marsh ( 5 ) . At present there are five commercial citrus wineries in Florida, and their combined output is probably less than 20,000 gallons per year. Because of the increasing interest in the subject, the present paper deaIs more fully with the possibilities of this new industry and discusses the results obtained during two years of research carried out by the Bureau of Chemistry and Soils at Winter Haven, Fla. Inasmuch as the work reported here was carried out in Florida, using fruit gron-n in that state, the results may not be strictly applicable to citrus fruits gron-n in other states or countries.
Varieties of Citrus Fruits Available Several classes of citrus fruits are available in Florida for the preparation of alcoholic beverages. They may be divided into three botanical species : Cilrus aurantium sinesis, the coiiimon orange; Citrus nobilis, the mandarin group; and Citrus decumunn, the grapefruit. Of these three groups, only tn o apparently give wines of characteristic differencesnamely, Citrus decuinana and Citrus atirnntiurn sinesis. These experiment3 failed to indicate any decided differences between the wines prepared from Pineapple, Seedling, and Valencia oranges. Furthermore, i t would not be comniercially practical to attempt to segregate Pineapple and Seedling oranges because of close similarity in many cases. None of the experimental citrus wines had a characteristic odor and taste which would afford positive identification of the fruit of origin. illthough grapefruit and orange wines differ in odor and taste, they do not possess a pronounced grapefruit or orange flavor, differing in this respect from grape wine, where the variety of grape makes a vast difference in the quality of the finished product. During the normal ripening of grapefruit, oranges, and 'tangerines, there is a decrease in acidity and an increase ill sugar. The composition of the fruit varies somewhat according to the location in which it is grown. The following data show the limits in the case of oranges as determined by analysis of the juice, and cover the season's pack lasting from January t o May, 1931: Reducing sugars, Yo Nonreducing sugars, 3' % Total sugars %, Total acid ( i s citric), %
.4sh,,% Speclfic gravity
2,20-6,87 2.60-5.77
4.80-12.64 0.53-1.61 0.32-0.55 1.03-1.06
Grapefruit contain less sugar than oranges or tangerines, and the following data represent the range in composition of grapefruit juice [Duncan) from September, 1933, to hpril, 1934: Reducing sugars % Nonreducing su&rs, % Total sugars 70, Total acid (a's citric), % Brix, %
2.00-4.39 2.91-3.75 4.91-8.14
1.51-1.96
9.7-12.1
The sugar content of the juice of oranges and grapefruit is never as high as that of grapes; the sugar content of grape must runs from 12 to 25 per cent. Because of the low sugar content of citrus fruits it is necessary, and permissible by lax, to add sugar to the juice prior to fermentation if wine is to be made. The high acid content apparently precludes the manufacture of a satisfactory dry wine. When attempts were made to prepare such a wine, the product was excessively sour. I n several instances the acid was partly neutralized by the addition of calculated amounts of calcium carbonate; although the resulting wine was less sour, it possessed a disagreeable musty flavor. 1224
INDUSTRIAL I N D ENGINEERING CHEMISTRY
OCTOBER, 1936
Extraction of the Juice Fruit used was third grade and culls. Both grades represented sound fruit which, because of size and color, is unsuitable for the market. Juice n-as extracted by cutting the fruit in half and reaming by hand on revolving burrs. The juice was passed through a revolving screen of 20-mesh monel metal to separate pulp and seeds. Tangerines, because of the comparatively brittle and thin peel, are not so readily adaptable for reaming. I n this work the whole fruit was disintegrated by means of a screw press, and the juice thus obtained was screened and passed through a supercentrifuge to separate the peel oil. This method was not entirely satisfactory because the juice still contained sufficient oil to retard fermentation. The juice