Livestock Research for Rural Development 31 (7) 2019 Guide for preparation of papers LRRD Newsletter

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Effect of post-mortem electrical stimulation and phenolic-rich plant extracts on breast meat quality of Muscovy ducks reared under the duck-rice integration system

Eddah Mtiti, Wycliff Mwanjasi, Jonathan Tanganyika and Gregory Chingala

Lilongwe University of Agriculture and Natural Resources, P O Box 219, Lilongwe Malawi


Duck production is getting popular in smallholder production systems in Africa due to better adaptability of ducks to wide environmental conditions than chickens. Nevertheless, the duck meat has lower consumer acceptance in Africa due to its toughness, less juiciness and darkness in colour than chicken meat. Electrical stimulation and plant extracts are used to enhance meat tenderness and colour. Their interactive effect was explored to improve meat tenderness and colour of breast meat of ducks under the duck-rice integration. A total of 28, twelve-week old female Muscovy ducks of similar weight (1.474 ± 0.128kg), were harvested and allocated to four treatment combinations ( n=7) arranged in a completely randomized design with 2×2 factorial treatment structure. The treatment combinations were: (1) electrical stimulation (ES) only; (2) ES+ Vachellia polycantha plant extracts,(3) plant extracts only and (4)without ES and plant extracts. Combination of the post-mortem-electrical stimulation (ES) and phenolic-rich plant extracts did not improve the physical meat quality characteristics of duck breast meat. However, electrical stimulation and plant extracts separately improved meat tenderness. The yellowness (b*) colour values were only improved by the addition of plant extracts only. The minimal effects of electrical stimulation in combination with plant extracts may have been due to inherently high iron, haem and type IIA in duck breasts which may have influenced physical meat quality parameter response to electrical stimulation and plant extracts.

Key words: colour, smallholder duck production, tenderness, Vachellia polyacantha


Duck production is getting popular in smallholder production systems in Africa due to better adaptability of ducks to wide environmental conditions than chickens (Faure et al 2003; Yakubu 2013). Ducks are less susceptible to many poultry diseases and have better foraging and scavenging ability than chickens (Raji et al 2009). In addition, ducks are often integrated with rice to control weeds and pests and as source of manure rice fields. Over 70% of ducks kept on African continent are Muscovy ducks (Yakubu, 2013). Muscovy ducks have comparable carcass and meat quality characteristics with Peking and other duck breeds (Tai and Tai 2001). Overall, the duck meat is tougher, less juice and darker in colour than chicken meat (Ali et al., 2007) making the duck meat not being readily accepted by consumers in African region (Yakubu 2013). However, duck production has enormous potential to contribute to food security and enhance smallholder livelihoods in Africa if duck meat acceptance could be improved (Yakubu 2013).

To improve consumer acceptance of the duck meat, there is need to move away from selling traditional selling of a whole carcass towards deboned breast fillets and/or portions (Smith et al 2015). On the other hand, application of post-mortem electrical stimulation and plant extracts are used to enhance meat tenderness and colour, respectively. Electrical stimulation enhances meat tenderness and meat colour through a rapid muscle acidification, inhibition of sarcomere shortening and induction of myofibrillar damage due to intense muscle contractions associated with pulsed electrical current passing through the carcass (Alvarado and Sams 2000; Sams 2002; Kahraman et al 2011) The direct application of phenolic-rich plant extracts to meat, improves its physical quality characteristics including colour stability in poultry meat (Karre et al 2013). Phenolic compounds possess the ability to avoid loss of the membrane integrity and protein cross-links by inhibiting/or reducing the rate of oxidation of the meat (Estévez 2011; Gómez-Cortés et al 2018). However, the synergy of the between the post-mortem electrical stimulation and plant extracts in improving breast muscle physical meat quality characteristics of poultry has not been explored. Therefore, the present study was conducted to evaluate the effect of post-mortem electrical stimulation and pre-rigor addition of Vachellia polyacantha leaf extracts on physical meat quality characteristics of breast meat of Muscovy ducks reared under the duck-rice integration system.

Materials and methods

The study site

The study was conducted at Bunda Campus of the Lilongwe University of Agriculture and Natural Resources in June 2018 with the approval of the Committee on Ethical and Care and Use in Experimental Animals, Department of Animal Science, Lilongwe University of Agriculture and Natural Resources.

