Browning of Dehydrated Vegetables during Storage R. R . LEGAULT, WILLIAM F. TALBURT, ANN XI. MYLNE, ISD LOUISE A. B R Y - J W e s t e r n Regional Resenrrh Laboratory, C‘. S.D e p a r t m e n t of Agricidtrire, A l b a n y , Calif. T h e rate of nonenzymatic browning of dehldrated regetables has been studied as a function of temperature. moisture content, and confining atmosphere. The brolc ning proceeds in linear fashion up to, and for a reasoriable distance bejond, the limit of palatability for dehldrated nonsulfited carrot, white potato, onion, and sweet potato. The effect of o n g e n on the browning rates of thebe four dehj drated legetables is relativel? small. The brow ninp rates lary exponentially with the reciprocal of the absolute temperature. The temperature coefficients are high. the QIOlalues larjing from 5.0 to 8.1 among these four regetables. The browning rates vary exponentiall? rc i t h the moisture content oler the range studied.
~ v i i aallo~vedto solidify, theii a 1- to 1.5-mm. holo \vats puiichid through both the was layer and the lid. Five cans, similiuly treated, were placed in a vacuum desiccator, and the whole evacuated to a gage pressure of about 0.1 mm. of mercury. Thi3ii t h r vacuuiii was broken with oxygen-free nitrogen gas. T h r (.-rk of reduction and restoration of pressure was repeated. Subsequrbiitly the tlcaicrntor 15-as opened and each can sealed immediately by t ourliing the was Isycr n-itha w i r m soldering iron. .lft(,r\vards a pc~rnianciitclosure was made by soldering thrpugh the \vas layer. I n tliii inariner the oxygen content of the atmosphere in the C H I I c m 1x1 reduced t o O.lYo or less, provided the nitrogen gas is of sat isfactory purity. T o adjust the osygeii conttwt to intermediate 1 partial rvacuation is needed. The other steps in the proc.cb(lure : t w as described for “complete” removal of oxygen. In this paper a nitrogen pack incans an osygeri coritciit of 1.5 0.5(>,in the atmospherr of the freshly packed can. -111 samples 1’roiii st orage, ivheiher air or nitrogen packed, were analyzcd for I )sygc.n and carbon diositle content of their atmospheres immediately before oth& test,s were conducted on the material. \Then t h e ohserved oxygen content of a nitrogen pack was high, iiitlicativr of leakage, the data for that, sample \veri’ discarded.
T
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HE brovaing of dehydrated vegetables has beeii noted 111 numerous contributions to recent literaturcx. The process is generally considered to be nonenzymatic in nature and is not t o be confused with reactions occurring in the browning of fresh apples and other fruits, or in the development of gray discoloration during the dehydration of white potatoes, Tvhich undoubtedly are of enzymatic origin. It, has been shown that the tendenq t o bron-n is aff ect,ed in positive manner by increasing temperature and moisture content (3,6, 11, 12, 1.5). I n the case of white potato the data show that the tendency of the product to turn hroivn is marliedly affected when the sugar contrnt esceeds a certain range ( 2 , 4 , 9, 1 0 ) . I t is knoivn that the addition of sulfite tends to inhibit the bro~vningprocess (I, 6, 7 , 8,13, 1 8 ) . Howrver, there is a paucity of quantitative data such as are required for the cvaluation of the relative e f f t of ~ ~the~ variables involved ill this bron-ning of dried vegetables. This paper is conci:riicd primarily ivith yuaiititative evaluation 1 ) t ’ broivnirig rate as a functjon of tcmpcrature, moisture content, and kind of vegetable. The quantitative relations shown hew may huggc’st models for devising furthw csperiments in this field. The experiments reported are a part of ari estensive investigation iindertakt,n for the purposca of inipi~)viiigtlw initial qualit!: airtl the storagi, stabilit>-of (i(~liytirat(~t1 vc~yc~tilhlri.