also possessed an objectionable bitter taste which rendered it unfit for wine purposes. Upon distillation, horn-ever, a product was obtained which had an unusually delicate and pleasant aroma, making the distillate ideal for the preparation of certain classes of cordials and liqueurs. Because of the oil i t contained, the product became milky when diluted to proof with distilled water and for this reason Tvould probably not be entirely satisfactory for a beverage similar to brandy. A satisfactory juice for wine making was obtained by h s t peeling the fruit by hand and extracting the pulp in a screw press. The yield of juice varied according to the size and condition of the fruit. Freezing tended t o cause the juice sacs to dry out (S),but otherwise such fruit, if not actually decayed, was suitable for fermentation. Fruit not frozen and therefore not exhibiting dryness yielded, on the average, about 85 gallons of juice per ton and might run a t times as high as 104 gallons per ton. Cruess ( 2 ) obtained 132.8 gallons of juice per ton in the case of sound Califcirriia Valencia oranges
$/KT1 so
100
IS0
200
250
300
GRAMS S U C 9 0 5 E A C OED PER LITER
FIGURE1. YIELDOF ALCOHOL OBTAINED BY ADDING DIFFERENTAMOUSTS OF SUCROSE TO ORANGE AND GR4PEFRUIT JUICE(CISLTURE 4097) Grapefruit juice possesies a bitter taste caused by a glucoside, naringin. During ripming this glucoside diminishes but never entirely disappears. Fermented grapefruit juice is more bitter than unfermented juice because in the latter case the sugars tend t o mask the bitterness. For this reason grapefruit wine may have such a bitter taste as to render it unpalatable. The bitterness may be decreased by (1) filtering the juice clear prior to fermentation with a filtering medium and a plate-and-frame press, or (2) treating the fermented juice with activated charcoal a t the rate of 4 grams of char per liter of juice (0.5 ounce per gallon) at 55" C. (130" F.) for 30 minutes. Treatment of the wine with char yields better results than mere filtration. Subsequent aging in barrels further improves the flavor.
1225
Fermentation Organisms Pure cultures of wine yeast were used in this work. With the exception of one culture (No. 104), they were obtained from the American Type Culture Collection at Chicago. Transfers to n-ort agar were made every three months and stored in the ice
box.
The cultures used mere as follows:
4123, Saccharomyces ellipsoideus, Burgundy wine yeast. 4097, Saccharomyces ellipsoideus, var. Champagne, "French champagne yeast .' ' 2338, Saccharomyces ellipsoideus, I. Hansen. 2575, S. spiritus vini. 4134, S. sakd Y a b e , Japanese sake yeast. 104, Saccharomyces ellipsoideus, isolated f r o m oranges by H a r r y E. Goresline, Bureau of Chemistry a n d Soils.
Two loops of the culture to be used were added to 50 cc. of sweetened sterile orange or grapefruit juice. The latter was allowed to incubate 48 hours at 24" C. (75' F.) and was then added to 500 cc. of sweetened sterile juice. After 48-hour incubation, the juice was in a c t i v e fermentation and was added to 5 liters of s w e e t e n e d orange juice which had been pasteurized by boiling for 15 minutes and t h e n c o o l e d . This quantity of "starter" juice was sufficient to inoculate 50, liters of unpasteurized juice. Several experiments were made where the main batch of juice was pasteurized, but this treatment had no apparent effect on the wine. Pasteurization would a l s o e n t a i l an additional step in the process with a resulting increase in cost of PER CENT ALCOHOL U I VOLUME production. FIGURE 2. CORRELATION BEIt was found advisable to TWEEN BRIX OF JUICE BEFORE use starters not older than FERMENTATION AND ALCOHOL IN 48 hours; older s t a r t e r s JUICE AFTER FERMENTATION caused a delay in fermenta( CCLTURE4097) tion. The sludge from a previous batch may be used as astarter even though such sludge is not a pure culture It was found inadvisable to use this sludge more than once. Citrus wines prepared by using different types of pure cultures of wine yeast failed to show any marked differences in either bouquet or taste.
