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Nutrient retention in foods after earth-oven cooking compared to other forms of domestic cooking☆
ARTICLE IN PRESS JOURNAL OF FOOD COMPOSITION AND ANALYSIS Journal of Food Composition and Analysis 19 (2006) 302–310 www.elsevier.com/locate/jfca
Original Article
Nutrient retention in foods after earth-oven cooking compared to other forms of domestic cooking$ 1. Proximates, carbohydrates and dietary fibre Shailesh Kumara,, Bill Aalbersbergb a
11 Cowin Close, Rowville, Victoria 3178, Australia Institute of Applied Sciences, University of the South Pacific, P.O. Box 1168, Suva, Fiji
b
Received 6 April 2004; received in revised form 23 June 2005; accepted 27 June 2005
Abstract Effects of Pacific traditional style of cooking in an earth-oven1 on proximate content of chicken, lamb chops, fish, cassava, taro and palusami2 were investigated. Retention of proximates in earth-oven-cooked samples was compared with the retention in microwaved and oven-roasted chicken and lamb chops, microwave-cooked fish, boiled cassava and taro, and steamed-cooked palusami, the nutrient analyses of all of which were conducted during the course of this study. Water content of the samples generally decreased most upon earth-oven cooking. As much as 32.9% moisture was lost from earth-oven-cooked lean of lamb chops. Loss of water from microwavecooked meat, ranging from 6.6 to 25.8 g/100 g, was second to the moisture loss in earth-oven-cooked meat and the least amount of moisture was lost from the gas-oven roasted meat with the values ranging from 4.4 to 22.2 g/100 g. Retention of protein ranged from 96% to 103% in all samples, the differences being not statistically significant. However, interestingly high retention values of fat were noted in separable lean of lamb chops ranging from 291% to 294%. A simple and logical explanation for this observation is adsorption of fat from separable fat, as it melted during cooking, into the muscle tissue of lamb chops. Retention of over 100% dietary fiber in all foods that had this component in the raw state was noted upon all types of cooking, except in steam-cooked palusami. This implied an increase in this component of food after cooking, whereas starch and sugars generally decreased after cooking. r 2005 Elsevier Inc. All rights reserved. Keywords: Earth-oven; Palusami; Proximate; Water; Protein; Fat; Sugars; Starch; Dietary fiber; Ash
Abbreviations: HPLC, High performance liquid chromatograph; na, Not analyzed Corresponding author. Tel.: +613 9753 9538; fax: +613 9764 9992. E-mail address: [email protected] (S. Kumar). $ Results of this work are printed as a technical report with limited distribution by the Institute of Applied Sciences at The University of the South Pacific, Suva, Fiji (Kumar et al., 2001). 1 Traditional Pacific style of cooking, lovo as it is known in Fijian, is performed in an oven in the earth prepared by digging a smooth sloping pit on the ground. The main source of heat is provided by the stones (once or twice the size of a fist), heated in an open fire (normally 1–2 h) until extremely hot. All food to be cooked is put in the pit at once on these extremely hot and dry stones (without water being poured to generate steam for cooking) and left in the pit, covered with earth, and left to cook for 1–2 h (1 h and 15 min in this research). 2 A vegetable-based dish that consists of a filling, made by mixing canned meat (fish, mutton or beef) with coconut cream, wrapped by a layer of taro leaves. Depending on the taste and likings of people, onion, chilies, carrot, tomatoes and salt are also added to the filling. 0889-1575/$ - see front matter r 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2005.06.006
1. Introduction When foods are prepared to be eaten, there are significant changes in the flavor as well as in the nutritional composition of the food. This paper focuses only on nutritional changes in different types of food preparation at home. Many of the data in the tables represent foods that are raw, but many foods are eaten after being processed, stored and/or prepared in various ways that may each affect at least some of the nutrient levels. Scientific literature contains results showing that nutrient losses differ for different nutrients and cooking methods. Some of the studies dealing with nutrient losses include work by researchers such as Hall and Lin (1981), Klein et al. (1981), Bertelsen et al. (1988), Unklesbay (1988), Kimura and Itokawa (1990), Rumm-Kreuter and Demmel
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(1990), Somogyi (1990), Dignos et al. (1992), El-Shimi (1992), Wanasundera and Ravindran (1992), Al-Khalifa and Dawood (1993), Uherova et al. (1993) and Thed and Phillips (1995). Numerous researches have been carried out on vitamins and minerals; however, research on effects of cooking on proximate nutrients is uncommon. This research compares the proximate composition in earth-oven-cooked foods, on which no previous work has been done, with several common practices of domestic cooking, namely microwaving, oven-roasting, boiling and steaming. Comparison is made on the basis of percentage nutrient retention calculated on dry-weight or dry-weightfat-free (DWFF) basis after cooking. 2. Materials and methods 2.1. Sample collection In general, sampling procedures were chosen so that the nutrient data obtained would represent the food as eaten in Fiji. To this end a composite sample of multiple food samples purchased at three different sites was analyzed. Fish (Lethrinus xanthochilus), taro leaves, cassava (Manihot esculenta) and taro (Colocasia esculenta), all fresh, were bought from three different vendors in Suva market. The most popular brand of frozen chicken, Crest, was bought from three leading outlets in Suva. Lamb chops (frozen), onion, corned mutton and coconut cream were also bought from these outlets. To avoid any variation in nutritional composition caused by different parts of lamb, only lamb chump chops, imported from New Zealand, were bought for analysis. Species identity for the fish was determined by reference to the ‘‘Species of Fiji’’ chart published by The Ministry of Fisheries of Fiji. This chart identifies all fishes of Fiji by their picture, common local name and species name. The local name of the fish used in this study is ‘‘Kacika’’. It is a reef finned-fish and commonly known as ‘‘yellowlip emperor’’. Three brands of corned mutton, Canterbury, Farmers and Golden Country, were mixed in equal amounts to make a composite for corned mutton that was used in palusami. Similarly, two brands of canned coconut cream, Solo’s Choice (made in Thailand) and Pacific Crown (made by Food Processors Fiji Limited), were mixed in equal amounts to make a composite of coconut cream that was used to prepare palusami. 2.2. General procedures for cooking methods In each case, cooking was carried out without the addition of any taste enhancers such as salt, soy sauce, garlic or ginger to the food except in the preparation of palusami, in which weighed amounts of salt and onion were added to the corned mutton and canned coconut cream filling.
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Microwave cooking: Chicken (whole), lamb chops and fish were cooked to a maximum internal temperature of 93 1C for 40, 20 and 15 min, respectively. Oven roasting: Chicken, whole, was cooked for 1 h 40 min to a maximum internal temperature of 88 1C. Lamb chops were cooked for 1 h to an internal temperature of 90 1C. Roasting of chicken and lamb chops in both of the ovens, microwave and conventional, produced considerable amount of drippings, which were discarded. Earth-oven cooking: All food to be cooked was put in the lovo pit at once and cooked for 1 h 15 min. The highest temperature reached in the oven was 126 1C. This was measured by leaving the probe of a thermocouple thermometer in the center of the pit while the foods were being cooked in the earth-oven. The internal temperatures of meat in microwave cooking, earth-oven cooking and oven roasting was measured by inserting the probe of the thermocouple thermometer in the respective meats while being cooked. Boiling: Taro and cassava were peeled, washed and cut into pieces of about half the size of an average-sized potato. These were then boiled separately in 1:3 root/water ratio (w/v), with the cooking liquor being discarded at the end of cooking. For cooking, the tubers were put into the pot once water had started boiling. Cooking was monitored and timed thereafter until the roots were cooked to softness (felt between fingers). Steaming: Palusami was steam-cooked for 1 h and 15 min. The temperature range for cooking was 103–107 1C. This was measured by leaving the probe of a thermocouple thermometer in the cooking pot, making sure the probe was not immersed in the boiling water. 2.3. Nutrient analysis Sufficient amount of each sample was homogenized and analyzed. Frozen samples were first thawed at room temperature before blending. Nutrients analyzed in this study were water, protein, fat, sugars (fructose, glucose, sucrose, maltose and lactose), starch, dietary fiber (DF) and ash. Official methods of analysis (AOAC, 1995) were used for the determination of moisture (method 925.04), protein (method 981.10), fat (method 954.02), total dietary fiber (TDF) (method 985.29) and ash (method 938.08). The factor used to convert nitrogen to the protein value was 6.25 for all foods. Sugars and starch were extracted and analyzed as described below. 2.3.1. Sugars Extraction of sugars was carried out in 85% ethanol/ water (v/v) (Chaplin, 1994). The ethanolic solution was added to the homogenized sample and boiled for a few minutes over a steam bath. The hot extraction solution was filtered through a pre-weighed filter paper and the extraction procedure was repeated another three times. The extract was concentrated on a rotary evaporator and
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made to volume with distilled water in a 10 mL volumetric flask. The quantification was carried out with high performance liquid chromatograph (HPLC) using acetonitrile/water, 80/20 (v/v), mobile phase with a 5 m Hypersil column (4.6 mm 250 mm) and refractive index detector. 2.3.2. Starch Starch was analyzed as glucose using HPLC after enzymatic digestion as described by Wills and Greenfield (1987). The residue remaining after extraction of sugars was dried in a vacuum oven at 75 1C to a constant weight (total residue). The dried residue, milled in a dry blender, was then weighed (300–400 mg), mixed in 10 mL water and digested with a-amylase in a boiling water-bath for 2 h. After cooling, this sample mixture was digested with amyloglucosidase in a shaking water-bath at 60 1C for 1 h. The solution was made to 25 mL with distilled water. A factor of 1/1.1 was used for the conversion of the value of glucose to starch.
Table 1 Weights before and after cooking for samples for which true retentions are calculated Sample
Weight after cooking (g)
Weight before cooking (g)
Cassava E/oven Boil
675 562
756 488
Taro E/oven Boil
950 500
1150 409
Fish E/oven M/wave
298 400
408 520
Palusami E/oven Steam
594 605
871 876
Abbreviations: E/oven, Earth-oven; M/wave, Microwave.
