Effects of Humectant and Roasting on Physicochemical and Sensory Properties of Jerky Made from Spent Hen Meat

Chilled carcasses of spent laying hens (approximately 80-90 wk of age including fattening period before slaughtering) were obtained from commercial slaughter plant, Jai Phue Phae Butchery, Nongjok, Bangkok, Thailand. These were manually deboned in the Department of Animal Production Technology and Fisheries, King Mongkut’s Institute of Technology Ladkrabang (KMITL). Chilled carcasses were dressed according to Thai style retailed cutting. Breast, tenderloin and boneless thigh and drum of spent hen were collected, removed the skin, packed in polyethylene bags, stored at −20℃ until further jerky processing (within 2 mon).

Five jerky treatments, control (no humectants added), 10% glycerol, 15% glycerol, 10% sorbitol and 15% sorbitol were performed. The lowering level of humectants was previously trial, but no succeed to prevent toughness resulting from raw meat and drying was found. Three replications of each treatment were made for any differences among batches. According to manufactured jerky, frozen spent hen meat was thawed overnight in refrigerator at 4℃. Meat was cut to a small pieces and ground with grinder using a die with 5 mm holes. Five treatments of jerky were processed by using ingredients including 0.8% w/w sodium chloride (Thai Refined Salt Co., Ltd., Thailand), 4% w/w sugar (Mitrphol Co., Ltd., Thailand), 2% w/w soy sauce (ThaiTheparos Co., Ltd., Thailand), 0.2% w/w monosodium glutamate (Ajinomoto Co., (Thailand) Ltd., Thailand), 0.5% w/w barbecue seasoning (McCormick & Company, Inc., USA), 0.5% w/w paprika powder (McCormick & Company, Inc., USA) and different amount of 10% or 15% w/w glycerol (Chemipan Corporation Co., Ltd., Thailand) or 10% or 15% w/w sorbitol (Siam Sorbitol Co., Ltd, Thailand). After thoroughly mixed, 5 formulated treatments were reformed strips (30 mm × 80 mm × 5 mm) by a jerky gum equipped with a flat nozzle (LEM Jerky Gun, LEM Products Direct, USA). All treatments were cooked using a convection type hot air oven (FP115, Binder Inc., USA) at a temperature of 85℃ until core temperature (CT) reached 71.1℃. Then the lower temperature, 60℃, was set for further dehydration till to a target a_w≤0.85. At 1 h intervals throughout drying, jerky samples were taken for monitoring temperature using insertion probe of Type-K thermocouple from a digital thermometer (52 Series II, Fluke Corp., USA) and recording a_w using a water activity meter (Novasina^® LabMasteraw, Axair Ltd., Switzerland). After drying and cooling to the ambient temperature (25-30℃), the sample was weighted. Determination of drying yield was calculated by dividing the dried weight by the initial weight before drying and multiplying by 100. Moisture content was determined according to AOAC methods (AOAC, 2012). Samples were packed in heat-sealed bags for analysis of jerky quality.

To ensure the microbiological safety of the present jerky process, total aerobic bacteria, yeast and mold and Staphylococcus aureus of raw meat and various 5 formulations of jerky products were carried out. Each sample (25 g) was transferred into 225 mL 0.1% peptone water (PW) and homogenized for 2 min with the Stomacher BagMixers 400 VW (Interscience Co., St. Nom, France). Appropriate ten-fold dilutions of the samples were prepared in PW and plated on growth media in duplicate to estimate microbial counts.

Total aerobic bacteria was determined by using pour plate method on Plate Count Agar (PCA) and the plates were incubated at 37℃ for 24-48 h (BAM, 2001a). The counts of mold and yeast were determined by surface plating on Malt agar (MA) acidified with lactic acid and plates were incubated at 26℃ for 3-5 d (BAM, 2001c). S. aureus was determined by surface plating on Baird-Parker Agar with added egg yolk tellurite emulsion and incubating the plates at 37℃ for 24-48 h (BAM, 2001b). The number of colonies was counted and expressed as logarithms of colony forming units per gram (Log CFU/g).