Sweetening the Juice Citrus juices contain such a small amount of sugar that additional sugar is necessary to yield a wine which will be palatable and possess satisfactory keeping qualities. The amount of sugar to be added depends upon the desired alcoholic content of the finished wine (Figure l). There is a limit to the quantity of sugar to be added, for when a certain amount of alcohol has been obtained by fermentation, the yeast is killed or rendered dormant by the alcohol so formed. However, as high as 17 per cent alcohol by volume has been obtained without taking special precautions. Numerous experiments carried out during this work showed that, if the Brix reading of the juice after the addition of sucrose is multiplied by 0.55, the result will approximate the amount of alcohol (in per cent by volume) in the finished product, provided fermentation has been complete. This figure was obtained from experiments using culture 4097; insufficient data are available to state whether this figure is valid when other cultures are used (Figure 2).1 Inasmuch as ordinary table wines contain approximately 13 per cent alcohol by volume, sufficient sugar was added to increase the Brix of the juice t o 24", which upon fermentation 1 I n t h e case of grape must, Bioletti found t h e alcohol yield to be 0.675 X Brix reading [Joslyn and Cruess, Calif. Agr. Bxpt Sta., Circ. 88 (1934)l.
VOL. 28, NO. 10
INDUSTRIAL AiXD ENGINEERING CHEMISTRY
1226
TABLE
I.
Run
h 0.
YIELD O F CITRUS WIXES CPON
Fruit Juice
Grapefruit Grapefruit Grapefruit" Orangea Tangeloa Orange Orangeb Orange Orang0 Orangeb Orangeb
.Juice
Orininai
Ton Frult
of Juice
Field/ Weight of Fruit
Kg. 1 2 3 4 5 6 7 8 9 10 11
ADDITION OF DIFFERENT AMOLXTS OF (CULTURE 4097)
109.5 107.3 39.6 41.4 39.6 67.8 200.3 58.6 212.0 258.0 246.0
Lb. 241 236 87 91 87 149 406 129 468 567 54 1
Gal. 81.0 76.1
104,O
84.2 97.4 78.2 56.7 82.1 73.8 63.5 63.2
~
Brix
~~
~~~~
B&
10.5 10.2 10.7 12.4 11.1 12.6 9.6 11.0 10.7 9.6 10.3
SUCROSE TO THE JUICE PRIOR TO FERMENTlTION
Sucrose Added Lb./ Grams/ gal. liter 2.34 2.00 1.84 1.60 1.80 1.14 2.00 1.53 1.79 1.78 1.59
280 240 220 192 215 136 240 183 2 14 213 190
after Adding Sucrose
26 23.6 26.0 24.9 25,l 24.0 26.0 23.8 25.0 24.3 23.6
~.
TemD. of Fermentation
Wine Yield/ Ton Fruit
O C .
Gal.
16 16 16 16
16 16 16 16 16 16 16
90.0 89.9 110.0 92.0 107.0 85.0 63.0 86.0 81.0 70.0 69.0
.Uro-
hol in Wine T-ol.
yo
14.8 13.7 13.1 13.8 12.7 13.7 14.4 13.0 13.8 13.6 13.5
Total Reducing Sugars in Wine Grams/ 100 c c .
0.35 0.60 1.96 1.96 3.04 0.40 0.40 0.44 1.09 0.30 0.40
* From Indian River section of Florida. Culls (froren fruit)
yielded a wine of approximately 13 per cent alcohol by volume (Table I). I n general, the amount of sucrose to be added runs from 180 to 210 grams per liter (1.5 to 1.75 pounds per gallon) of juice when wine is to be made. If hydrated corn sugar is used, 10 per cent more must be added to compensate for the water of crystallization in the sugar. Brown sugar was found undesirable because it imparted a molasses-like odor and taste t o the finished product. I n the preparation of citrus spirits, 120 grams per liter (1 pound per gallon) of sugar were added to the juice. This t r e a t m e n t yielded a fermented juice containing about 10 per cent alcohol by volume. The addition of the specified a m o u n t s of sugar i n c r e a s e d the volume of the liquid by 7 to 10 per cent. It app e a r e d t o make little difference in the ultimate results whether the sugar was added a t one time or in small quantities. Figure 3 gives approximate reFXQURE 3. GRAPH FOR OBTAINING s u l t s f o r t h e AMOUNTOF SUCROSE TO BE ADDEDTO amount of sucrose ORANGEOR GRAPEFRUIT JUICES to be a d d e d t o Multiply by 0.008 for pounds per gallon o r a n g e or g r a p e fruit juice to obtain a desired Brix. The original Brix of the juice and the Brix desired are read on the ordinate axis. The point where these values bisect the oblique line is noted, and then these values are read on the abscissa. The smaller value is subtracted from the larger, giving the amount of sucrose to be added. For example: The Brix of the original juice is 10'. This value bisects the line at 15 as read on the abscissa. It is desired to add sucrose to increase the Brix to 22". The reading of 22 on the ordinate bisects the line et 200 as read on the abscissa. Subtracting 15 from 200 gives 185, the number of grams of sucrose to be added per liter of juice.