2.4. Quality control procedures For each sample, duplicate measurements were used to determine the nutrient content which was obtained by calculating the mean of duplicates. Duplicates falling within 10% of their mean were accepted to be showing satisfactory agreement (Greenfield and Southgate, 1985). The analysis was repeated if the agreement was outside 10% of the mean for the duplicates, and the mean recalculated on the basis of all the replicates. Other quality assurance procedures included analyses of in-house reference materials for moisture (wheat flour), protein (wheat flour), sugars (Milo, a commercial product from Nestle, usually dissolved in milk to make an energy drink for children) and fat (soya bean oil), recovery samples and reagent blanks in each analytical batch of samples. The acceptable nutrient value for the in-house reference materials was 72 standard deviations of the mean, which was pre-determined by analyzing 10 replicates of the in-house reference material over 1 month by different analysts. Recoveries ranging from 90% to110% were classified satisfactory (Brubacher et al., 1986). Reagent blanks helped monitor purity of the reagent used. Any significant value for the blank determination was accounted for in the calculation of the results. In addition to the quality control procedures described above, the laboratory regularly participates in the testing of proximate nutrients as part of proficiency testing scheme of the Central Science Laboratory, UK. 2.5. Nutrient retention calculation To achieve a uniform baseline for comparison, it was necessary to calculate the percent retention, not on the ‘‘as assayed’’ basis but on the dry-weight basis or change of weight basis. All nutrient retention values, except for the lamb chop and chicken samples, reported in this work are
the true retentions (TRs) calculated on the weight change basis as described by Murphy et al. (1975). Table 1 lists the weights of foods before and after cooking. To ease calculation of retention values in palusami, the samples were prepared such that all palusami were identical with regard to the amount of each ingredient3 in them. (i) %TR ¼ (nutrient content per g cooked food g food after cooking)/(nutrient content per g raw food g food before cooking) 100. Nutrient retention factors in the chicken and lamb chop samples were calculated on DWFF basis according to Eq. (ii), given below. (ii) %R ¼ (nutrient content per g cooked food [DWFF basis])/(Nutrient content per g of raw food [DWFF basis]) 100. The use of the above formula limits the calculation of retention of water and fat in the cooked samples. To be able to report the latter, calculation of retention values of fat were done on dry-weight basis, described as apparent retention by Murphy et al. (1975). 3. Results and discussion 3.1. Nutrient composition Tables 2 and 3 contain the proximate content per 100 g of the foods analyzed in this study. Less than detection limit (oDL) is used where it is analytically shown that the 3 Each palusami was made by mixing together 161 g of corned mutton, 133 g of coconut cream, 53 g of onion and 0.6 g of salt and wrapping these mixed ingredients with 93 g of taro leaves. The whole palusami was then wrapped in aluminum foil.
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Table 2 Proximate content in 100 g of raw and cooked foods Foods
Water (g)
Protein (g)
Fat (g)
CHO total (g)
Dietary fibre (g)
Ash (g)
Chicken, whole, raw Chicken, whole, E/oven Chicken, whole, M/wave Chicken, whole, O/roast Chicken, whole, literaturea Chicken, lean, raw Chicken, lean, E/oven Chicken, lean, M/wave Chicken, lean, O/roast Chicken, lean, literaturea Chicken, skin, raw Chicken, skin, E/oven Chicken, skin, M/wave Chicken, skin, O/roast Chicken, skin, literaturea L/chops, whole, raw L/chops, whole, E/oven L/chops, whole, M/wave L/chops, whole, O/roast L/chops, whole, literaturea L/chops, lean, raw L/chops, lean, E/oven L/chops, lean, M/wave L/chops, lean, O/roast L/chops, lean, literaturea L/chops, fat, raw L/chops, fat, E/oven L/chops, fat, M/wave L/chops, fat, O/roast L/chops, fat, literaturea Fish, whole, raw Fish, whole, E/oven Fish, whole, M/wave Fish, whole, literatureb Cassava, raw Cassava, E/oven Cassava, boil Cassava, literaturea Taro, raw Taro, E/oven Taro, boil Taro, literaturea Palusami, raw Palusami, E/oven Palusami, steam
68.0 58.7 61.3 63.6 66.1 72.5 60.1 66.0 67.9 74.2 50.6 48.0 42.6 42.5 47.6 59.7 34.5 39.7 46.5 51.5 75.8 42.9 50.0 53.6 72.2 13.8 17.8 23.0 21.1 8.6 77.7 69.1 71.0 81.3 60.4 58.9 67.5 62.1 65.7 66.0 73.4 68.9 75.3 70.5 68.3
18.0 26.1 26.0 23.6 18.4 20.2 27.5 28.1 24.5 20.7 9.4 15.5 17.5 19.1 13.1 16.2 25.5 19.6 22.5 15.1 20.0 31.8 28.2 26.6 20.3 5.3 12.9 5.5 7.9 4.3 21.3 29.1 27.3 17.9 0.7 0.8 0.6 1.1 1.1 1.1 0.9 1.9 7.4 10.4 11.0
13.9 14.1 12.6 13.0 13.2 7.4 11.4 6.1 8.1 4.2 39.1 35.1 38.4 36.8 33.8 23.5 38.6 39.7 30.5 32.7 3.4 23.5 20.4 19.1 6.6 80.7 68.8 71.3 70.8 86.8 2.0 2.4 0.8 0.6 0.5 0.5 0.4 0.3 0.4 0.4 0.3 0.2 12.1 11.2 14.4
na na na na 0 na na na na 0 na na na na 0 na na na na 0 na na na na 0 na na na na 0 na na na 0 35.7 37.8 28.9 30.4 30.0 30.4 22.5 23.4 3.5 4.8 3.6
na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na 1.5 2.8 1.4 na 2.1 2.9 2.2 na 1.5 2.3 2.0
0.7 0.9 0.8 0.9 0.8 0.8 1.0 0.9 0.9 0.9 0.3 0.5 0.5 0.6 0.5 0.6 0.7 0.6 0.7 0.9 0.7 0.8 0.8 0.8 1.3 0.2 0.4 0.2 0.2 0.1 1.3 1.4 1.5 1.2 0.6 0.7 0.5 0.8 1.1 1.2 0.8 1.0 1.5 1.4 1.5
Abbreviations: E/oven, earth-oven; M/wave, microwave; O/roast, oven roast; L/chops, Lamb chops; na, not analyzed. a Literature values for raw foods adapted from Cashel et al. (1989). b Literature values for raw fin-fish (Pacific cod) adapted from Dickey (1991).
constituent was present, but below the DL, whereas zero (0) is used where there was no analytical indication of the presence of the analyte. Nutrients, which are known not to be found in a particular sample, were not analyzed (na) and are given as na in the tables. Table 2 shows that the proximate content in raw samples of chicken, lamb chops, cassava and taro were comparable to the proximate composition of the respective foods published by Cashel et al. (1989). Proximate composition
of raw fish was compared with the composition of raw Pacific cod, published in USDA handbook number 8 (Dickey, 1991). 3.2. Nutrient retention Table 4 below shows percentage retention of nutrients after cooking. For comparison of retention of nutrients, as appropriate, a t-test or F-test has been used (Morgan,
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1991). A dash (-) has been used in Table 4 to represent retentions that could not be calculated for those nutrients which were not present in a food. Table 3 Starch and individual sugar components in taro, cassava and palusami samples Sample
Starch Sugars (g/100 g) (g/100 g) Fructose Glucose
Sucrose
Maltose
Lactose
Raw Taro 27.4 Cassava 33.0 Palusami 0.0
0.1 0.1 1.3
0.1 0.1 0.7
0.4 1.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
E/oven Taro 26.8 Cassava 34.1 Palusami 0.0
0.0 0.0 0.8
0.0 0.2 0.6
0.7 0.7 0.0
0.0 0.0 0.0
0.0 0.0 0.0
Boil Taro Cassava
19.8 26.7
0.0 o0.1
0.0 0.0
0.5 0.8
0.0 0.0
0.0 0.0
Steam Palusami
0.0
1.0
0.6
0.0
0.0
0.0
Abbreviations: E/oven, earth-oven.
3.3. Water Though calculation of percent retention of water in chicken and lamb chop samples was limited by the formula used for calculation of nutrient retention in these samples, the comparison was based on the water content in these samples. Almost all previous researchers have found significantly more moisture to be lost by microwave cooking than by conventional cooking of meat (Janicki and Appledorf, 1974; Baldwin et al., 1976; Moore et al., 1980; El-Shimi, 1992). Except for the values in chicken skin and lamb fat, the content of moisture in microwave-cooked meat (chicken and lamb chops) was always lower than the moisture content of meat roasted in the oven. This agrees with the trend of a higher loss of moisture in microwave cooking compared to conventional roasting. However, apart from the moisture in chicken skin only, it was found that the moisture content of earth-ovencooked meat, including fish, was even lower than the moisture content of comparable samples cooked in the microwave. The loss in the moisture content of foods is due to evaporation and in the form of drippings, which is common in meat samples including fish (Headly and Jacobson, 1960). The content of water in samples cooked
Table 4 Percent retention of proximate nutrients in cooked samples Foods
Chicken, whole, E/ovena Chicken, whole, M/wavea Chicken, whole, O/roasta Chicken, lean, E/ovena Chicken, lean, M/wavea Chicken, lean, O/roasta Chicken, skin, E/ovena Chicken, skin, M/wavea Chicken, skin, O/roasta L/chops, whole, E/ovena L/chops, whole, M/wavea L/chops, whole, O/roasta L/chops, lean, E/ovena L/chops, lean, M/wavea L/chops, lean, O/roasta L/chops, fat, E/ovena L/chops, fat, M/wavea L/chops, fat, O/roasta Fish, whole, E/ovenb Fish, whole, M/waveb Cassava, E/ovenb Cassava, boilb Taro, E/ovenb Taro, boilb Palusami, E/ovenb Palusami, steamb
Protein
97 100 101 96 100 102 100 101 101 99 100 103 98 99 100 100 100 101 98 99 102 99 98 100 96 103
Fat
79 75 82 106 67 94 85 85 81 101 113 98 292 291 294 89 99 96 87 31 89 92 83 92 63 82
Starch
— — — — — — — — — — — — — — — — — — — — 92 93 81 89 — —
Sugars Fructose
Glucose
Sucrose
— — — — — — — — — — — — — — — — — — — — — — — — 43 58
— — — — — — — — — — — — — — — — — — — — 180 — — — 58 56
— — — — — — — — — — — — — — — — — — — — 63 92 103 80 — —
Dietary Fibre
Ash
— — — — — — — — — — — — — — — — — — — — 167 108 114 128 104 92
89 81 98 88 81 95 93 87 100 72 80 86 71 79 86 83 78 69 79 89 — — 90 89 64 69
Abbreviations: E/oven, earth-oven; M/wave, microwave; O/roast, oven roast; L/chops, Lamb chops; na, not analyzed. a Retention factor of fat for this food is calculated on dry weight basis and all other retention factors are calculated on a dry weight fat free basis. b Retention factors for this food are ‘‘true’’ retentions calculated on the change of weight basis.