f
RIOISTURE, SUGAR, AND SULFITE DETERMINATIONS
The moisture contents of the dried vegetables were determined arc-ordingto the method of Nakorver, Chastain, and Sielsr’n (14). 111 hricf, the method includes grinding the vegetahlrii: to pass a C. S. Standard 40-mesh sieve and drying in a vacuum o w n a t s, total prwsure of less than 5 mm. of mercury. Thc oven tcmpcraturcs (shelf) and drying time5 used wf’r(’ 60” C*. a n d 30 hours for
Sugar as Glucose (\I.F.B.)b, 5%
PREPARATIO\ OF X I ~ T E K I A I , ~
The dehydrated vegetahl(+ iihrd iii the-.(, studies nere prepareti in the laboratory pilot plant or tvtw obtained from commercial sources. Tests on t h r drird prodvcts showed that- the steam blanching (onions \veri’ not tilanched) liatt reduced the enzymatic. activity to the degree specified by the Quartermaster Corps (19). The sources and other details pertaining t,o the samples used arc listed in Table I. Samplrs for storage were usually packed iri 211 X 413 sanitary t i n n d cans: f o i , dried cabbage, however, 401 X 411 cans were used. The amount of sample in each call was held constant for each dried vegetable--125 grams for carrot! white potato, and sweet potato, 130 grams for cabbage, and 93 grams for onion. The cans were closed with a semiautomatic double seamer. Where a nitrogen atmosphere was desired, the procedure was >isfollows: An area of about 1 square inch a t one end of the sealed van was covered with a thin layer of high melting (160’ F.) wax, n p p l i d in the molten stat? to the slightly warmcd lid. The was
Carrot
1‘I
Carrot Carrot Carrot White
2 2
I ‘1
Onion Sweet potato Cabbage
... .(.
....
lIalf dice 3 7 . 5 Full dice 4 9 . 6 Full dice 4 9 . 3 Half dice 4.9
16.3 10.6 8.0 2 2
\l-hite Rose
2
Oreguri
2
Oregon Half dire 2.6 1.4 Setred (;ern 73.0 40.4 , . . Flakes .., Full dice 3 7 . 3 2 8 . 8
potati,
White potatv
17 3
1”
pot a t 0 k\ hite
61.8
Half dire
...
Setted Gem
,
Half dice
Shreds
2.2
48.4
1.3
47.0
1920
8.0 6.2 5.4 6.2
6.0
660
6.0 8.9 7.6 j.3
,.
8.0
* * * * * * * * * f
0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2
400
8.0 * 0,2
.. ..
3.6 7.4
930
3.2
*
* *
0.3
0.2 0.3
a Processed in pilot plant of Western Regional Research Laboratory. Vther dried commodities were f r o m commercial plants. b h1.F.B. = moisture-free basis. C Value of deviations (standard) refers t o uniformity of moisture content of subsamples.
1294
October 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
I295
to p H 3.8 * 0.2, a region where color intc,iiaity ir quite insenaitivr to pH change. For similar reasons carrot extracts are adjusted to pH 6.6 * 0.2 and sweet potato extracts t o p H 4.5 * 0.1. h s s of pigments, through adsorption on thr Seitz pad, is avoided by saturation of the pad with + 0.8 pigments of the vegetable under study. This ic k Le readily effected upon passage of about 100 ml. of a Z W standard extract from the undeteriorated vcyw n table. 0.4 J In all cast's 200 nil. ~ isolvent ' are used for the esc 0 traction. For d h t l onion a 4-gram sample is oxtQ tracted. For coiivt~iiii~iice. this will be referred to c n hereafter as a 2 7 e s t r a r t . On the same basis, thr SWEET POTATO VYHITE POTATO, LOT 2 concentrations of the other extracts are 2.5Yc for carrot,, cabl~agr,and ,wmlt,potato, and 5c7, for o potato. Tra1isriiissivitic.s arc' read on a photoelectric colorimeter equipped n-ith a 420-my filt,er for cabbage, 0 60 120 I80 240 0 60 120 I80 carrot, and sweet potato, and with a 390-my filter for u-hite potato and onion. Tvm-centimeter cells DAYS OF STORAGE are generally used for the sample, although somr Figure 1. Linear Nature of Browning Rate for Dried Vonsulfited early results were obtained with a 1-cm. cell. FoI Vegetables Yitropen-Packed and Stored at 38" C. cvnvenienct., the symbol for extinction coefficient Cnrrot. l o t 2: moisture c o n t e n t (M.F.B.), 6.0%; E 420 mM; u , 0.02 will be arbitrarily assigned a superscript and t n o 390 mM; e, 0.02 Onion: moisture content (M.F.B.), 3.6%; E subscript5 t o represent the extract' concentration, the cell, and wave lengths used, respectively. Sweet potato: moisture content (M.F.B.), 7.4%; E 420 mfi; u , 0.003 For example, E' 420 mM refers to the optical a h i t e potato, l o t 2: moisture rnntent (V.F.B.), 8.0%: E :&. 390 mu: o, 0.01 drnsity of a 2 . 5 5 extract measured in a 1-em. cell in a colorimeter equipped - .. with a 420-my light filter. thc cabbage and oiiion, and 70" C. and 40 hours for tile wther vegeThe standard deviations found in a series of eight determiiiatables. Details of the method are given in the original article. tions made on ground referenrt. samples are listed in Table I1 The sulfite contents of the dried vegetables xere determined The eight determinations rvere made on different days. l
