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The dehydration, shelf-life and potential uses of citrus pulps
Journal of Food Engineering 2 (1983)
19-34
The Dehydration, Shelf-life and Potential Uses of Citrus Pulps N. Passy and C. H. Mannheim
Department of Food Engineering and Biotechnology, Technion - Israel Institute of Technology, Haifa, Israel
ABSTRACT Technological treatments and drying conditions for citrus pulp, which accounts for lo-20% of the refuse from citrus juice plants, were established. Washed and unwashed pulp with 5 and 15% total solids (respectively) and partly washed material containing peel, with about 10% solids, served as raw materials. The flakes obtained from a drum drier (at about 4% moisture content) were non-uniform and had low bulk density. Water holding capacity was up to ten times their weight. Drying ratios of 7:l and l&:1 depending on the total solid content of the raw material, and drying rates of 15-22 kg h-’ m-’ dry material were obtained on a pilot double drum drier. Antioxidants added prior to drying prevented fading and development of rancidity during storage. The drum dried product, kept in darkness, at temperatures below 25°C. maintained good colour, flavour and flowability for over a year. Substituting some of the flour, in Snap cookies and yeast cake, with citrus fibres showed they could serve as binding and filling agents. Sensory evaluation of these products showed those with levels up to 7.5% fibres (containing peel) and 20% fibres (without peel) were tast_v.
INTRODUCTION One of the waste streams obtained after straining citrus juice is the juice sacs which account for lo-20% of the total refuse from citrus juice 19 Journal
Publishers
of Food
Engineering
Ltd, England,
0260-8774/83/0002-0019/$3.00
1983, Printed
in Great Britain
0 Applied
Science
20
N. Passy, C. H. bfannheim
plants. The juice sacs themselves may be washed or unwashed depending on the production procedure in the citrus plant and on the client’s demands. In many cases they are frozen and sold as raw material for various types of beverages or other products. In some plants the juice sacs are combined with membranes, ruptured vesicles or some peel residue ejected from the juice extractor, called pulp in this work. By drying this waste stream a by-product of high fibre content having high water and fat holding capacity is obtained (Kesterson and Braddock, 1976). The possibility of using this product in human food, instead of cattle feed, may result in substantial economic returns. Due to the presence of reducing sugars, the dried, unwashed pulp may undergo browning and produce a sticky product causing impairment of flow properties and caking. Therefore, the pulp is passed through a multiple stage pulp washing system to remove soluble solids yielding a final product with 2-4” Brix. Steger (1980) described a seven-stage pulp washing system to obtain a completely washed pulp and recover maximum soluble solids from it. Studies on our part showed that three counter current washing stages at a 1: 1.5 pulp to water ratio, were sufficient to give a washed pulp with 1-2” Brix. Autoxidation of lipids and oxidation of essential oils and pigments may result in a rancid flavour of the product and fading of its colour, especially if exposed to light. Oxidation is more pronounced in materials which contain peel, where the concentration of pigments is much higher. Antioxidant addition before drying retards the oxidation processes and results in a long shelf-life product if stored under suitable storage conditions (Kesterson and Braddock, 1976; Ferguson and Fox, 1978). In order to obtain a shelf-stable reconstitutable material 85 to 95% of the water must be removed. Drying procedures for citrus pulp were surveyed by Kesterson and Braddock (1976). A recent comparison (Ferguson and Fox, 1978) of various drying procedures, namely: tunnel belt, double drum drier, rotary kiln, spray and foam mat drying, has shown that a double drum drier was economically favourable. Although the rotary kiln appeared favourable both from capital standpoint and operating cost, the product agglomerated and darkened. The dried citrus fibres have some unusual functional properties due to their blandness and their very low bulk density (if not milled). They are low in calories and high in fibre and have a high water and fat
The dehydration,
shelf-life and potential uses of citrus pulps
21
holding capacity (Kesterson and Braddock, 1973, 1976; FMC Corporation, Lakeland, Florida, private comm., 1978; Belshaw, 1978). Due to these properties a number of potential applications were suggested. Some of the potential and existing uses are: cells for beverages, thickeners and gelling agents, dietary fibres for bread, cakes and cookies, cereal breakfast and health foods and low calorie bulk fillers and binders in meat products. The presence of peel particles in the dried pulp constitutes a problem due to its high vulnerability to oxidation and its bitterness. Information regarding juice sacs and pulp without peel is available. However, none exists concerning pulp products containing peel. This study was undertaken to establish optimum technological treatments and drying procedures for citrus pulps (containing peel) and to test their storage stability. Another object was to evaluate some food applications for the dried fibres and establish the maximum amount of peel components which can be incorporated to obtain satisfactory products.
EXPERIMENTAL Raw materials from a citrus plant (which contained juice sacs, core, membranes and peel) were collected and frozen in 5 kg quantities in polyethylene bags. These were transported and processed in our pilot plant. The sources of raw materials, and methods of preparation are shown in Fig. 1. Most of these were from oranges, but one experiment was conducted on pulp originating from grapefruit. The raw material was thawed at 4*C, and some of it was washed to remove solubles, filtered, disintegrated and drum dried. The finished dried product was stored in darkness and in light at various temperatures.
Equipment 1. A small double drum drier (made by L. A. Mitchell, Manchester, England) was used for preliminary experiments. The drums rotated at speeds up to 4 rpm. Their size was 152 mm in diameter and 152 mm in length; they were chrome plated, and had steel doctor blades. Steam pressures of l-2.5 atm were used.
N. Passy, C. H. Mannheim
22
I
EXTRACTION
RAW JUICE
-
CORES
I
SCREENING IN
GISIN;EGRATION
FINISHER
PRIME JUICE
NASilNG t SEPARATION
PULP 11%
LIQUOR
1
I
SEPAPiTING
SEPARATION
Fig. 1.
WASHED JUICE SACS E-h%
PEELS:C;UICE 44%
1 WASHING (1-6 steps)
SECONDARY JUICC
PEEL MEMBRANES
LIQUOR
JUICE SACS 4-6%x
Schematic diagram of raw material preparation.