ARTICLE IN PRESS S. Kumar, B. Aalbersberg / Journal of Food Composition and Analysis 19 (2006) 302–310 Table 5 Percent retention of water in fish, cassava, taro and palusami upon earthoven cooking, microwaving, boiling and steaming Sample
Earth-oven
Microwave
Boil
Steam
Fish Cassava Taro Palusami
65 87 83 64
70 — — —
— 129 137 —
— — — 63
by different methods could be attributed to the extent of moisture lost in each cooking method through drippings and evaporation. Microwave cooking has been known to cause more drip than conventional cooking, and it also causes more evaporation in chicken than roasting (Janicki and Appledorf, 1974). It seems from this study that earthoven cooking causes an even greater loss of moisture in the form of drippings and evaporation than microwave cooking. Moisture contents of earth-oven-cooked cassava and taro were lower than the moisture contents of boiled cassava and taro. It was seen that steaming caused a higher loss of water from palusami than earth-oven cooking. TR values for moisture in these samples and fish give a clearer view of loss or gain of moisture in the samples upon different types of cooking. Table 5 contains the retention values of water in cooked samples of fish, cassava, taro and palusami. The retention values also show a trend of higher loss of moisture from earth-oven-cooked samples compared to microwave-cooked fish or boiled tubers. There was, however, only 1% difference in the retention of moisture in the earth-oven and steam-cooked palusami. A gain of about 30–40% moisture in the boiled tubers was obviously because of the water that undoubtedly got absorbed by tubers during boiling. Water lost from earthoven-cooked cassava and taro could be due to evaporation and from palusami could largely be due to the loss in the form of drippings apart from evaporation losses. 3.4. Protein Protein retention in microwave-cooked meat, including fish, ranged from 99% to 101%. Retention of this nutrient had a bit higher range (100–103%) in oven-roasted meat samples. However, the differences in retention values were not statistically significant (P ¼ 0:05). A close to 100% retention of protein in microwave, together with the ovenroasted samples, is acceptable since neither conventional nor convenient food-handling procedures, such as the microwave, are known to significantly decrease protein nutritive values (Cross and Fung, 1982). Furthermore, Cross and Fung (1982) stated that microwave heating also does not alter the susceptibility of fish protein to proteolytic enzymes. Lang (1970) also commented that at 100 1C protein content of any type of meat is not altered.
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Some evidence that autoclaving lowers the value of meat protein with increasing temperatures and duration of heating have been shown by Lang (1970). This can also be true for cooking temperatures on meat. Low retention of protein (o100%) in earth-oven-cooked samples can be attributed to the internal temperature of the samples while being cooked. In earth-oven, chicken and lamb chops were cooked to an internal temperature of 99.5 1C compared to a maximum of only 88 1C in oven-roasting and 93 1C in microwave cooking. The rise in the internal temperature of meat was quite fast in earth-oven cooking compared to oven-roasting. Moreover, in the earth-oven all samples were cooked for a fixed time of one and a half hours whereas in other cooking methods the food samples were only cooked for such time that they were ready to be eaten. Over-cooking of earth-oven-cooked samples could probably be responsible for the decrease in the retention of protein compared to samples cooked by other methods. The more obvious path for the loss of protein from these samples is through drippings. Previous researchers have shown nitrogen to be present in the drippings of cooked meat (Lang, 1970; Baldwin et al., 1976; Cross and Fung, 1982). Protein is lost into the drippings through the loss of collagen in the form of gelatin in the drip (Lang, 1970). Cooking at this temperature in the earth-oven could have enhanced the loss of connective tissue into the drip, hence decreasing retention of protein in meat. Retention of protein in cassava and taro ranged from 98% to 102% regardless of the cooking method, but the difference in the retention of protein in these samples were not statistically significant at the 5% level.
3.5. Fat In all cases, cooking procedure significantly affected the fat content and hence its retention in cooked samples. Both types of boiled tubers retained the same amount of fat (92%) but retention in earth-oven-cooked taro was 83%, a bit lower than what was retained in earth-oven-cooked cassava (89%). Among the earth-oven-cooked samples fat retained in palusami was the least with a value of 63%. In comparison to this, steam-cooked palusami retained higher amount of fat (82%). Among all cooked samples, the lowest retention of fat was found to be in microwave-cooked fish, which showed a retention of only 31%. Earth-oven-cooked fish showed a retention of 87% fat. Retention of fat in whole chicken was 79% on earthoven cooking, 75% on microwave cooking and 82% on oven-roasting. Retention of fat in whole lamb chops was greater than its retention in whole chicken cooked by any method. Earth-oven cooking and oven-roasting retained similar amounts of fat in lamb chops, 101% and 98%, respectively, whereas microwave-cooked lamb chops had retention of 113%. Loss of fat from these samples was apparent as there was considerable amount of fat floating
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on the top of the drippings collected from the meat cooked in microwave and gas oven. A significantly low retention of fat in skin of chicken on all forms of cooking (81–85%) compared to a higher retention of fat in accordingly cooked separable fat of lamb chops (89–99%) could be due to the percent of fat that got drained into the drippings. The results suggest that the fat present in the chicken may have melted more readily and more of this fat got drained with the drips than the fat from the lamb chops. Another reason could be the amount of dripping, which probably was proportionally more from the chicken, washing out more fat from the chicken than the lamb chops. Baldwin et al. (1976) found that the percent fat in the drippings differed for pork and lamb cooked conventionally and by microwave. There seemed to have been no explanation for this difference in the trends between the two species of meat. The highest retention of fat was observed in the separable lean of lamb chops. Fat retention in lean of lamb chops ranged from 291% to 294% for the three methods of cooking. Such a high retention, about twice the amount (145%) reported by Anderson et al. (1989) could be due to adsorption of fat from separable fat as it melted during cooking, into the muscle tissue of lamb chops. This effect was less pronounced for the cooking of chicken. Almost all separable fat in chicken is present in and just beneath the skin. Therefore, during cooking, most of the fat that melted drained out with water that weeps out as drips rather than moving ‘‘backwards’’ into the flesh. It is easier for the melted fat to seep into the flesh if the separable fat is present in-between the muscle tissues, as is in the case of lamb chops. Due to this effect, the retention of fat in meat of chicken is only as high as 106% in earthoven cooking, 94% in Oven roasting and the least, 67% in microwave cooking. 3.6. Sugars The content of all sugars except for glucose in earthoven-cooked cassava and sucrose in earth-oven-cooked taro decreased after cooking. Decrease in the contents of reducing and non-reducing sugars on various cooking treatments of legumes were also reported by Jood et al. (1985, 1986). Retention of fructose and glucose was not calculated in taro and cassava samples, except for glucose in earth-oven-cooked cassava, because the content of these sugars were lower than 0.1 g/100 g of samples, which was the DL for determination of sugars. Retention of 180% glucose in earth-oven-cooked cassava could possibly be due to hydrolysis of starch to oligosaccharides and then to monosaccharides, resulting from cooking (Jood et al., 1988). Jood et al. (1988) and Rao and Belavady (1978) have observed similar increase in the level of sugars in cooked chickpea. Breakdown of starch to maltose has also been reported by Bradbury and Holloway (1988) in boiled, steamed and baked sweet potato.
Retention of sucrose in boiled cassava and taro were 92% and 80%, respectively. Earth-oven cooking retained 103% sucrose in taro but comparatively less (63%) in cassava. Loss of sugars during boiling can be explained by simple diffusion of sugars after being solubilized into the cooking water (Jood et al., 1988). The extent of diffusion of sugars from the tubers to the cooking medium may be a function of structure of the flesh of each tuber, hence the difference in the retention values of sucrose in these two types of tubers. Retention of fructose and glucose in palusami ranged from 43% to 58%. Retention of glucose in earth-oven and steam-cooked palusami was 58% and 56%, respectively. Steam-cooked palusami retained 58% of fructose whereas earth-oven cooking retained 43% fructose in palusami. The loss of these sugars can be attributed to their high solubility into the water that was lost as drippings from the palusami during cooking. In addition to the losses through leaching, fructose and glucose may have also been lost through the degradation and epimerization processes that occur at elevated temperatures (100–150 1C) in sugar solutions and foods (Lang, 1970). More than 100 different transformation products of glucose and fructose at elevated temperatures have been found, the most important being 5-hydroxymethylfurfural (HMF) (Bergdoll and Holmes, 1951; Muller and Taufel, 1953). At temperature of earth-oven and steam cooking of palusami, dehydration of sugars could have caused their conversion to HMF.
3.7. Starch There was a lower retention of starch in both cassava and taro on earth-oven cooking compared to boiling. Loss of starch in boiling treatment can be explained in two ways. The first explanation is based on the solubility of starch in plain water. Starch is composed of soluble and insoluble portions and on cooking in water the soluble portion might have been extracted out (Jood et al., 1986). The extraction of soluble starch into the cooking liquor was evident as after cooking, the water used for boiling the tubers gained viscosity. This implied a process of gelatinization, which is impossible without the presence of amylose and amylopectin units in the cooking water (Lang, 1970). The second process, known as the formation of resistant starch, also explains the loss of starch from tubers. This could be the major process by which starch is lost in earth-oven cooking. Resistant starch, which is a fraction of starch modified by the heat treatment and survives the exhaustive digestion by amylolytic enzymes, has been previously reported to be responsible for lowering the value of available starch in boiling, autoclaving and canning of legumes (Asp and Bjorck, 1992). Leaching out of soluble starch into the cooking medium and formation of resistant starch accounts for loss of starch upon boiling.