Animals, experimental design and treatments

Twenty-eight female ducks (12 weeks old and 1.67±0.13kg weight), harvested from duck-rice integration, were used in the current experiment. The ducks were put in a rice field from 09:00 to 15: 00 pm daily. The ducks were introduced in the rice field at the age of 4 weeks. The animals were laid in a completely randomised design with a 2×2 factorial treatment structure. The main factors were post-mortem electrical stimulation and addition of plant extracts before chilling. The treatment combinations were: electrical stimulation (ES) only, ES+ Vachellia polycantha plant extract (PE), PE only and without ES and PE. Each treatment combination had seven ducks.

Electrical stimulation and addition of plant extracts

The ducks were slaughtered at the Animal Science Department Mini-Abattoir, Bunda Campus, Lilongwe University of Agriculture and Natural Resources. The slaughter and electrical stimulation were done by modifying the procedure of Alvarado and Sams (2000). The birds were slaughtered by stunning them by immersing them in a 2% NaCl solution for 2 seconds that had been passed with 20 volts, followed by exsanguination. Ducks were electrically stimulated immediately after exsanguination. Electrical stimulation was conducted according to Alvarado & Sams (2000) by hanging the bled birds on steel shackles and immersed the necks in a 2% NaCl solution passed with 220V pulsated by 2s on and 1s off for 60 s. A successful electrical stimulation was shown by a wing flap movement caused by muscle contractions generated in the breast. After electrical stimulation, all birds were scalded in hot water of 63oC for 5 minutes and de-feathered followed by an evisceration. Breast muscles were removed from the carcasses. Skin and visible fat were removed from the muscles.

Pairs of muscles were packed in the Ziploc plastic bags. Plant extracts (1ml per cm2) were added to 14 breast muscles using a syringe. Muscles were sealed in Ziploc plastic bags and chilled at 4oC for 24 hours.

Preparation of plant extracts

Leaves of Vachellia polyacantha at full vegetative stage were hand plucked from the Bunda Experimental Farm, Lilongwe University of Agriculture and Natural Resources. Vachellia polyacantha (formerlyAcacia polyacantha) is a tannins-rich tree that belongs to the Mimosoidae family and is a characteristic tree of Savanna woodland of Africa and usually found in damp localities. Leaves Vachellia polyacantha were air dried for three days. The leaves were sieved using a 2 mm sieve plate, this was done to remove all stems and unwanted materials. Leaves were ground through 0.5 mm sieve. To extract the phenolic compounds from the Vachellia polyacantha powder, distilled water was used in ratio of 1: 4 to the powder and left for 24 h. Although distilled water is not efficient reagent for extracting phenolic compounds, it was used because other reagents including ethanol and acetone are toxic and cannot be added to the food (Shah et al 2014). The mixture was squeezed through a cheese cloth followed by a filtration by a filter paper (Bellés et al 2017) .

Phenolic assay

Filtrate of plant extracts was assayed for phenolic concentration using the Folin-Ciocalteu Method according to (Makkar 2000). About 20μl of the extract that previously diluted 10 times with distilled water, was added to 20% sodium carbonate and Folin-Ciocalteu and read at 725nm absorbance using Thermo Scientific UV Spectrophotometer. Tannic acid (Sigma-Aldrich Chemie GmbH, Germany) was used as a standard and total phenolic were expressed as tannic acid equivalent (TAE).

Measurements of physical meat quality characteristics

Measurement of pH for the muscles done at 24 h post-mortem (Lutron, PH205, Taiwan). Measurements were taken from three different locations per sample. A portion of breast muscles from each bird were sliced and packed in a polystyrene tray and cling-wrapped in a low-density polyethylene firm. Samples were stored in a chiller set at 4 °C for 15 days without any illumination for determination of colour stability. Colour measurements of steaks were made on day 0, 5, 10 and 15. Colour measurements were directly made on a meat surface after blooming for 30 min. Lightness (L*), redness (a*) and yellowness (b*) parameters were recorded using the CIELAB colour meter: Konica Minolta Colorimeter (CR-400, Japan). Measurements of colour were taken three times at different locations per sample. The hue angle (H°) and chroma (C*) values were calculated as:

Drip loss was measured by suspending the pre-weighed steaks in individual plastic bags according to Honikel (1998). Briefly, a small hole was made through a corner of a steak from which a green wire (50 cm in length) was attached. The steak was then suspended in an inflated plastic bag where the steak was not allowed to touch the sides of the bag during suspension. The bag was suspended and stored at 4 °C for 24 h. Steaks were removed from the bag and gently blotted dry using a paper towel and weighed again. The drip loss was expressed as a percentage ratio of weight loss to the initial weight of meat samples.