2. A medium sized type AZ 130 double drum drier (made by Esher Wyss, Switzerland) was used for the larger experiments. The drums rotated at 1.5-9 rpm; they were 3.50 mm in diameter and 600 mm in length and were chrome plated and had steel doctor blades. Steam pressures of up to 3.5 atm were used. Methods Ascorbic acid This was determined according to AOAC (1975). Four grams of dried fibres were dispersed in 80 ml distilled water. The solids were removed by centrifugation, and a titration was made with 2,6-dichlorophenolindophenol. Browning Four grams of fibres were dispersed in 80 ml distilled water. After centrifugation, the serum was diluted 1: 1 with 95% ethanol and filtered. The optical density was read at 420 nm in a Bausch and Lomb, Spectronic 2 1 (Meydav et al.; 1977).
The dehydration,
shelf-life and potential uses
of citrus
pulps
23
Co1ol.H Colour measurements were carried out on a Hunterlab Color Difference Meter model D25A-2. Standardisation was done using a yellow plate having the following characteristics: L = 78.8; b = -1.7; a = + 23.8. Area of lighting was 50 mm. In the experiment for the selection of the best antioxidant the dried flakes were stored in petri dishes (57 mm in diameter and 17 mm in height) and the reading was taken on the dish itself using the 12.5 mm lighting area.
Pectin content The pectic substances concentration was measured by a sulphuric acidcarbazole calorimetric reaction (Joslyn, 1970). The fibres were dispersed in ethanol, and the divalent ions were sequestered with EDTA. The pectin and pectinates were de-esterified by adding pectinase. This converted pectic substances into anhydrouronic acid which formed a coloured complex with carbazole. Standardisation was carried out with galacturonic acid. Cellulose Cellulose content was determined after removing the water, acid and hydroxide soluble materials. Then the material was washed and the cellulose content was obtained as a weight difference.
Total care tenoids This test was carried out for comparison, i.e. observing the change of total carotenoid colour with storage time (fading). The method according to Higby (1962) included extraction of the carotenoids with a hexane-isopropanol mixture, removing any traces of water with sodium sulphate and measuring optical density at 450 nm.
Water and oil holding capacity Five grams of flakes were dispersed in 75 ml oil or water and free liquids were filtered. The filter cake was left for 3 h and weighed.
Moisture content This was determined for 18 h.
at 50°C in a vacuum oven (28 in. Hg) after drying
N. Passy, C. H. Mannheim
24
Selection
of antioxidant
Antioxidants were added to raw materials containing peel which were then disintegrated in a Fitz Mill and dried on the small drum drier. The following antioxidants were tested: (1) 0.003% marked (2) 0.015% (3) 0.015% (4) 0.015% (5) None.
BHA (Butylated BHA. Tenox” 20, TBHQ Tenox* 2, marked Tenox” 6, marked
hydroxy
anisole)
in 0.03% Tween
80,
marked T-20. T-2. T-6.
The dried samples were introduced into small Petri dishes, stored at 25°C and exposed to light or darkness to evaluate the effect of light and type of antioxidant on colour stability of dried fibres. Hunter brightness value (L) was chosen for comparisons. Optimal conditions
for drum drying of citrus fibres
This experiment was done on the small drum drier with washed juice sacs (2” Brix). Steam pressure and speed of the drums were changed. The final product was visually evaluated as to its stickiness and flowability and moisture content was determined. The drier was adjusted to the desired conditions and after equilibrium was attained the raw material was fed manually into the drums. Shelf life of dried citrus fibres Both orange and grapefruit juice sacs with and without peel were drum dried without antioxidants and stored in darkness at 4, 15, 35 and 35°C. Samples were analysed periodically for optical density (at 420 nm), Hunter values and total carotenoids. Applications
of citrus fibres in baked products
The dried fibres were used in several baked products as a substitute some of the flour. This was done to evaluate water holding capacity
for of
* Tenox is a trademark for antioxidants supplied by Eastman Chemical Products Inc., Kingsport. Tenn., USA. Numbers indicate different mixtures of BHA and BHT.
The dehydration,
shelf-life and potential uses of citrus pulps
25
the fibres in dough and organoleptic acceptability of the products. The products evaluated were snap cookies and a yeast cake. The basic dough formula for snap cookies was as follows: 200g flour, 150 g sugar, 5 g baking powder, 0.8 g salt, 48 g eggs and 94 g margarine. While preparing the dough some of the flour was substituted with dried citrus fibres. The dough was prepared in a Kenwood kitchen mixer. The cookies were moulded to a certain height, diameter and weight and baked at 170°C for 13 min, cooled and packed. The objective measurements of the cookies were carried out according to Vratanina and Zabik t 1978) and included weight, diameter, height and colour by Hunter. The formula for yeast cake was: 350 g flour, 120 g sugar, 30 g yeast, 70 g margarine, one egg and 90 ml water. The flour was substituted with 5-20s of dried pulp containing peel. The dough was prepared in a Kenwood kitchen mixer. It was set to raise for 60min at 35’C, then divided into weighed portions of 250 g. At this stage about 2Og of (sugared) cacao was added and dough was put into baking pans, which were set to rise for another 40 min. Cakes were baked at 175°C for 30min. After cooling cakes were packed in polypropylene bags and stored at 25°C. Loaf volume was measured using the standard method for bread, and weight changes during storage were recorded.
RESULTS Selection
AND DISCUSSION
of antioxidants
The effect of four antioxidants was evaluated. Hunter (L) was chosen for comparison (Fig. 2). Results show was the most suitable antioxidant and inhibited colour drying and as well as in storage in light and darkness. darkness all samples, including the blank, only slightly during seven days. Optimal conditions
brightness value that Tenox 20 changes during When stored in changed colour
for drum drying citrus pulps
Table 1 and Fig. 3 show the results of these experiments. The best product with the lowest moisture content and good flowing properties was obtained at a steam pressure of 2-2.5 atm and 1.3 rpm.