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3.8. Total dietary fibre (TDF) In all cases, except for in steam-cooked palusami, the retention of TDF was more than 100% which meant that this component of food increased after cooking. The increase in the retention, which ranged from 108% to 167% in cooked cassava and taro samples, is most probably associated with the formation of resistant starch in these samples. Resistant starch formed upon cooking becomes analytically associated with non-starch polysaccharides (NSP), which is one component of DF (Englyst et al., 1982). Previous researchers have reported similar increase in the content of DF upon cooking (Bradbury and Holloway, 1988; Asp and Bjorck, 1992; Attia et al., 1994; Thed and Phillips, 1995) and all have attributed the formation of resistant starch to this increase. A retention factor of 92% in steam-cooked palusami implies a slight loss of TDF upon cooking. This loss could be attributed to the loss of water soluble fraction of DF into the drippings. Increase in retention of DF was not expected in palusami as it had no starch present in the raw form that could have changed to resistant starch and increased the value of DF. 3.9. Ash From the retention values it is disclosed that among almost all meat samples, whole chicken retained most ash followed by fish and then by whole lamb chops. The only case in which this situation was reversed was for microwave-cooked fish, which retained more ash (89%) than whole chicken (81%). The above trend could be associated with the size of the samples that were cooked. Lamb chops were in slices which were about 1 in thick, and this provided a greater surface area for dripping losses, compared to fish and chicken which were cooked whole. The shape of the whole chicken provided a lower surface area to mass ratio of chicken compared to fish, which because of its flat and long shape had a higher surface area to its mass ratio. A greater surface area to mass ratio would allow removal of a higher amount of water as dripping, evident from the data on moisture loss, which would have a direct relationship to the loss of any nutrients dissolved in it. Loss of ash from meat samples was mainly through the loss of inorganic salts dissolved in the drips. For high fat samples it could be true that the lowering in the retention of ash in the samples could be due to the loss of mineral elements drained with the melted fat into the drippings. Among the tubers, cassava seemed to be more resistant towards the loss of ash upon cooking. It showed 104% retention upon earth-oven cooking and 96% retention of ash upon boiling, whereas taro had lower retention of ash, 90% and 89%, on earth-oven cooking and boiling. Attia et al. (1994) also reported loss of ash upon boiling, but quite high values which ranged from 34% to 40% in chickpea. These losses compared to less than 10% lost in boiled
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tubers analyzed in this study could be due to the difference in the samples, length of cooking and the amount of water used for boiling. The decrease in the ash values in boiled tubers appears to be due to leaching of inorganic compounds from the tubers into the cooking water. Retention of ash in palusami cooked in the earth-oven and by steam did not differ much with the retention values of 64% and 69%, respectively. The loss in the value of ash could be from the leaching of inorganic compounds dissolved in the dripping, which was apparent in palusami. 4. Conclusions Retention of nutrients in the samples analyzed in this study varies with the nutrient, the nature of food and the heating methods used. Water content was found to be dependent on the cooking method and also on the food sample. Generally, the highest loss in the content of water was associated with earth-oven cooking of all samples. This could mean the dripping losses were the highest in earthoven cooking compared to microwave cooking or ovenroasting. Retention of protein in all samples ranged from 96% to 103%, implying the resistant nature of protein during cooking of foods. Oven-roasting, microwave cooking, boiling or steaming had very slight or no deterioration effect on the protein in the cooked samples with retention of 99–103%. Retentions in the lower range (96–100%), though not statistically significant, were only observed in the earth-oven-cooked samples. Protein is lost into the drippings via amino acids present in the connective tissues of meat which is lost in the form of gelatin (Lang, 1970). Cassava and taro samples, relatively low in fat compared to all other samples, retained almost equal amounts of fat irrespective of the type of cooking. However, in meat samples the cooked separable lean of chicken and lamb chops retained comparatively more fat, as much as 294% in oven-roasted lamb lean only, than separable fat of the two meat samples. This implies that fats, after melting on heating, diffuse along the concentration gradient into the flesh being cooked. This process and the loss into the drippings were responsible for the lower retention of fat in the ‘‘fat only’’ or ‘‘skin only’’ samples. Loss of fat into the drippings was the factor that reduced fat in other samples such as fish and palusami. Retention of fat did not show any particular trend. The total carbohydrate component of foods does not change or changes very slightly, but single carbohydrate components quite often change. Upon heating, in all samples which had starch the retention of TDF increased to more than a 100% with a simultaneous decrease in the retention of starch. This implies conversion of one form of carbohydrate (starch) to another (DF). In samples that excluded water in the form of drippings or which were cooked in water (boiling), the most commonly lost components were individual sugars because of their high solubility in water. Retention of total carbohydrate in
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earth-oven-cooked samples was comparable to the boiled or steam-cooked samples. It is impossible to draw any general trend from the retention of ash in the samples that were analyzed. In spite of this, it can be said that earth-oven cooking quite often caused the lowest retention of ash in meat samples (lamb chops, fish and palusami) compared to other methods of cooking such as microwaving, oven-roasting or steaming. References Al-Khalifa, A.S., Dawood, A.A., 1993. Effects of cooking methods on thiamin and riboflavin contents of chicken meat. Food Chemistry 48, 69–74. Anderson, B.A., Clements, M.L., Dickey, L.E., Jacob, E., Hoke, M., 1989. Composition of Foods. Lamb, Veal, and Game Products: Raw, Processed, Prepared. US Department of Agriculture. Agriculture Handbook No. 8-17. Asp, N.G., Bjorck, I., 1992. Resistant starch. Trends in Food Science and Technology 3, 111–114. AOAC, 1995. Official methods of analysis of the Association of Official Analytical Chemists. In: Cunniff, P. (Ed.), Methods, 925.04, 981.10, 954.02, 985.29, 938.08, 16th ed. Association of Official Analytical Chemists, Gaithersburg, USA. Attia, R.S., El-Tabey Shehata, A.M., Aman, M.E., Hemza, M.A., 1994. Effect of cooking and decortication on the physical properties, the chemical composition and the nutritive value of chickpea (Cicer arietinum L). Food Chemistry 50, 125–131. Baldwin, R.E., Korschgen, B.M., Russel, M.S., Mabesa, L., 1976. Proximate analysis, free amino acid, vitamin and mineral content of microwave cooked meat. Journal of Food Science 41, 762–765. Bergdoll, M.S., Holmes, E., 1951. Heating of sucrose solutions. (I). Relation of 5-(hydroxymethyl)-furfural to color formation. Food Research 16, 50. Bertelsen, G., Finglas, P.M., Loughridge, J., Faulks, R.M., Morgan, M.R.A., 1988. Investigation into the effects of conventional cooking on levels of thiamin (determined by HPLC) and pantothenic acid (determined by ELISA) in chicken. Food Sciences and Nutrition 42, 83–96. Bradbury, J.H., Holloway, W.D., 1988. Chemistry of Tropical Root Crops: Significance for Nutrition and Agriculture in the Pacific. Australian Center for International Agricultural Research (ACIAR), Canberra. Brubacher, G., Muller-Mulot, W., Southgate, D.A.T., 1986. Methods for Determination of Vitamins in Food. Elsevier Applied Science Publishers, England. Cashel, K., English, R., Lewis, J., 1989. Composition of Foods, Australia. Australian Government Publishing Service, Canberra. Chaplin, M.F., 1994. Monosaccharides. In: Chaplin, M.F., Kennedy, J.F. (Eds.), Carbohydrate Analysis—A Practical Approach. Oxford University Press, Great Britain, pp. 1–20. Cross, G.A., Fung, D.Y.C., 1982. The effect of microwaves on nutrient value of foods. CRC Critical Reviews in Food Science and Nutrition 16, 355–381. Dickey, L.E., 1991. Composition of Foods: Raw, Processed, Prepared. 1990 Supplement. US Department of Agriculture. Agriculture Handbook No. 8. Dignos, R.L., Cerna, P.F., Troung, V.D., 1992. Beta-carotene content of sweet potato and its processed products. ASEAN Food Journal 7 (3), 163–166. El-Shimi, N.M., 1992. Influence of microwave and conventional cooking and reheating on sensory and chemical characteristics of roast beef. Food Chemistry 45, 11–14.
Englyst, H.N., Wiggins, H.S., Cummings, J.H., 1982. Determination of the non-starch polysaccharides in plant foods by gas-liquid chromatography of constituent sugars as alditol acetates. Analyst 107, 307–318. Greenfield, H., Southgate, D.A.T., 1985. A pragmatic approach to the production of good quality food composition data. ASEAN Food Journal 1 (2), 46–55. Hall, K.N., Lin, C.S., 1981. Effect of cooking rates in electric microwave oven on cooking losses and retention of thiamin in broilers. Journal of Food Science 46, 1292–1293. Headly, M.E., Jacobson, M., 1960. Electronic and conventional cookery of lamb roasts. Journal of American Dietetic Association 36, 337–340. Janicki, L.J., Appledorf, H., 1974. Effect of broiling, grill frying and microwave cooking on moisture, some lipid components and total fatty acids of ground beef. Journal of Food Science 39, 715–717. Jood, S., Chauhan, B.M., Kapoor, A.C., 1988. Contents and digestibility of carbohydrates in chickpea and blackgram as affected by domestic processing and cooking. Food Chemistry 30, 113–127. Jood, S., Mehta, U., Singh, R., 1986. Effect of processing on available carbohydrates in legumes. Journal of Agriculture and Food Chemistry 34, 417–420. Jood, S., Mehta, U., Singh, R., Bhat, C.M., 1985. Effect of processing on flatus-producing factors in legumes. Journal of Agriculture and Food Chemistry 33, 268–271. Kimura, M., Itokawa, Y., 1990. Cooking losses of minerals in foods and its nutritional significance. Journal of Nutritional Science and Vitaminology 36, S25–S33. Klein, B.P., Kuo, C.H.Y., Boyd, G., 1981. Folacin and ascorbic acid retention in fresh raw, microwave, and conventionally cooked spinach. Journal of Food Science 46, 640–641. Kumar, S., Aalbersberg, W., English, R.M., Ravi, P., 2001. Pacific Island foods. Vol 2. Institute of Applied Sciences Technical Report 2001/1. University of the South Pacific, Suva, Fiji. Lang, K., 1970. Influence of cooking on foodstuffs. World Reviews of Nutrition and Dietetics 12, 266–317. Moore, L.J., Harrison, D.L., Dayton, A.D., 1980. Differences among top round steaks cooked by dry of moist heat in a conventional or a microwave oven. Journal of Food Science 45, 777–781. Morgan, E., 1991. Chemometrics: Experimental Design. Wiley, UK. Muller, K., Taufel, K., 1953. Changes in glucose or invert sugar solutions during sterilization. Biochemistry 324, 221. Murphy, E.W., Criner, P.E., Gray, B.C., 1975. Comparisons of methods for calculating retentions of nutrients in cooked foods. Journal of Agriculture and Food Chemistry 23 (6), 1153–1157. Rao, P.U., Belavady, B., 1978. Oligosaccharides in pulses: varietal differences and effects of cooking and germination. Journal of Agriculture and Food Chemistry 26 (2), 316–319. Rumm-Kreuter, D., Demmel, I., 1990. Comparison of vitamin losses of vegetables due to various cooking methods. Journal of Nutritional Science and Vitaminology 36, S7–S15. Somogyi, J.C., 1990. Influence of food preparation on nutritional quality. Journal of Nutritional Science and Vitaminology 36, S1–S6. Thed, S.T., Phillips, R.D., 1995. Changes of dietary fiber and starch composition of processed potato products during domestic cooking. Food Chemistry 52, 301–304. Uherova, R., Hozova, B., Smirnov, V., 1993. The effect of microwave heating on retention of some B vitamins. Food Chemistry 46, 293–295. Unklesbay, N.F., 1988. Selected B vitamin retention of beef, lamb, and pork after alternate heat processing. Catering and Health 1, 87–99. Wanasundera, J.P.D., Ravindran, G., 1992. Effects of cooking on the nutrient and antinutrient contents of yam tubers (Dioscorea alata and Dioscorea esculenta). Food Chemistry 45, 247–250. Wills, R.B.H., Greenfield, H., 1987. Laboratory Instruction Manual for Food Composition Studies. Department of Food Science and Technology, University of New South Wales.