Three randomly selected areas from an exterior color of jerky samples were measured in the L*a*b* mode of CIE. For WBSF measurement, the middle area of jerky samples approximately 10 mm × 30 mm × 5 mm were used to determine maximum force measured to cut the sample was expressed as Newton.

Pieces of dried meat samples measuring 10 cylindrical cores (Ø 10 mm with 10 mm long), reformed by the jerky gun with round nozzle, were placed parallel to the compression plate surface attached to an Instron universal testing machine model 3344. TPA textural parameters were measured at room temperature with the following testing conditions: crosshead speed was 60 mm/min and compressed twice to 40% of their original height. The Bluehill 2 software (Instron Engineering Corp., USA) was used to collect and process the data. TPA analyzes were defined and calculated as previously described by (Bourne, 1978). Hardness (N), cohesiveness (ratio), gumminess (N), springiness (ratio) and chewiness (N) were calculated from the force-time curves generated for each sample.

Prior to study on adsorption isotherm, dehydrated jerky samples were prepared according to the methods of Yang et al. (2007) and Kim et al. (2010). Jerky samples were frozen at 80℃ for 24 h and freeze-dried over 5 d. Dried samples were put into polystyrene weighing dishes and further dehydrated in vacuum desiccators over P₂O₅ for 5-7 d until constant weight was attained and the initial moisture content was recorded. Adsorption isotherm was carried out according to the operating instructions from Novasina^® LabMaster-aw using sorption isotherm set. Briefly, moisture adsorption isotherm was evaluated at 25℃ over a range of relative humidity based on saturated salt solutions (11%, 33%, 53%, 75%, 90% and 97%). Every 2 h after humidification in various salt solutions, a_w and weight gain of a dehydrated jerky sample was detected. Moisture content (g H₂O/g solid) was calculated from the weight gain after absorbed moisture and was plotted against a_w, exhibiting adsorption isotherm.

Sensory attributes in regard to color, appearance, texture, flavor and overall acceptability of the sample, were evaluated by 30 semi-trained panelists from undergraduate and graduate students of Department of Animal Production Technology and Fishery, KMITL using a sevenpoint hedonic scale. A score ranged from 1 to 7 with the following ratings: 7 = liked extremely, 6 = liked moderately, 5 = liked slightly, 4 = Indifferent, 3 = slightly disliked, 2 = moderately disliked and 1 = disliked extremely. Unsalted cracker and water were supplied to testers for refreshing their palates before tasting subsequent samples.

Because the best jerky formulation among various treatments was 15% glycerol (provided high jerky quality and sensory acceptance), this formulation was selected for studying the influenced of heat treatments. Jerky sample was processed in two types of heat treatments. The control treatment was dehydrated jerky in the convention oven until CT >71.1℃ and a_w≤0.85 as aforementioned processing detail. In the second heat method, dehydrated jerky was further roasted in a gas oven at the temperature of 180℃ for 6 min. Quality among two jerky samples regarding drying yield, a_w, moisture content, color, texture profile analysis and shear force was compared. Sensory evaluation was performed by 30 semi-trained panelists from undergraduate and graduate students of Department of Animal Production Technology and Fishery, KMITL using a seven-point hedonic scale. Samples were assessed for their color, appearance, texture, flavor and overall acceptability.

Jerky qualities as influenced by the addition of humectants, results were carried out by the analysis of variance (ANOVA) and mean comparison was performed by Duncan’s multiple-range test (Steel and Torrie, 1980). Regarding the effect of heat treatment on jerky qualities, results were subjected to independent-samples t-tests. These statistical analyzes were performed by using the Statistical Package for Social Science (SPSS for Windows Version 11.5, SPSS Inc., USA).