26
N. Passy, C. H. Mannheim
STORAGE
TIME
120 (hr)
160
I
40
STORAGE
Fig. 2.
120
60
TIME
160
(hr)
The effect of various antioxidants on Hunter brightness values of dried orange pulp during storage at 25°C with or without light.
Drying Conditions
TABLE 1 on the Small Drum Drier (Constant Steam pressure 0.5
Speed (rpm) Retention time (s) Moisture content (dried product) (%) a Sticky, non-flowing
product.
1.0
1.3 1.3 27 27 9
8
I.0
3 12 as
1.5
1.3 27
1.5
Drum Spacing) (atm) 2.0
3 1.3 12 27 a3
2.0
2.5
3 1.3 12 27 a3
2.5
3 12 a
The dehydration,
shelf-life and potential uses of citrus pulps
0.5 STEAM
Fig. 3.
1.0
1.5
PRESSURE
2.0
21
2.5
(at)
The effect of steam pressure in drums on final moisture content of dried
citrus pulps.
Material balance and steam consumption Two types of raw materials were evaluated for material balance and steam consumption, namely pulp with and without peel. The experimental results are presented in Table 2. Drying rates of 15-22 kg h-’ m-* dry material were obtained depending on the type of raw material. The higher the total solids content in the raw materials, the higher was the drying rate and the lower was the moisture content of the final product. There are two opposing factors here (Wadsworth et al., 1966). When the total solids content decreased, the amount of water to be removed was higher, and this lengthened the drying time and increased final moisture under constant drying conditions. On the other hand the resistance of the film was also reduced with decreasing total solids and therefore increase in drying time was relatively small. The decrease in drying rate, with lower initial total solids, may be explained by the smaller content of solids picked up by the drums. Drying ratios (kg wet material/kg dry product) of 7-l 8 were obtained in the experiments. The governing factor was the total solid content of the raw material.
N. Passy, C. H. Mannheim
28
Drying Parameters
TABLE 2 and Analyses of Orange Fibres Dried on the Escher Wyss Double Drum Drier (0.115 m’) Juice sacs
Juice sacs + peel
Juice sacs + peel
(ExP. 1)
(ExP. 2)
(ExP. 3)
2.5 5 4.7 0.6 3.2 38 18.1 15.4
6.5 13-4 2.6 0.5 3.5 33 7.2 22.5
6.5 13.4 2.6 0.5 3.5 33 7.3 21.6
Soluble solids in raw materials (’ Brix) Total solids in raw material (%) Moisture content (dried fibre) (%) Vitamin C (mg/g dry) Steam pressure (atm) Retention time (s) Drying ratioa Drying rate b
’ Drying ratio = kg raw materials/kg dry product. b Drying rate = kg h-r me2 dry product.
TABLE 3 Typical Analyses of Some Dried Orange Pulp Property
Soluble solids (in feed) (’ Brix) Moisture (%) Vitamin C (mg/g dry) Fibre (%) Pectin (%) Protein (%) Ash (%) Water holding capacity Fat holding capacity
Juice sacs Juice sacs Juice sacs Juice sacs washed unwashed twice + peel washed washed 4.2 6.7 Traces 17.6 13.5 6.5 7/l-8/1 311
11 l-2 3 9.1 11.2 4.8 6/l-7/1 2/l-3/1
1-2 2-3 Traces 16.3 10.6 4.8 8/l-9/1 311
4.5 3-4 0.8 17.9 13.5 6.1 2.5 9/1-10/l 311
The dehydration,
shelf-life and potential uses of citrus pulps
29
Regarding steam consumption, the value obtained was quite high (about 2 kg steam/kg water removed) and this was due to leakages in the system. In order to obtain accurate steam consumption data, larger amounts of raw material should be dried. Properties of some dried citrus pulps Table 3 gives typical analyses of dried pulps from various sources, with and without peel and washed to various degrees of soluble solids. From the results it can be seen that products containing peels have the highest water holding capacity 9/l-10/1. Values up to 13/l were found in the literature (Kesterson and Braddock, 1976). These values depend on the test method as well as on the origin of the fibres. Shelf-life studies Shelf-life of the dried pulp was evaluated by measurements of Hunter values (L, a, b), optical density and carotenoid content of the stored samples. Results are presented in Figs 4 and 5 and Table 4. All samples which were exposed to light faded very fast at all temperatures, as shown previously, regardless of antioxidant addition. Therefore, all samples for shelf-life tests were stored in darkness at 4, 15, 25 and 35°C. From Figs 4, 5 and 6 it can be seen that the dried pulps maintained their colour (Hunter L) and optical density at temperatures below 25°C for 100 days. Above 35’C browning products developed and the colour changed rapidly (after less than one month). This happened in spite of the very low concentration of ascorbic acid (less than 6 mg/g dried pulp) indicating that browning here may be also attributed to the Maillard reaction. Grapefruit fibres were more sensitive and their rate of change of colour with time was higher (Fig. 5). Analysing different washed pulps stored in darkness showed that there was hardly any change in their colour (as measured by Hunter L ) during storage for 60 days. The highly washed fibres (1” Brix raw material) developed a strong rancid flavour whereas the other products maintained a mild aromatic and pleasant flavour. The dried fibres stored at 4 and 15°C (in darkness) maintained their good quality for almost one year. This result can be seen in Fig. 6 indicating that hardly any browning products developed, and colour and flavour were maintained.
Fig. 4.
0.81
60
TIME
40
T IME
SToRACE
20
STORAGE
[days)
80
[days 1
100
Changes in Hunter brightness and browning values of dried orange pulp with pee1 during storage.
x
E
= 0.8’
(days)
days
Fig. 5. Changes in Hunter brightness and browning values of dried grapefruit pulp, containing peel, during storage.
TIME
TIME
STORAGE
STORAGE
o.220
1.0.