Original Article
Nutrient retention in foods after earth-oven cooking compared to other forms of domestic cooking$ 1. Proximates, carbohydrates and dietary fibre Shailesh Kumara,, Bill Aalbersbergb a
11 Cowin Close, Rowville, Victoria 3178, Australia Institute of Applied Sciences, University of the South Pacific, P.O. Box 1168, Suva, Fiji
b
Received 6 April 2004; received in revised form 23 June 2005; accepted 27 June 2005
Abstract Effects of Pacific traditional style of cooking in an earth-oven1 on proximate content of chicken, lamb chops, fish, cassava, taro and palusami2 were investigated. Retention of proximates in earth-oven-cooked samples was compared with the retention in microwaved and oven-roasted chicken and lamb chops, microwave-cooked fish, boiled cassava and taro, and steamed-cooked palusami, the nutrient analyses of all of which were conducted during the course of this study. Water content of the samples generally decreased most upon earth-oven cooking. As much as 32.9% moisture was lost from earth-oven-cooked lean of lamb chops. Loss of water from microwavecooked meat, ranging from 6.6 to 25.8 g/100 g, was second to the moisture loss in earth-oven-cooked meat and the least amount of moisture was lost from the gas-oven roasted meat with the values ranging from 4.4 to 22.2 g/100 g. Retention of protein ranged from 96% to 103% in all samples, the differences being not statistically significant. However, interestingly high retention values of fat were noted in separable lean of lamb chops ranging from 291% to 294%. A simple and logical explanation for this observation is adsorption of fat from separable fat, as it melted during cooking, into the muscle tissue of lamb chops. Retention of over 100% dietary fiber in all foods that had this component in the raw state was noted upon all types of cooking, except in steam-cooked palusami. This implied an increase in this component of food after cooking, whereas starch and sugars generally decreased after cooking. r 2005 Elsevier Inc. All rights reserved. Keywords: Earth-oven; Palusami; Proximate; Water; Protein; Fat; Sugars; Starch; Dietary fiber; Ash
Abbreviations: HPLC, High performance liquid chromatograph; na, Not analyzed Corresponding author. Tel.: +613 9753 9538; fax: +613 9764 9992. E-mail address: [email protected] (S. Kumar). $ Results of this work are printed as a technical report with limited distribution by the Institute of Applied Sciences at The University of the South Pacific, Suva, Fiji (Kumar et al., 2001). 1 Traditional Pacific style of cooking, lovo as it is known in Fijian, is performed in an oven in the earth prepared by digging a smooth sloping pit on the ground. The main source of heat is provided by the stones (once or twice the size of a fist), heated in an open fire (normally 1–2 h) until extremely hot. All food to be cooked is put in the pit at once on these extremely hot and dry stones (without water being poured to generate steam for cooking) and left in the pit, covered with earth, and left to cook for 1–2 h (1 h and 15 min in this research). 2 A vegetable-based dish that consists of a filling, made by mixing canned meat (fish, mutton or beef) with coconut cream, wrapped by a layer of taro leaves. Depending on the taste and likings of people, onion, chilies, carrot, tomatoes and salt are also added to the filling. 0889-1575/$ - see front matter r 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2005.06.006
1. Introduction When foods are prepared to be eaten, there are significant changes in the flavor as well as in the nutritional composition of the food. This paper focuses only on nutritional changes in different types of food preparation at home. Many of the data in the tables represent foods that are raw, but many foods are eaten after being processed, stored and/or prepared in various ways that may each affect at least some of the nutrient levels. Scientific literature contains results showing that nutrient losses differ for different nutrients and cooking methods. Some of the studies dealing with nutrient losses include work by researchers such as Hall and Lin (1981), Klein et al. (1981), Bertelsen et al. (1988), Unklesbay (1988), Kimura and Itokawa (1990), Rumm-Kreuter and Demmel
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(1990), Somogyi (1990), Dignos et al. (1992), El-Shimi (1992), Wanasundera and Ravindran (1992), Al-Khalifa and Dawood (1993), Uherova et al. (1993) and Thed and Phillips (1995). Numerous researches have been carried out on vitamins and minerals; however, research on effects of cooking on proximate nutrients is uncommon. This research compares the proximate composition in earth-oven-cooked foods, on which no previous work has been done, with several common practices of domestic cooking, namely microwaving, oven-roasting, boiling and steaming. Comparison is made on the basis of percentage nutrient retention calculated on dry-weight or dry-weightfat-free (DWFF) basis after cooking. 2. Materials and methods 2.1. Sample collection In general, sampling procedures were chosen so that the nutrient data obtained would represent the food as eaten in Fiji. To this end a composite sample of multiple food samples purchased at three different sites was analyzed. Fish (Lethrinus xanthochilus), taro leaves, cassava (Manihot esculenta) and taro (Colocasia esculenta), all fresh, were bought from three different vendors in Suva market. The most popular brand of frozen chicken, Crest, was bought from three leading outlets in Suva. Lamb chops (frozen), onion, corned mutton and coconut cream were also bought from these outlets. To avoid any variation in nutritional composition caused by different parts of lamb, only lamb chump chops, imported from New Zealand, were bought for analysis. Species identity for the fish was determined by reference to the ‘‘Species of Fiji’’ chart published by The Ministry of Fisheries of Fiji. This chart identifies all fishes of Fiji by their picture, common local name and species name. The local name of the fish used in this study is ‘‘Kacika’’. It is a reef finned-fish and commonly known as ‘‘yellowlip emperor’’. Three brands of corned mutton, Canterbury, Farmers and Golden Country, were mixed in equal amounts to make a composite for corned mutton that was used in palusami. Similarly, two brands of canned coconut cream, Solo’s Choice (made in Thailand) and Pacific Crown (made by Food Processors Fiji Limited), were mixed in equal amounts to make a composite of coconut cream that was used to prepare palusami. 2.2. General procedures for cooking methods In each case, cooking was carried out without the addition of any taste enhancers such as salt, soy sauce, garlic or ginger to the food except in the preparation of palusami, in which weighed amounts of salt and onion were added to the corned mutton and canned coconut cream filling.
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Microwave cooking: Chicken (whole), lamb chops and fish were cooked to a maximum internal temperature of 93 1C for 40, 20 and 15 min, respectively. Oven roasting: Chicken, whole, was cooked for 1 h 40 min to a maximum internal temperature of 88 1C. Lamb chops were cooked for 1 h to an internal temperature of 90 1C. Roasting of chicken and lamb chops in both of the ovens, microwave and conventional, produced considerable amount of drippings, which were discarded. Earth-oven cooking: All food to be cooked was put in the lovo pit at once and cooked for 1 h 15 min. The highest temperature reached in the oven was 126 1C. This was measured by leaving the probe of a thermocouple thermometer in the center of the pit while the foods were being cooked in the earth-oven. The internal temperatures of meat in microwave cooking, earth-oven cooking and oven roasting was measured by inserting the probe of the thermocouple thermometer in the respective meats while being cooked. Boiling: Taro and cassava were peeled, washed and cut into pieces of about half the size of an average-sized potato. These were then boiled separately in 1:3 root/water ratio (w/v), with the cooking liquor being discarded at the end of cooking. For cooking, the tubers were put into the pot once water had started boiling. Cooking was monitored and timed thereafter until the roots were cooked to softness (felt between fingers). Steaming: Palusami was steam-cooked for 1 h and 15 min. The temperature range for cooking was 103–107 1C. This was measured by leaving the probe of a thermocouple thermometer in the cooking pot, making sure the probe was not immersed in the boiling water. 2.3. Nutrient analysis Sufficient amount of each sample was homogenized and analyzed. Frozen samples were first thawed at room temperature before blending. Nutrients analyzed in this study were water, protein, fat, sugars (fructose, glucose, sucrose, maltose and lactose), starch, dietary fiber (DF) and ash. Official methods of analysis (AOAC, 1995) were used for the determination of moisture (method 925.04), protein (method 981.10), fat (method 954.02), total dietary fiber (TDF) (method 985.29) and ash (method 938.08). The factor used to convert nitrogen to the protein value was 6.25 for all foods. Sugars and starch were extracted and analyzed as described below. 2.3.1. Sugars Extraction of sugars was carried out in 85% ethanol/ water (v/v) (Chaplin, 1994). The ethanolic solution was added to the homogenized sample and boiled for a few minutes over a steam bath. The hot extraction solution was filtered through a pre-weighed filter paper and the extraction procedure was repeated another three times. The extract was concentrated on a rotary evaporator and
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made to volume with distilled water in a 10 mL volumetric flask. The quantification was carried out with high performance liquid chromatograph (HPLC) using acetonitrile/water, 80/20 (v/v), mobile phase with a 5 m Hypersil column (4.6 mm 250 mm) and refractive index detector. 2.3.2. Starch Starch was analyzed as glucose using HPLC after enzymatic digestion as described by Wills and Greenfield (1987). The residue remaining after extraction of sugars was dried in a vacuum oven at 75 1C to a constant weight (total residue). The dried residue, milled in a dry blender, was then weighed (300–400 mg), mixed in 10 mL water and digested with a-amylase in a boiling water-bath for 2 h. After cooling, this sample mixture was digested with amyloglucosidase in a shaking water-bath at 60 1C for 1 h. The solution was made to 25 mL with distilled water. A factor of 1/1.1 was used for the conversion of the value of glucose to starch.
Table 1 Weights before and after cooking for samples for which true retentions are calculated Sample
Weight after cooking (g)
Weight before cooking (g)
Cassava E/oven Boil
675 562
756 488
Taro E/oven Boil
950 500
1150 409
Fish E/oven M/wave
298 400
408 520
Palusami E/oven Steam
594 605
871 876
Abbreviations: E/oven, Earth-oven; M/wave, Microwave.