Two-step drying process of a jerky product from spent hen meat was developed in the present study. To produce microbiological safety of jerky as recommended by Nummer et al. (2004) and USDA-FSIS (2012), first cooked to attain CT above 71.1℃ and then dried until a_w≤0.85 was expected. The result showed that after heating formed jerky strips under convection oven at 85℃ for 2 h, the average of CT of all jerky was attained to about 73.55±0.45℃. Then jerky was subsequently dried in an oven at 60℃ for 1 h to further dehydration, where the average CT of samples was 57.50±0.14℃. Previously trials pointed out that if high temperature was used through the process, desirable hard surface and brittle texture was produced. Moreover, a short period of total drying time of 3 h was found in the present study, which all jerky products meet the USDA-FSIS (2012) a_w recommendation as details below.

Changes in a_w values of samples were monitored during drying as shown in Table 1. Overall, all a_w values of jerky samples decreased rapidly during 3 h of drying ranged from 0.993-0.982 at the beginning to 0.854-0.630 at final dehydration depending on their treatments. Although values of a_w≤0.85 were detected in all jerky products (3 h of dehydration), which can inhibit growth of important bacterial pathogen including Salmonella spp., Listeria monocytogenes, S. aureus and Escherichia coli O157:H7 (USDA-FSIS, 2012), significant differences in a_w among samples were observed (p<0.05). The addition of glycerol, especially 15% glycerol, into jerky formulation, provided the lowest a_w value (p<0.05) followed by treatments of sorbitol and control, respectively (p<0.05). Additionally, only the a_w value of 15% glycerol added jerky was 0.630, which was lower than a recommended a_w (≤ 0.70) to prevent mold growth during storage (USDA-FSIS, 2012). These results were similar to those previously reported by Chen et al. (2014) who study effects of glycerol and sorbitol on venison jerky and found that the a_w values of jerky treated with glycerol were significantly lower than those in sorbitol and control groups. Both of glycerol and sorbitol are categorized as humectants in which they are compound capable to bind free water and decrease a_w of a solution or a product (Brown, 2008). As a comparison to the empirical constant (K) values as given by Norrish (1964), glycerol showed a lower K value (K=0.38) than sorbitol (K=0.85) indicative of their greater effect in lowering a_w (Ergun et al., 2014). Thus, glycerol is more effective humectants in comparison with sorbitol to control a_w in jerky products processed from spent hen meat.

The thermal process is among the most important factors controlling the microbial content and stability of finished product (Huang and Nip, 2001). The results indicated that total aerobic bacteria, yeast and mold and S. aureus counts of spent hen raw meat were found as 2.79±0.29, 2.56±0.03 and 2.77±0.04 Log CFU/g, respectively (Table 2). After drying at 85℃ for 2 h (CT ~73.55℃) followed by 60℃ for 1 h, total aerobic bacteria, yeast and mold and S. aureus counts of all jerky samples were reduced to undetectable levels. The results confirmed that present drying condition could produce microbiological safety products because it eliminated most organisms presented in the raw meat. For IMM with a_w values within 0.60-0.90, microbiological spoilage is usually the most important problem during storage (Huang and Nip, 2001). S. aureus is important photogenic bacteria because it has the ability to produce toxin at an a_w as low as 0.85 (Jay et al., 2005). Due to the lowest a_w value of jerky added 15% glycerol, it may be safe from microbial growth.

Spent hen meat jerky samples showed significant differences in drying yield and moisture content (p<0.05) as depicted in Table 1. The samples added with glycerol and sorbitol exhibited higher drying yield than control (p<0.05). The moisture content of jerky sample significantly decreased with an increase level of either glycerol or sorbitol (p<0.05). This results are in agreement with several researches which reported that the moisture content of various kinds of jerky products were decreased by the addition of glycerol or sorbitol (Boyle et al., 1993; Chen et al., 2000; Chen et al., 2014) as well as sucrose (Chen et al., 2002). In fact, water in foods exists in three general forms: (1) free, unbound water; (2) free, immobilized water; (3) chemically bound water. However, simple measurements of the water content (using drying oven) report only free, unbound water and possibly a portion of free, immobilized water (Pennington et al., 1990). After adding humectants in jerky formulation, they attract water to itself, might be bound as chemically bound water, so they potentially reduced the moisture content of products due to underestimation of water content.