0
w
The dehydration,
Fading
shelf%fk and potential uses of citrus pulps
31
TABLE 4 of Carotenoids of Dried Citrus Pulp as Measured by Optical Density of Alcoholic Extracts’ Temperature
Storage time
(“C)
(days)
28 53 60 74 98 130 181
4
15
25
35
0.27 0.12 0.16 0.16
0.26 0.09 0.07 0.07 0.04 0.02 0.04
0.13 0.03 0.03 0.02 -0 0.02 -0
0.08 0.03 0.04 0.06 -0 0.02 0.02
0.17 0.07
a The tests are comparative showing the decrease of yellow colour of the carotenoids. Accuracy is 50.02 optical density units.
Table 4 shows the change in carotenoids, as measured by optical density (at 450 nm), as a function of time and temperature. The rate of fading (of the yellow colour) was low at 4°C but quite rapid at the other temperatures. At the higher storage temperatures the Hunter ‘a’ values showed a shift from red towards grey (a decrease in ‘0 and the ‘b’ values shifted from yellow towards grey. Material that was stored at -18°C was comparable in its colour and flavour to that stored at 4 and 15°C. At the higher storage temperatures the Hunter ‘a‘ values showed a shift from red towards grey (not shown here) and the ‘b’ values shifted from yellow towards grey (Fig. 6). The preservation of colour of the citrus fibres, as measured by Hunter L and as observed visually, can be explained as follows: there was a development of browned products due to Maillard reaction and to ascorbic acid oxidation, as can be seen from optical density data (Figs 4 and 5). On the other hand there was some fading due to oxidation of carotenoids (Table 4). The balance of these two effects, i.e. browning and fading, maintained the constant colour of the product. Total microbial counts of the dried pulp stored for six months at 15°C gave about 100 microorganisms/g material. meaning that the
32
N. Passy, C. H. Mannheim
0.8-
-I 0.6 9 !g 0.5
I 40
00
STORAGE
24’
40
a0
STORAGE
Fig. 6.
120 TIME
120 TIME
160
200
160
200
(days)
(days)
Changes in optical density and Hunter b values of dried orange pulp, with peel, during storage.
fibres were for all practical purposes microbially ‘clean’. The importance of this finding is that no pasteurisation of the raw material is needed prior to drying. Application
of citrus fibres baked products
Table 5 summarises the results obtained for the snap cookies. It can be seen that substituting flour with dried orange pulp decreased the spread factor of the cookies. It also changed the colour of the cookies as measured by Hunter brightness CL). In all cases the average weight of the cookies was higher than the blank, indicating probably the higher water holding capacity due to the pulp. Regarding organoleptic evaluation, substituting up to 15% of the flour with dried pulp gave a product with high score, but when fibres with peels were used the percentage had to be reduced due to the bitterness of the peels.
pulp
7.0 5.4 5.3
21.0 20-6 20.3
a The averages are on at least four cookies.
52.0 38.5 43.0
15-O (peel) 20.0 (peel) 30.0 (peel)
7-1 6.0
23.3 23.5
48.5 47.5
5.3 7.5
24.5 24.0
15.0 (no peel) 7.5 (peel)
W-d
(mm)
(g)
36 48
A verage height-T
,.AveWe diameter-W
Average weighta
0 (blank) 7.5 (no peel)
(%o)
&Qnge
3.0 3.8 3.8
3.3 3.9
4.6 3.2
IWIT)
Spread factor
Bitter Bitter Not edible
Good Good
Very good Good
Flavour
TABLE S Objective and Sensory Measurements of Snap Cookies with Orange Pulp
a 2.5 6.0 _ 5.3 _ -
L 71.0 64.1 _ 63.1 _ _
24.8 _ _
25.8 _
24.0 -
b
Hunter colour value
34
N. Passy, C. II. Mannheim
In yeast cakes, 5-l 5% of the flour was substituted by dried pulp with peel. There was no difference in volume, weight and organoleptic score between blanks and cakes with 5% pulp. All other cakes had considerably lower volumes, and slightly bitter off-tastes. Cakes with pulp maintained a better mouthfeel during storage. From the above results it seems that 5% flour can be substituted with dried pulp containing peel in both cookies and yeast cakes. ACKNOWLEDGEMENT This research was supported through a grant from the Citrus Products Board of Israel. The authors wish to thank Mrs M. Rauchverger for her extensive help in performing this study. REFERENCES AOAC (1975). Official Methods of Analyses. Association of Official Analytical Chemists, Washington, DC. Belshaw, F. (1978). Citrus flour - a new fiber, nutrient source. Food Prod. Devel., 12 (August), 60. Ferguson, R. R. and Fox, K. I. (1978). Dietary citrus fibers. Trans. ASME Citrus Eng. Conf., Winterhaven, Florida. Higby, W. K. (1962). A simplified method for determination of some aspects of the carotenoid distribution in natural orange juice. J. Food Sci., 27,42. Joslyn, M. A. (1970). Pectin. In Methods in Food Analysis. Academic Press, New York. Kesterson, J. W. and Braddock, R. J. (1973). Processing and potential uses for dried juice sacs. Food Tech., 27 (2), 50. Kesterson, J. W. and Braddock, R. J. (1976). By products and specialty products of Florida citrus. Bull. 784, Fla. Agr. Exp. Sta., Lake Alfred. Meydav, S., Saguy, I. and Kopelman, Y. (1977). Browning determination in citrus products. J. Agric. Food Chem., 25,602. Steger, E. S. (1980). Citrus solids recovery. Presented: 40th Annual IFT Meeting New Orleans, La. FMC Corp., Lakeland, Florida. Vratanina, D. L. and Zabik, M. E. (1978). Dietary fiber sources for baked products. J. Food Sci., 43, 1590. Wadsworth, J. I., Koltun, S. P., Gallo, A. S., Ziegler, G. M. and Spadaro, J. J. (1966). Instant sweet potato flakes: Factors affecting drying rate on doubledrum dryer. Food Tech., 20, 111.