2.4. Quality control procedures For each sample, duplicate measurements were used to determine the nutrient content which was obtained by calculating the mean of duplicates. Duplicates falling within 10% of their mean were accepted to be showing satisfactory agreement (Greenfield and Southgate, 1985). The analysis was repeated if the agreement was outside 10% of the mean for the duplicates, and the mean recalculated on the basis of all the replicates. Other quality assurance procedures included analyses of in-house reference materials for moisture (wheat flour), protein (wheat flour), sugars (Milo, a commercial product from Nestle, usually dissolved in milk to make an energy drink for children) and fat (soya bean oil), recovery samples and reagent blanks in each analytical batch of samples. The acceptable nutrient value for the in-house reference materials was 72 standard deviations of the mean, which was pre-determined by analyzing 10 replicates of the in-house reference material over 1 month by different analysts. Recoveries ranging from 90% to110% were classified satisfactory (Brubacher et al., 1986). Reagent blanks helped monitor purity of the reagent used. Any significant value for the blank determination was accounted for in the calculation of the results. In addition to the quality control procedures described above, the laboratory regularly participates in the testing of proximate nutrients as part of proficiency testing scheme of the Central Science Laboratory, UK. 2.5. Nutrient retention calculation To achieve a uniform baseline for comparison, it was necessary to calculate the percent retention, not on the ‘‘as assayed’’ basis but on the dry-weight basis or change of weight basis. All nutrient retention values, except for the lamb chop and chicken samples, reported in this work are
the true retentions (TRs) calculated on the weight change basis as described by Murphy et al. (1975). Table 1 lists the weights of foods before and after cooking. To ease calculation of retention values in palusami, the samples were prepared such that all palusami were identical with regard to the amount of each ingredient3 in them. (i) %TR ¼ (nutrient content per g cooked food g food after cooking)/(nutrient content per g raw food g food before cooking) 100. Nutrient retention factors in the chicken and lamb chop samples were calculated on DWFF basis according to Eq. (ii), given below. (ii) %R ¼ (nutrient content per g cooked food [DWFF basis])/(Nutrient content per g of raw food [DWFF basis]) 100. The use of the above formula limits the calculation of retention of water and fat in the cooked samples. To be able to report the latter, calculation of retention values of fat were done on dry-weight basis, described as apparent retention by Murphy et al. (1975). 3. Results and discussion 3.1. Nutrient composition Tables 2 and 3 contain the proximate content per 100 g of the foods analyzed in this study. Less than detection limit (oDL) is used where it is analytically shown that the 3 Each palusami was made by mixing together 161 g of corned mutton, 133 g of coconut cream, 53 g of onion and 0.6 g of salt and wrapping these mixed ingredients with 93 g of taro leaves. The whole palusami was then wrapped in aluminum foil.
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Table 2 Proximate content in 100 g of raw and cooked foods Foods
Water (g)
Protein (g)
Fat (g)
CHO total (g)
Dietary fibre (g)
Ash (g)
Chicken, whole, raw Chicken, whole, E/oven Chicken, whole, M/wave Chicken, whole, O/roast Chicken, whole, literaturea Chicken, lean, raw Chicken, lean, E/oven Chicken, lean, M/wave Chicken, lean, O/roast Chicken, lean, literaturea Chicken, skin, raw Chicken, skin, E/oven Chicken, skin, M/wave Chicken, skin, O/roast Chicken, skin, literaturea L/chops, whole, raw L/chops, whole, E/oven L/chops, whole, M/wave L/chops, whole, O/roast L/chops, whole, literaturea L/chops, lean, raw L/chops, lean, E/oven L/chops, lean, M/wave L/chops, lean, O/roast L/chops, lean, literaturea L/chops, fat, raw L/chops, fat, E/oven L/chops, fat, M/wave L/chops, fat, O/roast L/chops, fat, literaturea Fish, whole, raw Fish, whole, E/oven Fish, whole, M/wave Fish, whole, literatureb Cassava, raw Cassava, E/oven Cassava, boil Cassava, literaturea Taro, raw Taro, E/oven Taro, boil Taro, literaturea Palusami, raw Palusami, E/oven Palusami, steam
68.0 58.7 61.3 63.6 66.1 72.5 60.1 66.0 67.9 74.2 50.6 48.0 42.6 42.5 47.6 59.7 34.5 39.7 46.5 51.5 75.8 42.9 50.0 53.6 72.2 13.8 17.8 23.0 21.1 8.6 77.7 69.1 71.0 81.3 60.4 58.9 67.5 62.1 65.7 66.0 73.4 68.9 75.3 70.5 68.3
18.0 26.1 26.0 23.6 18.4 20.2 27.5 28.1 24.5 20.7 9.4 15.5 17.5 19.1 13.1 16.2 25.5 19.6 22.5 15.1 20.0 31.8 28.2 26.6 20.3 5.3 12.9 5.5 7.9 4.3 21.3 29.1 27.3 17.9 0.7 0.8 0.6 1.1 1.1 1.1 0.9 1.9 7.4 10.4 11.0
13.9 14.1 12.6 13.0 13.2 7.4 11.4 6.1 8.1 4.2 39.1 35.1 38.4 36.8 33.8 23.5 38.6 39.7 30.5 32.7 3.4 23.5 20.4 19.1 6.6 80.7 68.8 71.3 70.8 86.8 2.0 2.4 0.8 0.6 0.5 0.5 0.4 0.3 0.4 0.4 0.3 0.2 12.1 11.2 14.4
na na na na 0 na na na na 0 na na na na 0 na na na na 0 na na na na 0 na na na na 0 na na na 0 35.7 37.8 28.9 30.4 30.0 30.4 22.5 23.4 3.5 4.8 3.6
na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na 1.5 2.8 1.4 na 2.1 2.9 2.2 na 1.5 2.3 2.0
0.7 0.9 0.8 0.9 0.8 0.8 1.0 0.9 0.9 0.9 0.3 0.5 0.5 0.6 0.5 0.6 0.7 0.6 0.7 0.9 0.7 0.8 0.8 0.8 1.3 0.2 0.4 0.2 0.2 0.1 1.3 1.4 1.5 1.2 0.6 0.7 0.5 0.8 1.1 1.2 0.8 1.0 1.5 1.4 1.5
Abbreviations: E/oven, earth-oven; M/wave, microwave; O/roast, oven roast; L/chops, Lamb chops; na, not analyzed. a Literature values for raw foods adapted from Cashel et al. (1989). b Literature values for raw fin-fish (Pacific cod) adapted from Dickey (1991).
constituent was present, but below the DL, whereas zero (0) is used where there was no analytical indication of the presence of the analyte. Nutrients, which are known not to be found in a particular sample, were not analyzed (na) and are given as na in the tables. Table 2 shows that the proximate content in raw samples of chicken, lamb chops, cassava and taro were comparable to the proximate composition of the respective foods published by Cashel et al. (1989). Proximate composition
of raw fish was compared with the composition of raw Pacific cod, published in USDA handbook number 8 (Dickey, 1991). 3.2. Nutrient retention Table 4 below shows percentage retention of nutrients after cooking. For comparison of retention of nutrients, as appropriate, a t-test or F-test has been used (Morgan,
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1991). A dash (-) has been used in Table 4 to represent retentions that could not be calculated for those nutrients which were not present in a food. Table 3 Starch and individual sugar components in taro, cassava and palusami samples Sample
Starch Sugars (g/100 g) (g/100 g) Fructose Glucose
Sucrose
Maltose
Lactose
Raw Taro 27.4 Cassava 33.0 Palusami 0.0
0.1 0.1 1.3
0.1 0.1 0.7
0.4 1.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
E/oven Taro 26.8 Cassava 34.1 Palusami 0.0
0.0 0.0 0.8
0.0 0.2 0.6
0.7 0.7 0.0
0.0 0.0 0.0
0.0 0.0 0.0
Boil Taro Cassava
19.8 26.7
0.0 o0.1
0.0 0.0
0.5 0.8
0.0 0.0
0.0 0.0
Steam Palusami
0.0
1.0
0.6
0.0
0.0
0.0
Abbreviations: E/oven, earth-oven.
3.3. Water Though calculation of percent retention of water in chicken and lamb chop samples was limited by the formula used for calculation of nutrient retention in these samples, the comparison was based on the water content in these samples. Almost all previous researchers have found significantly more moisture to be lost by microwave cooking than by conventional cooking of meat (Janicki and Appledorf, 1974; Baldwin et al., 1976; Moore et al., 1980; El-Shimi, 1992). Except for the values in chicken skin and lamb fat, the content of moisture in microwave-cooked meat (chicken and lamb chops) was always lower than the moisture content of meat roasted in the oven. This agrees with the trend of a higher loss of moisture in microwave cooking compared to conventional roasting. However, apart from the moisture in chicken skin only, it was found that the moisture content of earth-ovencooked meat, including fish, was even lower than the moisture content of comparable samples cooked in the microwave. The loss in the moisture content of foods is due to evaporation and in the form of drippings, which is common in meat samples including fish (Headly and Jacobson, 1960). The content of water in samples cooked
Table 4 Percent retention of proximate nutrients in cooked samples Foods
Chicken, whole, E/ovena Chicken, whole, M/wavea Chicken, whole, O/roasta Chicken, lean, E/ovena Chicken, lean, M/wavea Chicken, lean, O/roasta Chicken, skin, E/ovena Chicken, skin, M/wavea Chicken, skin, O/roasta L/chops, whole, E/ovena L/chops, whole, M/wavea L/chops, whole, O/roasta L/chops, lean, E/ovena L/chops, lean, M/wavea L/chops, lean, O/roasta L/chops, fat, E/ovena L/chops, fat, M/wavea L/chops, fat, O/roasta Fish, whole, E/ovenb Fish, whole, M/waveb Cassava, E/ovenb Cassava, boilb Taro, E/ovenb Taro, boilb Palusami, E/ovenb Palusami, steamb
Protein
97 100 101 96 100 102 100 101 101 99 100 103 98 99 100 100 100 101 98 99 102 99 98 100 96 103
Fat
79 75 82 106 67 94 85 85 81 101 113 98 292 291 294 89 99 96 87 31 89 92 83 92 63 82
Starch
— — — — — — — — — — — — — — — — — — — — 92 93 81 89 — —
Sugars Fructose
Glucose
Sucrose
— — — — — — — — — — — — — — — — — — — — — — — — 43 58
— — — — — — — — — — — — — — — — — — — — 180 — — — 58 56
— — — — — — — — — — — — — — — — — — — — 63 92 103 80 — —
Dietary Fibre
Ash
— — — — — — — — — — — — — — — — — — — — 167 108 114 128 104 92
89 81 98 88 81 95 93 87 100 72 80 86 71 79 86 83 78 69 79 89 — — 90 89 64 69
Abbreviations: E/oven, earth-oven; M/wave, microwave; O/roast, oven roast; L/chops, Lamb chops; na, not analyzed. a Retention factor of fat for this food is calculated on dry weight basis and all other retention factors are calculated on a dry weight fat free basis. b Retention factors for this food are ‘‘true’’ retentions calculated on the change of weight basis.