The amount of alcohol in the finished wine not only depends upon the Brix of the juice and temperature of fermentation, but also upon the culture used and kind of sugar added.
Temperature of Fermentation Juice from grapes is allowed to ferment a t temperatures as high as 32" C. (90' F.). It was impossible to ferment citrus wines a t such high temperatures because, owing to the presence of acetic acid organisms, high fermentation temperatures yielded a product with a pronounced aroma of ethyl acetate. It was found desirable, in making citrus wines, to maintain the temperature a t about 16' C. (60' F.), and withbrandies,between24' and 27" C. (75' and 80' F.). Fermentation was started at about 24' C.; when it became active the must was cooled to the desired temperature by pumping the fermenting juice through a tin coil immersed in an ice bath. This method is more efficient than immersing cooling coils in the fermenting vat itself, and circulation of the juice permits aeration and hence lessens the danger of "sticking" (arresting of fermentation caused either by too high a temperature, disease, or insufficient oxygen). Addition of ice to the must is not good practice. The active fermentation takes place during the first three days. Without m e a n s of cooling, the temperature may rise to as high as 35' C. (95' F.) and even high enough to be d e t r i m e n t a l to the y e a s t , but not sufficiently high to hinder b a c t e r i a l action. The alcoholic fermentation of 1 gram of sugar liberates 120 calories; 1gram of sugar per 100 cc. juice will theoretically i n c r e a s e the temperature 1.2'C. (2.2' F.). Since citrus juice to be fermented contains about 23 per cent sugar, there will be a rise i n t e m p e r a t u r e of 27.6' C. (50.6' F.), provided no heat is lost by radiation. The yield of alcohol at different temperatures nf fermentation is plotted in FigFIGURE 4. YIELDOF ALCOure 4. If fermentation was HOL OBTAINEDBY FERMENTcarried out a t a temperature INQ SWEETENED ORANGE higher than 25' C. (77' F.), JUICE (24" BRIX)AT DIFFERthe alcoholic content of the ENT TEMPERATURES (CULTURE. 4097) finished w i n e d e c r e a s e d
FIGURE 5 (Right). ALUMINUM PRESS FOR INITIALFILTRATIOK OF CITRUSWINES This is a 7-inch square, two-eyed, closeddelivery type with flush plate and frame pattern, inrluding eight chambers. The wine is forced through the press by means of compressed air. I n a commercial installation the l i uid would he forced through by a t u h n e pump.
FIQURE6 (Below). E X P E R I WENT.AL BR.LTDYSTILL
rapidly with increase in temperature. Besides being low in alcohol, wines fermented a t the higher temperatures were also darker in color. The lots of wine from which these data were obtained were fermented in flasks plugged with cotton. Probably the loss of alcohol caused by the higher temperatures was less than would be experienced if fermentation were carried out in large open tanks.
