1. Introduction
Pork sausages are popular, frequently consumed meat products that occupy a large proportion of the meat market for their smooth taste, fine texture, nutrition-rich properties, and convenient use [
1]. Commercial sausages contain approximately 30% fat, which contributes to the characteristics of the quality of meat products, such as flavour, texture, juiciness, colour, and mouthfeel, thus enhancing the acceptability of these products [
2]. However, high animal fat content in meat products increases the incidence of diet-related diseases, such as obesity, hypertension, type-2 diabetes, cardiovascular diseases, and coronary heart diseases, owing to high amounts of saturated fatty acids (SFAs) and serum cholesterol [
3]. The meat industry is entering a new era where products are characterised by low fat, low calorie, and high fibre [
4]. Such characteristics are driven by consumer demands for healthier and more nutritious diets, leading to the development of innovative products with reduced fat in traditional formulations.
In recent years, a variety of plant oils [
2,
5,
6,
7,
8,
9,
10], such as olive oil, sesame oil, sunflower oil, corn oil, canola oil, grape seed oil, hazelnut oil, and soybean oil have been added to various meat products as partial substitutes for animal fats, which can reduce the proportion of SFAs and increase the proportion of unsaturated fatty acids (UFAs). Among vegetable oil sources, soybean is widely planted worldwide with a high yield. Therefore, soybean oil is a low-cost and easily obtained vegetable oil that contains a high proportion of UFAs (52–56% linoleic acid, 20–25% oleic acid, and 7–11% linolenic acid) [
11]. However, vegetable oil added solely to meat batter may affect its appearance, flavour, texture, and physical stability, significantly decreasing the acceptability of meat products [
2,
12]. Consequently, emulsifiers and/or stabilisers that function as meat binders and texture stabilisers have been added to different meat products to counter the problem caused by fat substitutes, to reduce cooking loss, and to improve physicochemical and sensory properties [
13,
14].
Polysaccharides, including dietary fibre [
5,
15,
16,
17,
18,
19] (such as sugarcane, pumpkin, pineapple, rye bran, mushrooms, and rice bran), konjac [
20], carrageenan [
21], guar gum [
22], regenerated cellulose [
6], xylooligosaccharide [
23], carboxymethyl cellulose, and microcrystalline cellulose [
24], are widely used candidates of importance. Moreover, dietary fibre derived from plant polysaccharides is widely accepted as an important nutrient in the human diet, based on its ability to improve intestinal functions, decrease the incidence rates of cardiovascular and gastrointestinal diseases, and have a protective effect against weight gain and obesity without reducing the feeling of satiety [
4,
14,
16,
25].
Eggplant (
Solanum melongena L.) is widely planted worldwide [
26], and is reported to have a rich source of essential nutrients, with a high content of dietary fibre (33.79% in dry weight) and protein (11.65%), and low level of fat [
27]. Its incorporation in meat products may improve the dietary fibre content of meat products, thus providing well-balanced meals for meat eaters. In our previous study [
27], it was found that the mechanically homogenised eggplant flesh pulp (EFP) can effectively emulsify soybean oil in water emulsions (3:7,
v/
v) with good viscoelasticity by forming an interfacial film adhered to the oil droplets and the coherent three-dimensional network in the continuous phase. Rheological analysis illustrated that the EFP emulsion had typical gel-like nature. Moreover, the whole eggplant without chemical treatment can be utilised as food supplement, which leads to an environmentally friendly way for food production. Considering the features of eggplant, it could be a promising naturally functional ingredient to improve the physicochemical and sensory properties of fat-reduced meat products. In addition, the use of eggplant as an additive in meat products has not been previously investigated.
As a contribution to the development of handier and more versatile procedures for replacing animal fat with soybean oil, we conducted a study to assess the effect of 1, 2 and 3% eggplant powder (EP) addition on physicochemical properties (proximate composition, fat and water-binding properties, colour, water distribution, and texture) and sensory characteristics of reduced-fat pork sausages. The results of this study may provide a basis for studies on the preparation of EP sausages rich in UFAs and dietary fibre.