The cooking loss was determined by boiling pre-weighed steaks which were put into thin-walled pre-labelled plastic bags with a permanent marker. Steaks were boiled at 80 şC in a water-bath for 1 h (Honikel 1998). After boiling, samples were removed from the water-bath. Water that had accumulated in the bags during boiling was removed and samples were cooled in an ice slurry. The steaks were removed from the bag, gently blotted dry using a paper towel, and re-weighed. Cooking loss was expressed as the percentage ratio of weight loss due to boiling of meat to the initial weight of samples.

Warner-Bratzler (WB) shear force was determined using the cooked meat samples from cooking loss analysis. The cooked samples were cooled at 4 °C for 24 h before the determination of WB shear force. Three circular cores (diameter = 10 mm) were bored using a core borer. The cores were sheared using a V-shaped, 1 mm thick WB cutting blade (speed; 33 mm/s) attached to Texture Analyser (TA.XT plus, Stable Micro Systems, UK) equipped with a 500 N load cell. The shear force was determined in kg.

All data were analysed using the General Linear Model (GLM) procedure of SAS (SAS Inc, Cary, NC). Analysis of variance (ANOVA) was used to test the effects of electrical stimulation, plant extract and their interaction. Data of colour stability were analysed using the Mixed Model, where a day of storage was fitted as repeated measures. Separation of the means was achieved by pairwise difference option imbedded in SAS with Tukey-Cramer Adjustment. A significance was declared at P < 0.05. The following statistical model was used:

Yij= µ + αi + βj + (αβ)ij + Ɛ ij


Yij = observed meat quality characteristics

µ = overall mean

αi = effect of electrical stimulation

Aj = effect of plant extract

(αβ)ij = the interactive effect of electrical stimulation and plant extract

Ɛij = random error component

Results and discussion

In the current study total phenolic content of Vachellia polyacantha leaf extracts was 85.3mg TEA /ml. The total phenolic content was lower than in previous report by (Chingala et al 2018) because in the current study the phenolics were extracted using water. Vachellia polyacantha leaf extracts also contain high levels of condensed tannins (Rubanza et al 2005). Several studies revealed that polyphelic compounds have a high antioxidant capacity (Luciano et al 2011).

The pulsation of the electrical current during electrical stimulation depletes energy reserves in the muscle thereby reducing amount of time associated with rigor mortis (Alvarado and Sams 2000). However, neither post-mortem electrical stimulation, nor plant extracts nor their interaction had an effect (p <0.05) on ultimate pH of the duck breast muscle (Table 1). A prior study by Alvarado and Sams (2000) found no effect of post-mortem electrical stimulation on ultimate pH of duck breast muscle. In addition, lack of influence of electrical stimulation on ultimate pH was also reported by Hoffman et al (2009) in ostriches which have a dark muscle similar to ducks. Electrical stimulation did not also influence pH of chicken fillets (Zhuang et al 2010)

Ability of meat to keep its intrinsic water content is called the water-holding capacity (WHC; Puolanne 2017). The WHC is measured as drip or purge loss and affects quality and yield of meat products (Puolanne 2017). Interaction of electrical stimulation and plant extracts had an effect ( p<0.05) on drip loss (Table 1). Muscles added with plant extracts only had the highest drip loss (p <0.05) while the electrically stimulated muscles combined with plant extracts had the lowest drip loss (Table 1). Muscle pH directly influences drip loss (Puolanne 2017). Muscles that have ultimate pH close to isoelectric point (pH=5.51) have higher drip loss than the muscles with pH away from the isoelectric point (Warriss 2000). Although there were no differences in ultimate pH among the treatments, the lowest drip loss in electrically stimulated breast muscles combined with addition the plant extracts may be because it had a pH which was much farther from isoelectric point than the other three treatments.

