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1、文献翻译How blanching and drying affect the colour and functional characteristics of yam (Dioscorea cayenensis-rotundata) flourNol Akisso, Joseph Hounhouigan, Christian Mestresb, Mathurin NagoaAbstract: Colour and texture are important features of amala, a traditional thick paste obtained from dry yam f

2、lour. Tubers from eight yam cultivars were blanched at 65 C for 20 min and dried at 40 C for 5 days to test biochemical, thermal and pasting changes occurring during traditional yam flour processing. Blanching reduced peroxidase activity and drying reduced polyphenoloxidase activity, but total pheno

3、l content and the brown index of flour and of amala increased dramatically during the latter operations. The brown index of amala was significantly correlated with the total phenol content of the flour (r = 0.84) and the peroxidase activity of the fresh tubers (r = 0.75). Amylose content and starch

4、gelatinization enthalpy remained stable. For all cultivars, drying significantly increased the onset gelatinization temperature, suggesting the occurrence of starch annealing. The latter leads to a reduction in swelling power and solubility during pasting, and hence to a lower paste viscosity. 2003

5、Elsevier Science Ltd. All rights reserved.Keywords: Yam; Phenols; Polyphenoloxidase; Peroxidase; Blanching; Viscosity1 IntroductionYams (Dioscorea spp.) are an important source of carbohydrate for many people of the sub-Sahara region, especially in the yam zone of west Africa. To overcome the high p

6、erishability of fresh yam tubers, due to their high moisture content and the seasonal nature of their production, in some west African countries (Benin and Nigeria) yams are processed into flour using a well established method. Tubers are peeled, sometimes sliced, blanched in hot water (around 65 fo

7、r 1550 min) and sun-dried (Akisso, Hounhouigan, Bricas, Vernier, Nago, & Olorunda, 2001). The resulting dried tubers or chips are then milled into flour, which is used to make a thick paste known as amala with a different sensory quality from that of pounded yam (Ajibola, Abonyi, & Onayemi, 1988). T

8、he main quality attributes of amala are texture and colour (Hounhouigan, Kayode, Bricas, & Nago, 2002), which may be affected by yam cultivar characteristics and/or by processing conditions.The flesh of the yam species used is white, whereas the colour of the processed flour ranges from creamy white

9、 to dark brown. The discoloration phenomenon has long been studied on fresh tubers and has mainly been associated with enzymatic browning, due to the action of polyphenoloxidase (Almenteros & Del Rosatio, 1985; Ozo, Caygill, & Coursey, 1984) and peroxidase (Asemota, Wellington, Odutuga, & Ahmad, 199

10、2) and to the production of polyphenols and derived products (Osagie & Opoku, 1984). Various phenolic constituents have been reported to be responsible for the discoloration of edible yams: catecholamine was first suspected (Franklin & Ruth, 1972), then cyanidin-3-glucoside (Imbert & Sea-forth, 1968

11、; Rasper & Coursey, 1967), while Ozo et al. (1984) reported ( + )catechol and procyanidin oligomers as contributing to the discoloration of D. alata (Ozo et al., 1984). In the case of dried yam flour and amala, Mestres, Dorthe, Akissoe, and Hounhouigan (2002) reported a close positive correlation be

12、tween the amala brown index and the total phenol content of the flour. Izundu (1995), on the other hand, pointed out the role of peroxidase in amala browning. The actual contribution of poly-phenoloxidase and peroxidase to amala discoloration is, however, still unknown: polyphenoloxidase activity ca

13、n be partly inactivated during blanching at 65 C (Ozo & Caygill, 1985), and polyphenoloxidase and peroxidase activities are nearly nil in yam flour (Mestres et al., 2002).In addition, changes in pasting properties, occurring during blanching and drying, have not been fully investigated, although the

14、y have been identified as an essential and critical stage of yam chip processing (Akissoe et al., 2001; Ige & Akintunde, 1981).The aim of this work was to assess the changes in colour and pasting behaviour occurring during blanching and drying, using various yam cultivars, and to investigate the phy

15、sicochemical bases for these changes.2 Material and methods2.1 Plant materialEight cultivars of Dioscorea cayenensis-rotundata (Banioure, Deba, Gnidou, Kagourou, Porchekbim, Tam- sam,Terlunto, Yakarango) were obtained from the International Institute of Tropical Agriculture (IITA- Benin). According

16、to farmers, five of them (Deba, Kagourou, Porchekbim, Tam-sam, Yakarango) are very suitable for processing into dry yam chips, two (Banioure, Gnidou) are very unsuitable and one (Ter- lunto) is moderately suitable (Dansi, 2001). The yam tubers were harvested in the first half of December 2000 and pr

17、ocessed 1 week after harvesting.2.2 Experimental designThe tubers (5-7 for each cultivar) were hand-peeled in a water bath (28-30 C) and sliced into 30 mm thick cylinders using a cutting box. One sample of slices (referred to as A) was freeze-dried and used as a control to assess the physicochemical