2. Materials and Methods
2.1. Materials
Pork leg muscle (71.82% moisture, 3.26% lipids, and 23.64% protein) and pork back fat (8.62% moisture, 89.64% lipids, and 2.11% protein) were purchased from Yurun chilled meat (Nanjing, China); purple eggplant fruits (Solanum melongena L.) with accordant maturity (length 20 cm and diameter 6 cm), soybean oil, salt, sugar, and white pepper from local supermarket (Nanjing, China); sodium caseinate (protein > 90%) from Henan Wanbang Industrial (Zhengzhou, China); sodium tripolyphosphate (food grade) from Nanjing Zhonghua Seasoning (Nanjing, China); and commercial cholesterol analysis test kit (F002-1, Zhejiang Dongou, Wenzhou, China) from Nanjing Jiancheng Bioengineering Institute (Nanjing, China).
2.2. Preparation of Eggplant Powder
The peeled eggplant was sliced and dried in a fan-forced air oven (DGG-9240A, Senxin Company, Guangzhou, China) at 60 °C for 36 h. Then, the dried eggplant was ground in a centrifugal mill (TLG-08, Tianlihengcheng, Beijing, China) fitted with a 30-mesh screen. The EP was stored in a vacuum pressure desiccator at room temperature until use.
2.3. Preparation of Pork Sausages
The pork leg muscle (all visible connective tissue and fat were trimmed) and back fat were cut into pieces and separately passed through a mincer (MM-12, Fengwei Product, Zhengzhou, China) using a 6 mm plate. The formulations of pork sausages are listed in
Table 1. Briefly, the minced meat was chopped in a bowl chopper (UMC-5C, Stephan, Karlsruhe, Germany) at a low speed (1500 rpm) for 30 s, followed by the addition of salt, sodium tripolyphosphate, pepper, and sugar, and chopped at a high speed (3000 rpm) for 60 s. After a 120 s pause, back fat, soybean oil, sodium caseinate, EP, and 1/2 ice water were added and the mixture chopped for 120 s at high speed, followed by the addition of the remaining ice water and chopping for another 120 s after a 60 s pause. The temperature of the meat batter was kept below 12 °C in all samples.
A portion of ground meat was drawn (about 20 g sample) to analyse its water- and fat-binding properties, and the rest was used to prepare sausages. The meat batter was stuffed into nylon/PE casings with a diameter of 26 mm, manually linked into approximately 15 cm lengths, and heated in a water bath (HH-42, Guohua, Changzhou, China) at 80 °C for 20 min. The sausages were cooled at room temperature and stored in a freezer at 4 °C until subsequent analysis within a week of production.
2.4. Proximate Composition of the Pork Sausages
Moisture content was determined by weight loss after drying at 105 °C for 12 h in a drying oven (DGG-9240A, Senxin, Guangzhou, China) according to AOAC 950.46 [
28]. Fat content was determined using the Soxhlet method (SOX406, Hanon, Jinan, China) according to AOAC 960.39 [
28]. Protein content was determined using the Kjeldahl method (Kjeltec 2300, Foss, Beijing, China) according to AOAC 981.10 [
28]. The ash content was determined using a muffle furnace (MF0910Pa, Huagangtong Technology, Beijing, China) according to AOAC 920.153 [
28]. The cholesterol content was determined using a commercial cholesterol analysis test kit.