19-34
The Dehydration, Shelf-life and Potential Uses of Citrus Pulps N. Passy and C. H. Mannheim
Department of Food Engineering and Biotechnology, Technion - Israel Institute of Technology, Haifa, Israel
ABSTRACT Technological treatments and drying conditions for citrus pulp, which accounts for lo-20% of the refuse from citrus juice plants, were established. Washed and unwashed pulp with 5 and 15% total solids (respectively) and partly washed material containing peel, with about 10% solids, served as raw materials. The flakes obtained from a drum drier (at about 4% moisture content) were non-uniform and had low bulk density. Water holding capacity was up to ten times their weight. Drying ratios of 7:l and l&:1 depending on the total solid content of the raw material, and drying rates of 15-22 kg h-’ m-’ dry material were obtained on a pilot double drum drier. Antioxidants added prior to drying prevented fading and development of rancidity during storage. The drum dried product, kept in darkness, at temperatures below 25°C. maintained good colour, flavour and flowability for over a year. Substituting some of the flour, in Snap cookies and yeast cake, with citrus fibres showed they could serve as binding and filling agents. Sensory evaluation of these products showed those with levels up to 7.5% fibres (containing peel) and 20% fibres (without peel) were tast_v.
INTRODUCTION One of the waste streams obtained after straining citrus juice is the juice sacs which account for lo-20% of the total refuse from citrus juice 19 Journal
Publishers
of Food
Engineering
Ltd, England,
0260-8774/83/0002-0019/$3.00
1983, Printed
in Great Britain
0 Applied
Science
20
N. Passy, C. H. bfannheim
plants. The juice sacs themselves may be washed or unwashed depending on the production procedure in the citrus plant and on the client’s demands. In many cases they are frozen and sold as raw material for various types of beverages or other products. In some plants the juice sacs are combined with membranes, ruptured vesicles or some peel residue ejected from the juice extractor, called pulp in this work. By drying this waste stream a by-product of high fibre content having high water and fat holding capacity is obtained (Kesterson and Braddock, 1976). The possibility of using this product in human food, instead of cattle feed, may result in substantial economic returns. Due to the presence of reducing sugars, the dried, unwashed pulp may undergo browning and produce a sticky product causing impairment of flow properties and caking. Therefore, the pulp is passed through a multiple stage pulp washing system to remove soluble solids yielding a final product with 2-4” Brix. Steger (1980) described a seven-stage pulp washing system to obtain a completely washed pulp and recover maximum soluble solids from it. Studies on our part showed that three counter current washing stages at a 1: 1.5 pulp to water ratio, were sufficient to give a washed pulp with 1-2” Brix. Autoxidation of lipids and oxidation of essential oils and pigments may result in a rancid flavour of the product and fading of its colour, especially if exposed to light. Oxidation is more pronounced in materials which contain peel, where the concentration of pigments is much higher. Antioxidant addition before drying retards the oxidation processes and results in a long shelf-life product if stored under suitable storage conditions (Kesterson and Braddock, 1976; Ferguson and Fox, 1978). In order to obtain a shelf-stable reconstitutable material 85 to 95% of the water must be removed. Drying procedures for citrus pulp were surveyed by Kesterson and Braddock (1976). A recent comparison (Ferguson and Fox, 1978) of various drying procedures, namely: tunnel belt, double drum drier, rotary kiln, spray and foam mat drying, has shown that a double drum drier was economically favourable. Although the rotary kiln appeared favourable both from capital standpoint and operating cost, the product agglomerated and darkened. The dried citrus fibres have some unusual functional properties due to their blandness and their very low bulk density (if not milled). They are low in calories and high in fibre and have a high water and fat
The dehydration,
shelf-life and potential uses of citrus pulps
21
holding capacity (Kesterson and Braddock, 1973, 1976; FMC Corporation, Lakeland, Florida, private comm., 1978; Belshaw, 1978). Due to these properties a number of potential applications were suggested. Some of the potential and existing uses are: cells for beverages, thickeners and gelling agents, dietary fibres for bread, cakes and cookies, cereal breakfast and health foods and low calorie bulk fillers and binders in meat products. The presence of peel particles in the dried pulp constitutes a problem due to its high vulnerability to oxidation and its bitterness. Information regarding juice sacs and pulp without peel is available. However, none exists concerning pulp products containing peel. This study was undertaken to establish optimum technological treatments and drying procedures for citrus pulps (containing peel) and to test their storage stability. Another object was to evaluate some food applications for the dried fibres and establish the maximum amount of peel components which can be incorporated to obtain satisfactory products.
EXPERIMENTAL Raw materials from a citrus plant (which contained juice sacs, core, membranes and peel) were collected and frozen in 5 kg quantities in polyethylene bags. These were transported and processed in our pilot plant. The sources of raw materials, and methods of preparation are shown in Fig. 1. Most of these were from oranges, but one experiment was conducted on pulp originating from grapefruit. The raw material was thawed at 4*C, and some of it was washed to remove solubles, filtered, disintegrated and drum dried. The finished dried product was stored in darkness and in light at various temperatures.
Equipment 1. A small double drum drier (made by L. A. Mitchell, Manchester, England) was used for preliminary experiments. The drums rotated at speeds up to 4 rpm. Their size was 152 mm in diameter and 152 mm in length; they were chrome plated, and had steel doctor blades. Steam pressures of l-2.5 atm were used.
N. Passy, C. H. Mannheim
22
I
EXTRACTION
RAW JUICE
-
CORES
I
SCREENING IN
GISIN;EGRATION
FINISHER
PRIME JUICE
NASilNG t SEPARATION
PULP 11%
LIQUOR
1
I
SEPAPiTING
SEPARATION
Fig. 1.
WASHED JUICE SACS E-h%
PEELS:C;UICE 44%
1 WASHING (1-6 steps)
SECONDARY JUICC
PEEL MEMBRANES
LIQUOR
JUICE SACS 4-6%x
Schematic diagram of raw material preparation.