Treatment after Fermentation ]Then a vigorous starter of yeast was used and the temperature mas properly controlled, active fermentation was completed in 10 or 11 days, provided tanks with loosely fitting covers were used. In containers fitted with a water seal, where carbon dioxide was not readily lost, the period of fermentation was longer. The cloudy, supernatant wine was yiphoned from the sludge and mixed with a filtering medium, rising 1 or 2 per cent based on the weight of the wine. In the experimental work an aluminum plate-and-frame press (Figure .5) was used which gave satisfactory results although the filtered product was slightly hazy. This is not important \ince the wine must be passed through a "polisher" after aging and before bottling. rlfter initial filtration, alcohol and total sugars were determined in the wine. If fermentation had proceeded properly, the residual sugar amounted to about 0.4 gram per 100 cc. (as reducing sugars), and the alcohol to 13 or 14 per cent by volume. Such a product is a dry wine and has a more or less musty, harsh flavor which does not improve upon aging. Therefore it was found advisable to add sugar (either corn or sucrose but preferably the former) to increase the final sugar content to either 3 or 5 per cent. Such a wine is not excessively sweet and resembles a sauterne. I n no case did the addition of sugar cause secondary fermentation. Fortification of Wines Fortified citrus wines were prepared by increasing the alcoholic content to 18 or 22 per cent by the addition of citrus wine spirits. If the wine contained 18 per cent alcohol, corn sugar was added to increase the sugar content to 7 per cent. If the wine contained 22 per cent alcohol, the sugar content was increased to 10 per cent by adding 7 per cent sucrose and 3 per cent corn sugar. The fortified wines were heated to 52-55' C. (125-130'' F.) for about 60 days. Heating was carried out in plain oak barrels; the wine darkened and assumed a sherry-like flavor 1227
1228
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
VOL. 28, NO. 10
which served to return part of the c o n d e n s a t e a n d w a s h ascending vapors, was attached to the top of the column. The still had a capacity of about 12 liters (3 gallons) of 120-proof spirits per hour. Table I1 indicates the yields obJUICE tained from grapefruit j u i c e wr h e n different amounts of sugar were added YEAST SCREW FERMENTERS FERMENTERS to the juice prior to fermentation. The juice used was extracled from cannery waste consisting of cores and DISTILLING EMULSION broken sections left in the preparation of grapefruit for canning. Each run I I consisted of 189 to 303 liters (50 to 80 CENTRIFUGE DRVER SUGAR gallons) of juice. GRAPEFRUIT I Storage was carried out in both plain and charred oak kegs of 3- t o 5-gallon CORDIAL COLDDAIRY MIXING PRESSED SPIRITS capacity. Because of the larger surface exposed in such small containers in proportion to the bulk of liquor, AGING DISTILLED BLENDING AGING VACUUM TANK WATER TANK STILL TANK t h e p r o d u c t had a "woody" taste. This defect, which would not be encountered in large casks, was later CONCLNTRATED POLISHER BARRELS partly corrected by treating the barrels with ethyl alcohol vapor in a reflux i system and then s t e a m i n g . The BOTTLER product aged in charred barrels pos1 1 sessed a whisky taste but appeared to i age more rapidly than the brandy in LIGHT CDRDWLS BRANDY WINE plain oak barrels. N e i t h e r g r a p e f r u i t nor orange FIGURE 7. FLOW SHBETFOR MANUFACTURE OF LIGHTAND FORTIFIED ORANGE AND GRAPEFRCIT WINES, BRANDY,AND CITRUSCORDIALS, WITH PROVISIONS FOR MAKIXG brandy has an aroma or taste suggesL C O L D - P R E S S E D AND COSCENTRATED C I T R U S OILS .4ND DRIEDC I T R U S W A S T E tive of the fruit or origin, although some tasters were able to detect an and color. Heating had to be carried out with care to preorange flavor in orange brandy. Grapefruit and orange brandy vent local overheating and excessive loss of alcohol. may be distinguished from one another; the former possec;ses a characteristic aroma and taste difficult to dewribe accurately. CITWS
p 1 7 y " i
I"'.:*"I
i
I I
I
111
I-
i--;...14-1
,
Aging of Wines Both the light and fortified citrus wines were aged in plain and charred oak barrels between 27' and 32" C. (80' and 90" F.). Citrus wines stored a t these temperatures are palatable after as short a storage period as seven months.