18、 characteristics of the raw material. The remainder (B) was blanched in a thermostat-controlled water bath set at 65 C for 20 min, then equilibrated in water at 28-30 C for 5 min. The blanched slices were divided into two groups (B1 and B2). Group B1 was freeze-dried and used to assess the effect of

19、 blanching. Group B2 was dried in a forced air oven at 40 C for 5 days to assess the effect of drying. Each group (A, B1, B2) was crushed in a mortar, then ground in a laboratory centrifuge mill (Retsch, Haan, Germany) fitted with a 0.2 mm screen. The flour was then stored at 4 C until analysis.2.3

20、Methods2.3.1 Physicochemical characterization of fresh tubersThe fresh tubers were weighed, their dimensions were measured with a Vernier caliper and their volume was determined by millet displacement. In each case, a mean value was calculated for eight tubers of each cultivar. The dry matter conten

21、t of the fresh tubers was measured after drying 10 g yam pieces at 105 C for 48 h. The mean value of three tubers of each cultivar was calculated.2.3.2 Polyphenoloxidase (PPO) and peroxidase (POD) activity and total phenol (TP) contentPPO, POD and TP were determined using the methods described by Me

22、stres et al. (2002), measuring the oxygen consumption kinetic at 460 nm with catechol as substrate and the discoloration kinetic with an optical density at 760 nm after reaction with Folin reagent.2.3.3 Hunter Lab colour coordinatesThe colour of the flours and the pastes (obtained with a Rapid Visco

23、 Analyser as described below) was measured using a Minolta CR-210 portable chromameter (illuminant D65 CIE 1976). The Hunter Lab colour coordinate system L*, a* and b* values were recorded and the brown index was calculated as (100-L*).2.3.4 Pasting behaviourPasting properties were determined using

24、a Rapid Visco Analyser (RVA, Newport Scientific, Narrabeen, Australia) on 8% dry matter suspension. The suspension was heated from, 35 C to 95 C at a rate of 6 C min , maintained at 95 C for 4 min then cooled to 50 C at the same rate. Viscosity at the start of the 95 C plateau (V95b), viscosity at t

25、he end of the 95 C plateau (V95e) and end viscosity after cooling to 50 C (V50) were measured.2.3.5 Swelling power and solubilityThe swelling power and solubility procedures described by Mestres, Nago, Akissoe;, and Matencio (1997) were modified, with a dry matter concentration of 4% (wb; 1.2 g of d

26、ry matter dispersed in distilled water to give a total mass of 28 g) being used. The suspension was heated from 35 C to 95 C at a rate of 6 C min1 and held at 95 C for 1 min using the RVA. The heated suspension was then centrifuged at 3000 g for 15 min at ambient temperature.2.3.6 Thermal behaviourA

27、mylose content and starch gelatinization properties were determined by differential scanning calorimetry on a Perkin Elmer DSC7 device (Perkin-Elmer, Norwalk, USA), as described by Mestres, Matencio, Pons, Yajid, and Fliedel (1996) and Mestres, Boungou, Akissoe:, and Zakhia, (2000).2.4 Statistical a

28、nalysisAnalysis of variance and correlation and regression analyses were performed using Statitcf software (ITCF, Boigneville, France).3 Results3.1 Physicochemical characterization of fresh tubersDry matter content ranged from 24.9 to 32.1% (wb, Table 1). There was great heterogeneity in the weight

29、and size of the fresh tubers: volume ranged from 202 ml for Tamsam to 955 ml for Gnidou. It should be noted that the lowest-rated cultivars (Banioure and Gnidou) had the largest tubers, at least twice the size of the others.Table 1 Physicochemical characteristics of yam cultivarsYam cultivars(local

30、name)Dry matter（% wb）Weight（g）Volume（ml）Length（cm）Head-diameter（cm）Tail-diameter（cm）Banioure27.472583034.36.35.9Deba26.338138523.04.34.1Gnidou28.3115095531.76.36.2Kagourou32.146846424.34.84.1Porchekbim30.331332420.74.83.8Tam-sam24.918220219.72.93.3Terlounto29.645443922.05.55.0Yakarango26.533836617.5

31、5.24.83.2 Contrasting behaviour of polyphenoloxidase and peroxidase activities and total phenols during yam chip processingPolyphenoloxidase (PPO) and peroxidase (POD) activities and total phenol (TP) content varied to a large extent with cultivar and unit operation (Tables 2 and 3). Fresh Gnidou an

32、d Banioure cultivars (cvs) showed contrasting behaviour for PPO and POD activities:Table 2 Effect of processing stage and yam cultivaron polyphenol oxidase activity (mol O2 min-1 g-1)Yam cultivars(local name)Freeze-driedOven-driedFresh slicesBlanched slicesBlanched slicesBanioure10.410.21.2Deba32.219.80.6Gnidou72.651.43.7Kagourou35.724.72.5Porchekbim59.542.54.5Tam-sam13.611.01