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Earth-oven
Microwave
Boil
Steam
Fish Cassava Taro Palusami
65 87 83 64
70 — — —
— 129 137 —
— — — 63
by different methods could be attributed to the extent of moisture lost in each cooking method through drippings and evaporation. Microwave cooking has been known to cause more drip than conventional cooking, and it also causes more evaporation in chicken than roasting (Janicki and Appledorf, 1974). It seems from this study that earthoven cooking causes an even greater loss of moisture in the form of drippings and evaporation than microwave cooking. Moisture contents of earth-oven-cooked cassava and taro were lower than the moisture contents of boiled cassava and taro. It was seen that steaming caused a higher loss of water from palusami than earth-oven cooking. TR values for moisture in these samples and fish give a clearer view of loss or gain of moisture in the samples upon different types of cooking. Table 5 contains the retention values of water in cooked samples of fish, cassava, taro and palusami. The retention values also show a trend of higher loss of moisture from earth-oven-cooked samples compared to microwave-cooked fish or boiled tubers. There was, however, only 1% difference in the retention of moisture in the earth-oven and steam-cooked palusami. A gain of about 30–40% moisture in the boiled tubers was obviously because of the water that undoubtedly got absorbed by tubers during boiling. Water lost from earthoven-cooked cassava and taro could be due to evaporation and from palusami could largely be due to the loss in the form of drippings apart from evaporation losses. 3.4. Protein Protein retention in microwave-cooked meat, including fish, ranged from 99% to 101%. Retention of this nutrient had a bit higher range (100–103%) in oven-roasted meat samples. However, the differences in retention values were not statistically significant (P ¼ 0:05). A close to 100% retention of protein in microwave, together with the ovenroasted samples, is acceptable since neither conventional nor convenient food-handling procedures, such as the microwave, are known to significantly decrease protein nutritive values (Cross and Fung, 1982). Furthermore, Cross and Fung (1982) stated that microwave heating also does not alter the susceptibility of fish protein to proteolytic enzymes. Lang (1970) also commented that at 100 1C protein content of any type of meat is not altered.
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Some evidence that autoclaving lowers the value of meat protein with increasing temperatures and duration of heating have been shown by Lang (1970). This can also be true for cooking temperatures on meat. Low retention of protein (o100%) in earth-oven-cooked samples can be attributed to the internal temperature of the samples while being cooked. In earth-oven, chicken and lamb chops were cooked to an internal temperature of 99.5 1C compared to a maximum of only 88 1C in oven-roasting and 93 1C in microwave cooking. The rise in the internal temperature of meat was quite fast in earth-oven cooking compared to oven-roasting. Moreover, in the earth-oven all samples were cooked for a fixed time of one and a half hours whereas in other cooking methods the food samples were only cooked for such time that they were ready to be eaten. Over-cooking of earth-oven-cooked samples could probably be responsible for the decrease in the retention of protein compared to samples cooked by other methods. The more obvious path for the loss of protein from these samples is through drippings. Previous researchers have shown nitrogen to be present in the drippings of cooked meat (Lang, 1970; Baldwin et al., 1976; Cross and Fung, 1982). Protein is lost into the drippings through the loss of collagen in the form of gelatin in the drip (Lang, 1970). Cooking at this temperature in the earth-oven could have enhanced the loss of connective tissue into the drip, hence decreasing retention of protein in meat. Retention of protein in cassava and taro ranged from 98% to 102% regardless of the cooking method, but the difference in the retention of protein in these samples were not statistically significant at the 5% level.
3.5. Fat In all cases, cooking procedure significantly affected the fat content and hence its retention in cooked samples. Both types of boiled tubers retained the same amount of fat (92%) but retention in earth-oven-cooked taro was 83%, a bit lower than what was retained in earth-oven-cooked cassava (89%). Among the earth-oven-cooked samples fat retained in palusami was the least with a value of 63%. In comparison to this, steam-cooked palusami retained higher amount of fat (82%). Among all cooked samples, the lowest retention of fat was found to be in microwave-cooked fish, which showed a retention of only 31%. Earth-oven-cooked fish showed a retention of 87% fat. Retention of fat in whole chicken was 79% on earthoven cooking, 75% on microwave cooking and 82% on oven-roasting. Retention of fat in whole lamb chops was greater than its retention in whole chicken cooked by any method. Earth-oven cooking and oven-roasting retained similar amounts of fat in lamb chops, 101% and 98%, respectively, whereas microwave-cooked lamb chops had retention of 113%. Loss of fat from these samples was apparent as there was considerable amount of fat floating
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on the top of the drippings collected from the meat cooked in microwave and gas oven. A significantly low retention of fat in skin of chicken on all forms of cooking (81–85%) compared to a higher retention of fat in accordingly cooked separable fat of lamb chops (89–99%) could be due to the percent of fat that got drained into the drippings. The results suggest that the fat present in the chicken may have melted more readily and more of this fat got drained with the drips than the fat from the lamb chops. Another reason could be the amount of dripping, which probably was proportionally more from the chicken, washing out more fat from the chicken than the lamb chops. Baldwin et al. (1976) found that the percent fat in the drippings differed for pork and lamb cooked conventionally and by microwave. There seemed to have been no explanation for this difference in the trends between the two species of meat. The highest retention of fat was observed in the separable lean of lamb chops. Fat retention in lean of lamb chops ranged from 291% to 294% for the three methods of cooking. Such a high retention, about twice the amount (145%) reported by Anderson et al. (1989) could be due to adsorption of fat from separable fat as it melted during cooking, into the muscle tissue of lamb chops. This effect was less pronounced for the cooking of chicken. Almost all separable fat in chicken is present in and just beneath the skin. Therefore, during cooking, most of the fat that melted drained out with water that weeps out as drips rather than moving ‘‘backwards’’ into the flesh. It is easier for the melted fat to seep into the flesh if the separable fat is present in-between the muscle tissues, as is in the case of lamb chops. Due to this effect, the retention of fat in meat of chicken is only as high as 106% in earthoven cooking, 94% in Oven roasting and the least, 67% in microwave cooking. 3.6. Sugars The content of all sugars except for glucose in earthoven-cooked cassava and sucrose in earth-oven-cooked taro decreased after cooking. Decrease in the contents of reducing and non-reducing sugars on various cooking treatments of legumes were also reported by Jood et al. (1985, 1986). Retention of fructose and glucose was not calculated in taro and cassava samples, except for glucose in earth-oven-cooked cassava, because the content of these sugars were lower than 0.1 g/100 g of samples, which was the DL for determination of sugars. Retention of 180% glucose in earth-oven-cooked cassava could possibly be due to hydrolysis of starch to oligosaccharides and then to monosaccharides, resulting from cooking (Jood et al., 1988). Jood et al. (1988) and Rao and Belavady (1978) have observed similar increase in the level of sugars in cooked chickpea. Breakdown of starch to maltose has also been reported by Bradbury and Holloway (1988) in boiled, steamed and baked sweet potato.
Retention of sucrose in boiled cassava and taro were 92% and 80%, respectively. Earth-oven cooking retained 103% sucrose in taro but comparatively less (63%) in cassava. Loss of sugars during boiling can be explained by simple diffusion of sugars after being solubilized into the cooking water (Jood et al., 1988). The extent of diffusion of sugars from the tubers to the cooking medium may be a function of structure of the flesh of each tuber, hence the difference in the retention values of sucrose in these two types of tubers. Retention of fructose and glucose in palusami ranged from 43% to 58%. Retention of glucose in earth-oven and steam-cooked palusami was 58% and 56%, respectively. Steam-cooked palusami retained 58% of fructose whereas earth-oven cooking retained 43% fructose in palusami. The loss of these sugars can be attributed to their high solubility into the water that was lost as drippings from the palusami during cooking. In addition to the losses through leaching, fructose and glucose may have also been lost through the degradation and epimerization processes that occur at elevated temperatures (100–150 1C) in sugar solutions and foods (Lang, 1970). More than 100 different transformation products of glucose and fructose at elevated temperatures have been found, the most important being 5-hydroxymethylfurfural (HMF) (Bergdoll and Holmes, 1951; Muller and Taufel, 1953). At temperature of earth-oven and steam cooking of palusami, dehydration of sugars could have caused their conversion to HMF.
3.7. Starch There was a lower retention of starch in both cassava and taro on earth-oven cooking compared to boiling. Loss of starch in boiling treatment can be explained in two ways. The first explanation is based on the solubility of starch in plain water. Starch is composed of soluble and insoluble portions and on cooking in water the soluble portion might have been extracted out (Jood et al., 1986). The extraction of soluble starch into the cooking liquor was evident as after cooking, the water used for boiling the tubers gained viscosity. This implied a process of gelatinization, which is impossible without the presence of amylose and amylopectin units in the cooking water (Lang, 1970). The second process, known as the formation of resistant starch, also explains the loss of starch from tubers. This could be the major process by which starch is lost in earth-oven cooking. Resistant starch, which is a fraction of starch modified by the heat treatment and survives the exhaustive digestion by amylolytic enzymes, has been previously reported to be responsible for lowering the value of available starch in boiling, autoclaving and canning of legumes (Asp and Bjorck, 1992). Leaching out of soluble starch into the cooking medium and formation of resistant starch accounts for loss of starch upon boiling.