Spirits and Brandy I n the preparation of citrus spirits and brandy, sufficient sugar was added to obtain a fermented juice containing approximately 10 per cent alcohol by volume. This amounted to about 120 grams of sucrose per liter (1.0 pound per gallon) of juice. The more sugar added, the greater the yield of brandy upon distillation, but there is a limit to the amount of sugar that can be introduced. It has been determined that large amounts of sugar may yield a brandy of less aroma and flavor, and for that reason the amount added is kept to a minimum consistent with profitable recovery of alcohol. Preparatory to distilling, the cloudy wine was skimmed and siphoned from the yeast sludge. Failure to do this sometimes resulted in a brandy with a yeasty taste and odor. The still used (Figure 6) did not represent a commercial installation but was constructed mainly in the laboratory. Its efficiency was therefore not equal to t h a t of better stills specially designed for this purpose. The fractionating column consisted of a copper pipe 7.6 cm. (3 inches) in diameter and about 3 meters (10 feet) high. The column was lagged with the exception of about 30 cm. (1foot), which was cooled with sprays of water. Carbon Raschig rings were used in place of the conventional plate and bubble caps. A separator,
Cordials The finest imported cordials are prepared with cognac as a base; the cheaper domestic cordials are made with grain or molasses alcohol. I n preparing citrus cordials, citrus spirits was used as a base, with the addition of sugar sirup and oil expressed from the peel of the fruit. il better product was obtained from citrus spirits than from ordinary molasses alcohol. The citrus cordials were standardized to contain 33 per cent alcohol
TABLE 11. YIELD OF GRAPEFRUIT BR.4XDY UP09 .4DDITIOII O F DIFFEREXT AMOUXTSOF SUCROSE TO THE JUICEPRIOR TO FERMENTATION (CULTURE 4097) Original Brix
Run
of
No.
Juice
1
9.5 10.3 9.0 10.2
Brix Temp. Alcoafter of hol Adding Fermenin Sucrose Added Sucrose tation Mash
Lb./ Crams/ gal.
2
10.2 10.2 10.2
9.4
10
11 12 13 14 a I
9.4
9 7
9.7 10.1 10.1
9.7
0 0
1.4
1.4 1.4 1.0 1.0 1.0
1.0 1.0 1.0
1.0 1.5
0.5
liter 0 0 167 167 167 120 120 120 120 120 120 120 180 60
OC.
.. 2;:5 23.2 23.2 17.3 19.3 19.2 20.9 19.0 18.5 19.6 22.7
gallons per 100 gallons of juice.
..
24 26 24 22 19 26 26 24 23 18 20 24 25 20
Vol. %
Yield of 100Proof Brandy
Literslhee-
11.0 10.0 11.3 10.1 9.9
Lolitera 7.6 8.0 21.9 22.6 20.0 19.1 19.8 17.7 18.3 17.8 17.1
12.7
23.1
3.85 4.35 14.4 13.8 12.7 10.9
10.6 7.4
18.2
14.8
OCTOBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
by volume, and about 37 per cent total sugars. The following is a typical formula for grapefruit: Sucrose, grams Corn sugar, grams 164-proof citrus spirits, cc. Distilled water, cc. Naringin. gram Grapefruit oil (cold-pressed), cc. Certified yellon dye
paratus and processes to yield the products discuqced here, as well as a number of by-products.
Literature Cited
888
888 1510 1165 0.90
5.8 Su5oient quantity
The oil and nari,lgill nere dissolved in the spirits by violent and corn agitation and then added to the sirup Of wear. The mixture \vas allowed to stand a t about 27” to 30’ C. (80” to 85” F.)for :iweek and then filtered, using a filter aid. Aging was carried out in glass. Tangerine, lime, lemon, and orange cordials were made in a similar manner by the use of the cold-pressed and distilled oils from the peel of the particular fruit. Figure 7 shows in schematic form the arrangement of ap-
1229
(1) Congress of U. S. (74th), Public S o . 401 (H. R. 8870), pp. 12-14, See. 11 and ff. (2) Cruess, W. V., Calif. Agr. Expt. Sta., BULL244, 157-70 (1914). (3) Gary, W. Y . , Fla. Dept. Agr., Chem. Lab. Div., p. 17, March. 1 Q25 _---.