Although the color of final products slightly differences among samples as shown in Fig. 1A, all humectants added jerky, except for 10% glycerol, possessed higher CIE L* value than control (p<0.05) (Table 1). Values of CIE a* and CIE b* among samples were notably increased with an increase level of glycerol and sorbitol. Fernandez-Lopez et al. (2003) reported that the meat color is thought to be due to 3 factors, the concentration of heme pigments, chemical state of these pigments, and the physical light-scattering properties of the meat structure. Generally, humectants could attract water to itself resulting in low moisture levels and helps to prevent IMM from drying out, leading to increasing light-scattering and producing lighten finish product.

Fig. 1. Pictures of jerky products made from spent hen meat as influenced by humectants (A) and heat treatments (B).

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According to textural characteristics of jerky made from spent hen meat, only glycerol added jerky (10% and 15%) showed lower hardness and higher springiness as compared to control and sorbitol added treatment (p<0.05) (Table 1). Lower gumminess and chewiness than control and sorbitol-treated jerky was detected in 15% glycerol treated sample (p<0.05). Shear value of jerky samples was greatest decreased by the addition of 15% glycerol, followed by 10% of glycerol or 10% of sorbitol and 15% sorbitol or control, respectively (p<0.05). Among the textural characteristics, hardness and chewiness are the most important sensory attributes of jerky-type foods and determines the uniqueness and markedly attractive of this type of products (Lee et al., 2004). The results indicated that spent hen meat jerky, possessing tough meat background, should incorporate with 15% of glycerol for maintaining low chewiness, gumminess and hardness with high springiness value indicating soft and springy texture. These results are in agreement with Barrett et al. (1998), indicating that the glycerol can act as an effective textural plasticizer in certain meat products. Chen et al. (2000) reported that the shear force value of pork jerky decreased significantly with increased levels of glycerol rather than sorbitol. Vagenende et al. (2009) proposed that glycerol prevents protein aggregation by inhibiting protein unfolding and by stabilizing aggregation-prone intermediates through preferential interactions with hydrophobic surface regions that favor amphiphilic interface orientations of glycerol. However, 20% to 30% glycerol was reported to impart a bitter taste (Boyle et al., 1993).

Moisture adsorption curves of various jerky treatments processed from spent hen meat measured at 25℃ were classified as Type III isotherms (Fig. 2) as found in several other foodstuffs rich insoluble components (Labuza and Altunakar, 2007). The resultant curve is due to high protein and carbohydrate content of jerky (data not shown). At low a_w, water content could be absorbed only at the surface sites. Increasing of a_w, generally higher than 0.8, results in the dissolution of the soluble component and brings about sharp increase in moisture content (Labuza and Altunakar, 2007). In the present study, jerky added with 10% and 15% glycerol could significantly modify the shape of the isotherm. In the range of 0.3 to 0.9, which is equivalent to an open storage relative humidity of 30% to 90%, jerky treated with glycerol had higher moisture content than those treated with sorbitol and control (p<0.05), indicating good adsorption properties probably due to loosely packed protein network. These results are similar to those of Chen et al. (2000), who reported that glycerol increased absorption and sorbitol slightly decreased absorption of Chinese-style pork jerky. Since glycerol plasticized and expanded the protein network, the consequence of action was favorable to the adsorption of water molecules to jerky products. On the contrary, more aggregated protein from control and also sorbitol added jerky might act as barrier them to rehydration. Good humectants used in semi-dried meat products acts as both potential control and plasticized protein network, which was observed in glycerol regardless of sorbitol, could provide softer and high elastic texture of jerky. However, additional research is needed to elucidate the exact mechanism for fulfillment this observation.