2. A medium sized type AZ 130 double drum drier (made by Esher Wyss, Switzerland) was used for the larger experiments. The drums rotated at 1.5-9 rpm; they were 3.50 mm in diameter and 600 mm in length and were chrome plated and had steel doctor blades. Steam pressures of up to 3.5 atm were used. Methods Ascorbic acid This was determined according to AOAC (1975). Four grams of dried fibres were dispersed in 80 ml distilled water. The solids were removed by centrifugation, and a titration was made with 2,6-dichlorophenolindophenol. Browning Four grams of fibres were dispersed in 80 ml distilled water. After centrifugation, the serum was diluted 1: 1 with 95% ethanol and filtered. The optical density was read at 420 nm in a Bausch and Lomb, Spectronic 2 1 (Meydav et al.; 1977).
The dehydration,
shelf-life and potential uses
of citrus
pulps
23
Co1ol.H Colour measurements were carried out on a Hunterlab Color Difference Meter model D25A-2. Standardisation was done using a yellow plate having the following characteristics: L = 78.8; b = -1.7; a = + 23.8. Area of lighting was 50 mm. In the experiment for the selection of the best antioxidant the dried flakes were stored in petri dishes (57 mm in diameter and 17 mm in height) and the reading was taken on the dish itself using the 12.5 mm lighting area.
Pectin content The pectic substances concentration was measured by a sulphuric acidcarbazole calorimetric reaction (Joslyn, 1970). The fibres were dispersed in ethanol, and the divalent ions were sequestered with EDTA. The pectin and pectinates were de-esterified by adding pectinase. This converted pectic substances into anhydrouronic acid which formed a coloured complex with carbazole. Standardisation was carried out with galacturonic acid. Cellulose Cellulose content was determined after removing the water, acid and hydroxide soluble materials. Then the material was washed and the cellulose content was obtained as a weight difference.
Total care tenoids This test was carried out for comparison, i.e. observing the change of total carotenoid colour with storage time (fading). The method according to Higby (1962) included extraction of the carotenoids with a hexane-isopropanol mixture, removing any traces of water with sodium sulphate and measuring optical density at 450 nm.
Water and oil holding capacity Five grams of flakes were dispersed in 75 ml oil or water and free liquids were filtered. The filter cake was left for 3 h and weighed.
Moisture content This was determined for 18 h.
at 50°C in a vacuum oven (28 in. Hg) after drying
N. Passy, C. H. Mannheim
24
Selection
of antioxidant
Antioxidants were added to raw materials containing peel which were then disintegrated in a Fitz Mill and dried on the small drum drier. The following antioxidants were tested: (1) 0.003% marked (2) 0.015% (3) 0.015% (4) 0.015% (5) None.
BHA (Butylated BHA. Tenox” 20, TBHQ Tenox* 2, marked Tenox” 6, marked
hydroxy
anisole)
in 0.03% Tween
80,
marked T-20. T-2. T-6.
The dried samples were introduced into small Petri dishes, stored at 25°C and exposed to light or darkness to evaluate the effect of light and type of antioxidant on colour stability of dried fibres. Hunter brightness value (L) was chosen for comparisons. Optimal conditions
for drum drying of citrus fibres
This experiment was done on the small drum drier with washed juice sacs (2” Brix). Steam pressure and speed of the drums were changed. The final product was visually evaluated as to its stickiness and flowability and moisture content was determined. The drier was adjusted to the desired conditions and after equilibrium was attained the raw material was fed manually into the drums. Shelf life of dried citrus fibres Both orange and grapefruit juice sacs with and without peel were drum dried without antioxidants and stored in darkness at 4, 15, 35 and 35°C. Samples were analysed periodically for optical density (at 420 nm), Hunter values and total carotenoids. Applications
of citrus fibres in baked products
The dried fibres were used in several baked products as a substitute some of the flour. This was done to evaluate water holding capacity
for of
* Tenox is a trademark for antioxidants supplied by Eastman Chemical Products Inc., Kingsport. Tenn., USA. Numbers indicate different mixtures of BHA and BHT.
The dehydration,
shelf-life and potential uses of citrus pulps
25
the fibres in dough and organoleptic acceptability of the products. The products evaluated were snap cookies and a yeast cake. The basic dough formula for snap cookies was as follows: 200g flour, 150 g sugar, 5 g baking powder, 0.8 g salt, 48 g eggs and 94 g margarine. While preparing the dough some of the flour was substituted with dried citrus fibres. The dough was prepared in a Kenwood kitchen mixer. The cookies were moulded to a certain height, diameter and weight and baked at 170°C for 13 min, cooled and packed. The objective measurements of the cookies were carried out according to Vratanina and Zabik t 1978) and included weight, diameter, height and colour by Hunter. The formula for yeast cake was: 350 g flour, 120 g sugar, 30 g yeast, 70 g margarine, one egg and 90 ml water. The flour was substituted with 5-20s of dried pulp containing peel. The dough was prepared in a Kenwood kitchen mixer. It was set to raise for 60min at 35’C, then divided into weighed portions of 250 g. At this stage about 2Og of (sugared) cacao was added and dough was put into baking pans, which were set to rise for another 40 min. Cakes were baked at 175°C for 30min. After cooling cakes were packed in polypropylene bags and stored at 25°C. Loaf volume was measured using the standard method for bread, and weight changes during storage were recorded.
RESULTS Selection
AND DISCUSSION
of antioxidants
The effect of four antioxidants was evaluated. Hunter (L) was chosen for comparison (Fig. 2). Results show was the most suitable antioxidant and inhibited colour drying and as well as in storage in light and darkness. darkness all samples, including the blank, only slightly during seven days. Optimal conditions
brightness value that Tenox 20 changes during When stored in changed colour
for drum drying citrus pulps
Table 1 and Fig. 3 show the results of these experiments. The best product with the lowest moisture content and good flowing properties was obtained at a steam pressure of 2-2.5 atm and 1.3 rpm.