Cooking loss measures ability of meat to bind water and fat after protein denaturation and aggregation due to cooking (Hayes et al 2011). High cooking loss reduces nutritional quality and yield of the meat. The present study found that neither post-mortem electrical stimulation nor plant extracts nor their interaction had influence on the cooking loss of the duck breast meat. This is consistent with the findings by Alvarado and Sams (2000) and Kahraman et al (2011) who reported that electrical stimulation did not affect cooking loss of duck breast meat. Similar results were also observed in ostriches (Hoffman et al 2009) and bison (Ding et al 2016).

The WB shear force values are shown in Table 1. Shear force values of duck breast meat obtained in the current study were higher than those reported by (Omojola 2007) for female Muscovy ducks of 3.91 kg which may be due to differences in ages at slaughter and production systems. In the present study ducks were slaughtered at 12 weeks old. The WB shear force was affected (p<0.05) by interaction between electrical stimulation and plant extracts. Ducks that were electrically stimulated only and those whose breast muscles were added with plant extracts pre-rigor mortis only had lower WB shear force values (p <0.05) than those with electrical stimulation combined with plant extract and without plant extracts. Electrical stimulation elongates sarcomeres besides the rigor mortis muscle pH acceleration which is a significant determinant of tenderness (Kahraman et al 2011). The meat tenderisation caused by addition of plant extracts only may have been due to the effect of phenolic compounds which maintain the membrane integrity of myofibrils thereby facilitating a normal proteolysis (Estévez 2011). However, high WB shear force in duck breasts that received both electrical stimulation and plant extract muscle were related to ultimate pH which was high in the treatment compared to other treatments.

Table 1. Influence of post-mortem electrical stimulation and addition of plant extract on meat quality
characteristics of Muscovy ducks

Elec. stimulation




Plant ext


Plant ext.














DL %









CL %









WBSF, kg









abc Means with different superscripts within a row differ at p< 0.05
PE= plant extract; ES= electrical stimulation; pHu, ultimate pH; DL, drip loss; CL, cooking loss,
WBSF, Warner-Blatzer Shear Force

Meat colour is an important quality attribute of meat products as it is regarded as an indicator of meat freshness by consumers (Faustman et al 2010). Day of storage affected all colour measurements except the hue angle.

The changes in a* values of duck breast meat over 15-day storage are shown in Figure 1. Duck breast samples with ES and plant extracts or their combination had lower a* values than the control (without ES and plant extracts) during refrigerated storage. However, progressively increased trend of a* from Day 10 to Day 15 (the end of the storage) was observed in duck breast meat samples for ES and plant extracts only while the meat samples without ES and plant extracts showed stagnation. The finding of minimal effects of ES on discolouration (a*) in refrigerated storage were consistent with prior report by (Ding et al 2016) in bison meat. The duck breast has inherently high iron, haem and type IIA Smith et al 1993) which may have influenced the response of a*to electrical stimulation. Similarly, these physical characteristics may have influenced the duck meat’s response to plant extract addition. Ali et al (2007) also observed minimal changes of the a* values in duck breast during refrigerated storage period.

On the other hand, duck breast meat treated with plant extract had higher b* values than other treatments at the end of storage. Better meat colour including b* measures consumer acceptance of meat (Holman et al 2017). The results of more stability of b* values indicated that duck breast meat treated with plant extracts had more freshness than breast meat from other treatments.

Figure 1. Effect of electrical stimulation (ES) and Vachellia polyacantha plant extracts (PE) on red
colour stability of breast meat of Muscovy ducks over a 15-day refrigerated storage.
Different letters for the same day depict statistically different means (p <0.05)

Figure 2. Effect of post-mortem electrical stimulation (ES) and Vachellia polyacantha plant extracts (PE) on
yellow colour stability of breast meat of Muscovy ducks over a 15-day refrigerated storage.
Different letters for the same day depict statistically different means (p< 0.05)



The World Bank and Government of Malawi through Agricultural Productivity Programme for Southern Africa (APPSA) for funding the research


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Received 26 April 2019; Accepted 13 June 2019; Published 2 July 2019

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