Fig. 2. Sorption isotherm of jerky products made from spent hen meat treated with humectants. Bars represent the standard deviation (n=2).

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Sorption isotherms are important for more than one reason including useful in the prediction of shelf-life. Molds can cause spoilage of jerky products during shelf life if the product acquires moisture from the environment (Nummer et al., 2004). Although the product gained the moisture from humid conditions, for example, it gained moisture up to 0.20% dry basis, the addition of glycerol (both 10% and 15%) could maintain the a_w<0.70. Meanwhile, at 20% moisture content, sorbitol added jerky and control sample showed a_w>0.75, which is a_w concerning of mold growth. It might indicate that glycerol could control the finished product from moisture pick-up, so that the a_w does not increase to an unsafe level.

The result of the sensory acceptance test is presented in Table 1. Jerky prepared by spent hen meat with 15% glycerol showed better acceptances in terms of color, flavor and overall acceptability than control sample (p<0.05) and tended to be highest appearance and texture scores among treatments. There were no significant differences in sensory attributes between jerky treated with sorbitol (both 10% and 15%) and control (p>0.05). It was observed that all values of the evaluated attributes were above the acceptance limit for the product (score 4 = Indifferent and 5 = liked slightly). Trindade et al. (2005) reported that the overall product quality decreased with the increase of spent hen meat percentage in the formulation of sausage. To achieve a higher sensory score of product in the present study, the process of roasting was used to combine into jerky processing as the details below.

The superior jerky quality processed from sent hen meat, defined as the lowest a_w, soft and chewy textures, better sensory acceptance, was found in jerky formulated with 15% glycerol. Therefore, this formulation was selected for further improvement of product quality, specifically sensory acceptance. The effect of two different thermal treatments of jerky processed from spent hen meats on drying yield, physicochemical properties and sensory attributes were evaluated. These results are shown in Table 3. There were no significant differences in drying yield, a_w, moisture content among samples (p>0.05). Darker color as considered by lower CIE L* value was found in jerky produced by drying plus roasting as compared to one step of drying process (p<0.05). More Maillard reaction between amino acid compounds and reducing sugar could promote darker and browned color of roasted jerky. Drying plus roasting process provided higher hardness and shear value of the jerky product than only drying process alone (p<0.05). This result was influenced by the two thermal preparation accelerating water loss from the jerky product as concomitant with a tendency for low moisture content.

Higher sensory scores of color, appearance, flavor and overall acceptability was found in jerky processed by drying followed by roasting rather than drying alone (p<0.05) (Table 3). There was no significant detrimental effect on texture score as resulted from roasting (p>0.05). During roasting, high Maillard reaction occurred and some of oil from glycerol that exuded from the jerky strips also increased, providing more desirable color, shining of jerky surface as illustrated in Fig. 1B as well as intense flavor, thus increase palatability. Theses sensory attribute of roasted jerky scored between 5 (liked slightly) and 6 (like moderately) on the hedonic scale, which indicated higher acceptance than drying process alone. Dyubele et al. (2010) also reported a significant effect of thermal preparation on sensory scores of chicken where the roasted meat possessed higher sensory scores of aroma intensity, initial impression of juiciness and overall flavor intensity than the boiled meat. The Maillard reaction, thermal degradation of lipids and Maillard-lipid interactions, are the main pathways by which a large number of flavor and aroma compounds responsible for desirable chicken meat flavor are formed during cooking (Jayasena et al., 2013). Cooking methods such as roasting, grilling, frying, and pressure cooking generates many pyrazines, pyridines, pyrroles and thiazoles compared to boiling of chicken meat (Shi and Ho, 1994).