26
N. Passy, C. H. Mannheim
STORAGE
TIME
120 (hr)
160
I
40
STORAGE
Fig. 2.
120
60
TIME
160
(hr)
The effect of various antioxidants on Hunter brightness values of dried orange pulp during storage at 25°C with or without light.
Drying Conditions
TABLE 1 on the Small Drum Drier (Constant Steam pressure 0.5
Speed (rpm) Retention time (s) Moisture content (dried product) (%) a Sticky, non-flowing
product.
1.0
1.3 1.3 27 27 9
8
I.0
3 12 as
1.5
1.3 27
1.5
Drum Spacing) (atm) 2.0
3 1.3 12 27 a3
2.0
2.5
3 1.3 12 27 a3
2.5
3 12 a
The dehydration,
shelf-life and potential uses of citrus pulps
0.5 STEAM
Fig. 3.
1.0
1.5
PRESSURE
2.0
21
2.5
(at)
The effect of steam pressure in drums on final moisture content of dried
citrus pulps.
Material balance and steam consumption Two types of raw materials were evaluated for material balance and steam consumption, namely pulp with and without peel. The experimental results are presented in Table 2. Drying rates of 15-22 kg h-’ m-* dry material were obtained depending on the type of raw material. The higher the total solids content in the raw materials, the higher was the drying rate and the lower was the moisture content of the final product. There are two opposing factors here (Wadsworth et al., 1966). When the total solids content decreased, the amount of water to be removed was higher, and this lengthened the drying time and increased final moisture under constant drying conditions. On the other hand the resistance of the film was also reduced with decreasing total solids and therefore increase in drying time was relatively small. The decrease in drying rate, with lower initial total solids, may be explained by the smaller content of solids picked up by the drums. Drying ratios (kg wet material/kg dry product) of 7-l 8 were obtained in the experiments. The governing factor was the total solid content of the raw material.
N. Passy, C. H. Mannheim
28
Drying Parameters
TABLE 2 and Analyses of Orange Fibres Dried on the Escher Wyss Double Drum Drier (0.115 m’) Juice sacs
Juice sacs + peel
Juice sacs + peel
(ExP. 1)
(ExP. 2)
(ExP. 3)
2.5 5 4.7 0.6 3.2 38 18.1 15.4
6.5 13-4 2.6 0.5 3.5 33 7.2 22.5
6.5 13.4 2.6 0.5 3.5 33 7.3 21.6
Soluble solids in raw materials (’ Brix) Total solids in raw material (%) Moisture content (dried fibre) (%) Vitamin C (mg/g dry) Steam pressure (atm) Retention time (s) Drying ratioa Drying rate b
’ Drying ratio = kg raw materials/kg dry product. b Drying rate = kg h-r me2 dry product.
TABLE 3 Typical Analyses of Some Dried Orange Pulp Property
Soluble solids (in feed) (’ Brix) Moisture (%) Vitamin C (mg/g dry) Fibre (%) Pectin (%) Protein (%) Ash (%) Water holding capacity Fat holding capacity
Juice sacs Juice sacs Juice sacs Juice sacs washed unwashed twice + peel washed washed 4.2 6.7 Traces 17.6 13.5 6.5 7/l-8/1 311
11 l-2 3 9.1 11.2 4.8 6/l-7/1 2/l-3/1
1-2 2-3 Traces 16.3 10.6 4.8 8/l-9/1 311
4.5 3-4 0.8 17.9 13.5 6.1 2.5 9/1-10/l 311
The dehydration,
shelf-life and potential uses of citrus pulps
29
Regarding steam consumption, the value obtained was quite high (about 2 kg steam/kg water removed) and this was due to leakages in the system. In order to obtain accurate steam consumption data, larger amounts of raw material should be dried. Properties of some dried citrus pulps Table 3 gives typical analyses of dried pulps from various sources, with and without peel and washed to various degrees of soluble solids. From the results it can be seen that products containing peels have the highest water holding capacity 9/l-10/1. Values up to 13/l were found in the literature (Kesterson and Braddock, 1976). These values depend on the test method as well as on the origin of the fibres. Shelf-life studies Shelf-life of the dried pulp was evaluated by measurements of Hunter values (L, a, b), optical density and carotenoid content of the stored samples. Results are presented in Figs 4 and 5 and Table 4. All samples which were exposed to light faded very fast at all temperatures, as shown previously, regardless of antioxidant addition. Therefore, all samples for shelf-life tests were stored in darkness at 4, 15, 25 and 35°C. From Figs 4, 5 and 6 it can be seen that the dried pulps maintained their colour (Hunter L) and optical density at temperatures below 25°C for 100 days. Above 35’C browning products developed and the colour changed rapidly (after less than one month). This happened in spite of the very low concentration of ascorbic acid (less than 6 mg/g dried pulp) indicating that browning here may be also attributed to the Maillard reaction. Grapefruit fibres were more sensitive and their rate of change of colour with time was higher (Fig. 5). Analysing different washed pulps stored in darkness showed that there was hardly any change in their colour (as measured by Hunter L ) during storage for 60 days. The highly washed fibres (1” Brix raw material) developed a strong rancid flavour whereas the other products maintained a mild aromatic and pleasant flavour. The dried fibres stored at 4 and 15°C (in darkness) maintained their good quality for almost one year. This result can be seen in Fig. 6 indicating that hardly any browning products developed, and colour and flavour were maintained.
Fig. 4.
0.81
60
TIME
40
T IME
SToRACE
20
STORAGE
[days)
80
[days 1
100
Changes in Hunter brightness and browning values of dried orange pulp with pee1 during storage.
x
E
= 0.8’
(days)
days
Fig. 5. Changes in Hunter brightness and browning values of dried grapefruit pulp, containing peel, during storage.
TIME
TIME
STORAGE
STORAGE
o.220
1.0.