(4) Hill, H. P.. Fruit Products J . , 14, 138-42, 156 (1935) (5) J o s h , M.A., and sfarsh, G. L , I b i d , 1 3 , 3 0 7 (1934). (6) McNair, J. B , “Citrus Products,” Pt. 1, pp. 138-9, 1926. (7) U. S. Dept. Agr , Service and Regulatory Announcement, Food and Drun S o . 2. Rev. 3. D. 18. June. 1932. (8) U.S. Treasury Dept., Bur.’Prohibition Reg. 7, May, 1930, Sec. 610 (Act of Feb. 24, 1919: 40 Stat. 1057). (9) VonLoesecke, H. W., Cztrus Ind., 15, S o . 7 , 8-9, 20-1 (1934); Proc. Fla. State Hort. SOC.,1934, 85-90. (io) Von Loesecke, H. w., F L ~Grower, . 43, 5-6 (1935). RBCEIVED
July 11, 1936.
Department of igriculturs, Food Kesearch 131-
vision Contribution 293.
Combustion Qualities of Diesel Fuel
0 . I). BOERLAGE AND J. J. BROEZE N. V. de Bataafsche Petroleum Maatschappij, The Hague, Holland
DEAL conibustion in a C. I. (compression ignition) engine means instantaneous and complete combustion of every particle of fuel as soon as i t is injected (Figure 1, I). I n practice, combustion lags behind the theoretical curve at two points: ( a ) Mainly because of insufficient reaction velocity during the pre5ame or ignition period (phase 1, Figure 1, 11); this lag, however, is overcome by the rapid spread of flame (phase 2) so that even fuel injected later (phase 3) burns with great rapidity; the reaction velocity is meanwhile greatly increased by the high temperature. Mainly because of inefficient mixing of fuel and air; this lag(b) increases towards the end of phases 2 and 3.
Combustion is not finished when fuel injection stops; it continues as long as fuel particles find oxygen. This is called “after-burning” (phase 4); i t lowers the efficiency and performance of t h e engine because the expansion ratio is decreasing a t the same time. As the fuel quantity per cycle is increased, after-burning increases and results in incomplete combustion a t the end of the expansion stroke, although air is still present. The imperfection of the mixture is due to two causes: defective micromixture (insufficient atomization and evaporation) and defective macromixture (insufficient distribution) (2, 3). As the fuel is injected in the liquid phase, there is insufficient evaporation during the ignition period. The latter is represented as follows (Figure 2) : (a) Heating of the droplets, partial evaporation, and rapid subse uent heating of the vapors to air temperature (600’ to 800’ by d’irect contact. (b) Development of heat from reaction of the vapors, which causes locally increased temperatures (beginning with one or more points, where the vapor concentration and other conditions are most favorable), until flame temperature is attained which spreads rapidly.
8.)
Period a may be called the “physical delay” (endothermic), and period b the “chemical delay” (exothermic); actually the two overlap. For normal light Diesel fuels the chemical
character governs the total ignition period, showing that the physical delay is short; heavy fuels such as oil residues have relatively lower cetene numbers than the gas oil fractions from their crudes (Table I). Evaporation before the flame is formed depends mainly on the temperature of the compressed air and of the walls and on the nature of the spray. When starting cold, i t may be insufficient in many types of engine; sometimes an excess of fuel may be helpful (the equivalent of choking in carburetor engines)
The ignition period consists of a physical and a chemical delay. The former becomes important with heavy fuels, the latter is normally predominant. After-burning is mainly due to uneven distribution and to slow evaporation of fuel deposits on combustion chamber walls : the most favorable mixing conditions are, at best, a compromise between these two. The fuel influences the mixing process by its viscosity, its volatility, and its ignition quality; the optimum value of each property varies with the engine type. A better criterion for volatility is needed. Combustion in C. I. engines is mostly of the destructive type: under certain conditions there is evidence that it may be partially an oxidation process-for example, according to the hydroxylation theory.