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3.8. Total dietary fibre (TDF) In all cases, except for in steam-cooked palusami, the retention of TDF was more than 100% which meant that this component of food increased after cooking. The increase in the retention, which ranged from 108% to 167% in cooked cassava and taro samples, is most probably associated with the formation of resistant starch in these samples. Resistant starch formed upon cooking becomes analytically associated with non-starch polysaccharides (NSP), which is one component of DF (Englyst et al., 1982). Previous researchers have reported similar increase in the content of DF upon cooking (Bradbury and Holloway, 1988; Asp and Bjorck, 1992; Attia et al., 1994; Thed and Phillips, 1995) and all have attributed the formation of resistant starch to this increase. A retention factor of 92% in steam-cooked palusami implies a slight loss of TDF upon cooking. This loss could be attributed to the loss of water soluble fraction of DF into the drippings. Increase in retention of DF was not expected in palusami as it had no starch present in the raw form that could have changed to resistant starch and increased the value of DF. 3.9. Ash From the retention values it is disclosed that among almost all meat samples, whole chicken retained most ash followed by fish and then by whole lamb chops. The only case in which this situation was reversed was for microwave-cooked fish, which retained more ash (89%) than whole chicken (81%). The above trend could be associated with the size of the samples that were cooked. Lamb chops were in slices which were about 1 in thick, and this provided a greater surface area for dripping losses, compared to fish and chicken which were cooked whole. The shape of the whole chicken provided a lower surface area to mass ratio of chicken compared to fish, which because of its flat and long shape had a higher surface area to its mass ratio. A greater surface area to mass ratio would allow removal of a higher amount of water as dripping, evident from the data on moisture loss, which would have a direct relationship to the loss of any nutrients dissolved in it. Loss of ash from meat samples was mainly through the loss of inorganic salts dissolved in the drips. For high fat samples it could be true that the lowering in the retention of ash in the samples could be due to the loss of mineral elements drained with the melted fat into the drippings. Among the tubers, cassava seemed to be more resistant towards the loss of ash upon cooking. It showed 104% retention upon earth-oven cooking and 96% retention of ash upon boiling, whereas taro had lower retention of ash, 90% and 89%, on earth-oven cooking and boiling. Attia et al. (1994) also reported loss of ash upon boiling, but quite high values which ranged from 34% to 40% in chickpea. These losses compared to less than 10% lost in boiled
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tubers analyzed in this study could be due to the difference in the samples, length of cooking and the amount of water used for boiling. The decrease in the ash values in boiled tubers appears to be due to leaching of inorganic compounds from the tubers into the cooking water. Retention of ash in palusami cooked in the earth-oven and by steam did not differ much with the retention values of 64% and 69%, respectively. The loss in the value of ash could be from the leaching of inorganic compounds dissolved in the dripping, which was apparent in palusami. 4. Conclusions Retention of nutrients in the samples analyzed in this study varies with the nutrient, the nature of food and the heating methods used. Water content was found to be dependent on the cooking method and also on the food sample. Generally, the highest loss in the content of water was associated with earth-oven cooking of all samples. This could mean the dripping losses were the highest in earthoven cooking compared to microwave cooking or ovenroasting. Retention of protein in all samples ranged from 96% to 103%, implying the resistant nature of protein during cooking of foods. Oven-roasting, microwave cooking, boiling or steaming had very slight or no deterioration effect on the protein in the cooked samples with retention of 99–103%. Retentions in the lower range (96–100%), though not statistically significant, were only observed in the earth-oven-cooked samples. Protein is lost into the drippings via amino acids present in the connective tissues of meat which is lost in the form of gelatin (Lang, 1970). Cassava and taro samples, relatively low in fat compared to all other samples, retained almost equal amounts of fat irrespective of the type of cooking. However, in meat samples the cooked separable lean of chicken and lamb chops retained comparatively more fat, as much as 294% in oven-roasted lamb lean only, than separable fat of the two meat samples. This implies that fats, after melting on heating, diffuse along the concentration gradient into the flesh being cooked. This process and the loss into the drippings were responsible for the lower retention of fat in the ‘‘fat only’’ or ‘‘skin only’’ samples. Loss of fat into the drippings was the factor that reduced fat in other samples such as fish and palusami. Retention of fat did not show any particular trend. The total carbohydrate component of foods does not change or changes very slightly, but single carbohydrate components quite often change. Upon heating, in all samples which had starch the retention of TDF increased to more than a 100% with a simultaneous decrease in the retention of starch. This implies conversion of one form of carbohydrate (starch) to another (DF). In samples that excluded water in the form of drippings or which were cooked in water (boiling), the most commonly lost components were individual sugars because of their high solubility in water. Retention of total carbohydrate in
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earth-oven-cooked samples was comparable to the boiled or steam-cooked samples. It is impossible to draw any general trend from the retention of ash in the samples that were analyzed. In spite of this, it can be said that earth-oven cooking quite often caused the lowest retention of ash in meat samples (lamb chops, fish and palusami) compared to other methods of cooking such as microwaving, oven-roasting or steaming. References Al-Khalifa, A.S., Dawood, A.A., 1993. Effects of cooking methods on thiamin and riboflavin contents of chicken meat. Food Chemistry 48, 69–74. Anderson, B.A., Clements, M.L., Dickey, L.E., Jacob, E., Hoke, M., 1989. Composition of Foods. Lamb, Veal, and Game Products: Raw, Processed, Prepared. US Department of Agriculture. Agriculture Handbook No. 8-17. Asp, N.G., Bjorck, I., 1992. Resistant starch. Trends in Food Science and Technology 3, 111–114. AOAC, 1995. Official methods of analysis of the Association of Official Analytical Chemists. In: Cunniff, P. (Ed.), Methods, 925.04, 981.10, 954.02, 985.29, 938.08, 16th ed. Association of Official Analytical Chemists, Gaithersburg, USA. Attia, R.S., El-Tabey Shehata, A.M., Aman, M.E., Hemza, M.A., 1994. Effect of cooking and decortication on the physical properties, the chemical composition and the nutritive value of chickpea (Cicer arietinum L). Food Chemistry 50, 125–131. Baldwin, R.E., Korschgen, B.M., Russel, M.S., Mabesa, L., 1976. Proximate analysis, free amino acid, vitamin and mineral content of microwave cooked meat. Journal of Food Science 41, 762–765. Bergdoll, M.S., Holmes, E., 1951. Heating of sucrose solutions. (I). Relation of 5-(hydroxymethyl)-furfural to color formation. Food Research 16, 50. Bertelsen, G., Finglas, P.M., Loughridge, J., Faulks, R.M., Morgan, M.R.A., 1988. Investigation into the effects of conventional cooking on levels of thiamin (determined by HPLC) and pantothenic acid (determined by ELISA) in chicken. Food Sciences and Nutrition 42, 83–96. Bradbury, J.H., Holloway, W.D., 1988. Chemistry of Tropical Root Crops: Significance for Nutrition and Agriculture in the Pacific. Australian Center for International Agricultural Research (ACIAR), Canberra. Brubacher, G., Muller-Mulot, W., Southgate, D.A.T., 1986. Methods for Determination of Vitamins in Food. Elsevier Applied Science Publishers, England. Cashel, K., English, R., Lewis, J., 1989. Composition of Foods, Australia. Australian Government Publishing Service, Canberra. Chaplin, M.F., 1994. Monosaccharides. In: Chaplin, M.F., Kennedy, J.F. (Eds.), Carbohydrate Analysis—A Practical Approach. Oxford University Press, Great Britain, pp. 1–20. Cross, G.A., Fung, D.Y.C., 1982. The effect of microwaves on nutrient value of foods. CRC Critical Reviews in Food Science and Nutrition 16, 355–381. Dickey, L.E., 1991. Composition of Foods: Raw, Processed, Prepared. 1990 Supplement. US Department of Agriculture. Agriculture Handbook No. 8. Dignos, R.L., Cerna, P.F., Troung, V.D., 1992. Beta-carotene content of sweet potato and its processed products. ASEAN Food Journal 7 (3), 163–166. El-Shimi, N.M., 1992. Influence of microwave and conventional cooking and reheating on sensory and chemical characteristics of roast beef. Food Chemistry 45, 11–14.
Englyst, H.N., Wiggins, H.S., Cummings, J.H., 1982. Determination of the non-starch polysaccharides in plant foods by gas-liquid chromatography of constituent sugars as alditol acetates. Analyst 107, 307–318. Greenfield, H., Southgate, D.A.T., 1985. A pragmatic approach to the production of good quality food composition data. ASEAN Food Journal 1 (2), 46–55. Hall, K.N., Lin, C.S., 1981. Effect of cooking rates in electric microwave oven on cooking losses and retention of thiamin in broilers. Journal of Food Science 46, 1292–1293. Headly, M.E., Jacobson, M., 1960. Electronic and conventional cookery of lamb roasts. Journal of American Dietetic Association 36, 337–340. Janicki, L.J., Appledorf, H., 1974. Effect of broiling, grill frying and microwave cooking on moisture, some lipid components and total fatty acids of ground beef. Journal of Food Science 39, 715–717. Jood, S., Chauhan, B.M., Kapoor, A.C., 1988. Contents and digestibility of carbohydrates in chickpea and blackgram as affected by domestic processing and cooking. Food Chemistry 30, 113–127. Jood, S., Mehta, U., Singh, R., 1986. Effect of processing on available carbohydrates in legumes. Journal of Agriculture and Food Chemistry 34, 417–420. Jood, S., Mehta, U., Singh, R., Bhat, C.M., 1985. Effect of processing on flatus-producing factors in legumes. Journal of Agriculture and Food Chemistry 33, 268–271. Kimura, M., Itokawa, Y., 1990. Cooking losses of minerals in foods and its nutritional significance. Journal of Nutritional Science and Vitaminology 36, S25–S33. Klein, B.P., Kuo, C.H.Y., Boyd, G., 1981. Folacin and ascorbic acid retention in fresh raw, microwave, and conventionally cooked spinach. Journal of Food Science 46, 640–641. Kumar, S., Aalbersberg, W., English, R.M., Ravi, P., 2001. Pacific Island foods. Vol 2. Institute of Applied Sciences Technical Report 2001/1. University of the South Pacific, Suva, Fiji. Lang, K., 1970. Influence of cooking on foodstuffs. World Reviews of Nutrition and Dietetics 12, 266–317. Moore, L.J., Harrison, D.L., Dayton, A.D., 1980. Differences among top round steaks cooked by dry of moist heat in a conventional or a microwave oven. Journal of Food Science 45, 777–781. Morgan, E., 1991. Chemometrics: Experimental Design. Wiley, UK. Muller, K., Taufel, K., 1953. Changes in glucose or invert sugar solutions during sterilization. Biochemistry 324, 221. Murphy, E.W., Criner, P.E., Gray, B.C., 1975. Comparisons of methods for calculating retentions of nutrients in cooked foods. Journal of Agriculture and Food Chemistry 23 (6), 1153–1157. Rao, P.U., Belavady, B., 1978. Oligosaccharides in pulses: varietal differences and effects of cooking and germination. Journal of Agriculture and Food Chemistry 26 (2), 316–319. Rumm-Kreuter, D., Demmel, I., 1990. Comparison of vitamin losses of vegetables due to various cooking methods. Journal of Nutritional Science and Vitaminology 36, S7–S15. Somogyi, J.C., 1990. Influence of food preparation on nutritional quality. Journal of Nutritional Science and Vitaminology 36, S1–S6. Thed, S.T., Phillips, R.D., 1995. Changes of dietary fiber and starch composition of processed potato products during domestic cooking. Food Chemistry 52, 301–304. Uherova, R., Hozova, B., Smirnov, V., 1993. The effect of microwave heating on retention of some B vitamins. Food Chemistry 46, 293–295. Unklesbay, N.F., 1988. Selected B vitamin retention of beef, lamb, and pork after alternate heat processing. Catering and Health 1, 87–99. Wanasundera, J.P.D., Ravindran, G., 1992. Effects of cooking on the nutrient and antinutrient contents of yam tubers (Dioscorea alata and Dioscorea esculenta). Food Chemistry 45, 247–250. Wills, R.B.H., Greenfield, H., 1987. Laboratory Instruction Manual for Food Composition Studies. Department of Food Science and Technology, University of New South Wales.