0
w
The dehydration,
Fading
shelf%fk and potential uses of citrus pulps
31
TABLE 4 of Carotenoids of Dried Citrus Pulp as Measured by Optical Density of Alcoholic Extracts’ Temperature
Storage time
(“C)
(days)
28 53 60 74 98 130 181
4
15
25
35
0.27 0.12 0.16 0.16
0.26 0.09 0.07 0.07 0.04 0.02 0.04
0.13 0.03 0.03 0.02 -0 0.02 -0
0.08 0.03 0.04 0.06 -0 0.02 0.02
0.17 0.07
a The tests are comparative showing the decrease of yellow colour of the carotenoids. Accuracy is 50.02 optical density units.
Table 4 shows the change in carotenoids, as measured by optical density (at 450 nm), as a function of time and temperature. The rate of fading (of the yellow colour) was low at 4°C but quite rapid at the other temperatures. At the higher storage temperatures the Hunter ‘a’ values showed a shift from red towards grey (a decrease in ‘0 and the ‘b’ values shifted from yellow towards grey. Material that was stored at -18°C was comparable in its colour and flavour to that stored at 4 and 15°C. At the higher storage temperatures the Hunter ‘a‘ values showed a shift from red towards grey (not shown here) and the ‘b’ values shifted from yellow towards grey (Fig. 6). The preservation of colour of the citrus fibres, as measured by Hunter L and as observed visually, can be explained as follows: there was a development of browned products due to Maillard reaction and to ascorbic acid oxidation, as can be seen from optical density data (Figs 4 and 5). On the other hand there was some fading due to oxidation of carotenoids (Table 4). The balance of these two effects, i.e. browning and fading, maintained the constant colour of the product. Total microbial counts of the dried pulp stored for six months at 15°C gave about 100 microorganisms/g material. meaning that the
32
N. Passy, C. H. Mannheim
0.8-
-I 0.6 9 !g 0.5
I 40
00
STORAGE
24’
40
a0
STORAGE
Fig. 6.
120 TIME
120 TIME
160
200
160
200
(days)
(days)
Changes in optical density and Hunter b values of dried orange pulp, with peel, during storage.
fibres were for all practical purposes microbially ‘clean’. The importance of this finding is that no pasteurisation of the raw material is needed prior to drying. Application
of citrus fibres baked products
Table 5 summarises the results obtained for the snap cookies. It can be seen that substituting flour with dried orange pulp decreased the spread factor of the cookies. It also changed the colour of the cookies as measured by Hunter brightness CL). In all cases the average weight of the cookies was higher than the blank, indicating probably the higher water holding capacity due to the pulp. Regarding organoleptic evaluation, substituting up to 15% of the flour with dried pulp gave a product with high score, but when fibres with peels were used the percentage had to be reduced due to the bitterness of the peels.
pulp
7.0 5.4 5.3
21.0 20-6 20.3
a The averages are on at least four cookies.
52.0 38.5 43.0
15-O (peel) 20.0 (peel) 30.0 (peel)
7-1 6.0
23.3 23.5
48.5 47.5
5.3 7.5
24.5 24.0
15.0 (no peel) 7.5 (peel)
W-d
(mm)
(g)
36 48
A verage height-T
,.AveWe diameter-W
Average weighta
0 (blank) 7.5 (no peel)
(%o)
&Qnge
3.0 3.8 3.8
3.3 3.9
4.6 3.2
IWIT)
Spread factor
Bitter Bitter Not edible
Good Good
Very good Good
Flavour
TABLE S Objective and Sensory Measurements of Snap Cookies with Orange Pulp
a 2.5 6.0 _ 5.3 _ -
L 71.0 64.1 _ 63.1 _ _
24.8 _ _
25.8 _
24.0 -
b
Hunter colour value
34
N. Passy, C. II. Mannheim
In yeast cakes, 5-l 5% of the flour was substituted by dried pulp with peel. There was no difference in volume, weight and organoleptic score between blanks and cakes with 5% pulp. All other cakes had considerably lower volumes, and slightly bitter off-tastes. Cakes with pulp maintained a better mouthfeel during storage. From the above results it seems that 5% flour can be substituted with dried pulp containing peel in both cookies and yeast cakes. ACKNOWLEDGEMENT This research was supported through a grant from the Citrus Products Board of Israel. The authors wish to thank Mrs M. Rauchverger for her extensive help in performing this study. REFERENCES AOAC (1975). Official Methods of Analyses. Association of Official Analytical Chemists, Washington, DC. Belshaw, F. (1978). Citrus flour - a new fiber, nutrient source. Food Prod. Devel., 12 (August), 60. Ferguson, R. R. and Fox, K. I. (1978). Dietary citrus fibers. Trans. ASME Citrus Eng. Conf., Winterhaven, Florida. Higby, W. K. (1962). A simplified method for determination of some aspects of the carotenoid distribution in natural orange juice. J. Food Sci., 27,42. Joslyn, M. A. (1970). Pectin. In Methods in Food Analysis. Academic Press, New York. Kesterson, J. W. and Braddock, R. J. (1973). Processing and potential uses for dried juice sacs. Food Tech., 27 (2), 50. Kesterson, J. W. and Braddock, R. J. (1976). By products and specialty products of Florida citrus. Bull. 784, Fla. Agr. Exp. Sta., Lake Alfred. Meydav, S., Saguy, I. and Kopelman, Y. (1977). Browning determination in citrus products. J. Agric. Food Chem., 25,602. Steger, E. S. (1980). Citrus solids recovery. Presented: 40th Annual IFT Meeting New Orleans, La. FMC Corp., Lakeland, Florida. Vratanina, D. L. and Zabik, M. E. (1978). Dietary fiber sources for baked products. J. Food Sci., 43, 1590. Wadsworth, J. I., Koltun, S. P., Gallo, A. S., Ziegler, G. M. and Spadaro, J. J. (1966). Instant sweet potato flakes: Factors affecting drying rate on doubledrum dryer. Food Tech., 20, 111.