2.5. Water- and Oil-Binding Properties
Water- and oil-binding properties were determined by measuring water and oil loss according to a previously described procedure [
5]. Raw batter (~20 g) was weighed (M
1), transferred to 50 mL centrifuge tubes, and centrifuged at 3000 rpm for 15 min at 4 °C to remove air bubbles (Allegra 64R, Beckman Coulter, California, USA). The tubes were heated in an 80 °C water bath (HH-42, Guohua, Changzhou, China) for 20 min and then immediately uncapped and left inverted for 1 h to release fat and water exudate (total fluid release, TR) onto a weighing bottle (M
2). Water release (WR) was determined as weight loss after heating the total exudate in an air oven (DGG-9240A, Senxin, Guangzhou, China) at 105 °C for 16 h (M
3). Oil release (OR, ignored minor protein or salt component) was calculated as the difference between TR and WR:
where M
0, M
1, M
2, and M
3 are the weight of the weighing bottle, raw batter, total exudate together with the weighing bottle, and oil release together with the weighing bottle, respectively.
2.6. Colour Measurements
The colour of the sausages was measured using a colorimeter (CR-400, Minolta Camera, Tokyo, Japan) and Illuminate C, calibrated with a white plate (L* = 96.86, a* = −0.15, b* = 1.87). Lightness (L*), redness (a*), and yellowness (b*) values of the five measurements were recorded.
2.7. Low-Field Nuclear Magnetic Resonance (NMR) Relaxation Measurements
NMR relaxation measurement was performed on an NMR Analyzer (MesoMr23, Niumag, Suzhou, China) with the magnetic field strength of 0.5 Tesla, and with corresponding resonance frequency for protons of 21 MHz. Sausages (approximately 2.0 g) were placed into cylindrical glass tubes (15 mm in diameter) and transverse relaxation (T2) was measured at 32 °C using the Carr-Purcell-Meiboom-Gill pulse sequence. T2 measurements were performed with a τ-value (interval between 90° and 180° pulse width) of 140 µs, 6500 echoes as eight scan repetitions, and repetition time of 5 s. The obtained T2 data were subjected to multi-exponential fitting analysis using MultiExp Inv Analysis software (Niumag, Suzhou, China). The measurements were carried out with five replicates for each sample and expressed as T2a, T2b, T21, and T22.
2.8. Texture Profile Analysis (TPA)
The sausages were cut into five cylindrical segments (height 20 mm, diameter 26 mm) after equilibration to room temperature at 25 °C for 3 h. TPA was performed using a texture analyser (TA-XT2i, Stable Micro System, Surrey, UK) fitted with the loadcell of 50 kg and a cylindrical probe (P/50, 50 mm stainless cylinder). The TPA measurement conditions were, pre-test speed 2.0 mm/s, test speed 2.0 mm/s, post-test speed 5.0 mm/s, strain 50%, time 5.0 s, trigger type auto, and trigger force of 5 g [
5]. All measurements were carried out at room temperature, and the hardness, springiness, cohesiveness, adhesiveness, and chewiness of the sausage were evaluated.
2.9. Sensory Analysis
Sensory evaluation was performed by 10 panellists who were trained according to the Chinese standard GB/T22210-2008 (criterion for sensory evaluation of meat and meat products) and had a common consensus for each point of the evaluation index. Sausages were warmed and served to the panellists for assessment of appearance, flavour, texture, mouthfeel, and overall acceptability using a 9-point hedonic scale [
5,
29]. The samples were cut into bite-sized pieces, placed on a plate, encoded with random numbers, and placed randomly. Sensory evaluation was performed at 2–3 h after the panellists’ meal, and the panellists had to gargle with distilled water before evaluating each sample. There was no communication among the panellists during the entire evaluation process. The sensory score was averaged after deducting the outliers (9, like extremely; 8, like very much; 7, like moderately; 6, like slightly; 5, neither like nor dislike; 4, dislike slightly; 3, dislike moderately; 2, dislike very much; 1, dislike extremely).
2.10. Statistical Analysis
Experiments were performed in triplicate (except for specially declared) for each sample. The data were analysed using the SAS v9.2 Windows program by an analysis of variance (one-way ANOVA) and Duncan’s multiple-range test. The results are reported as mean values ± standard deviations, and differences were considered to be significant when p < 0.05.
4. Conclusions
Reducing animal fat levels from 30% to 15% by adding soybean oil and water reduced the fat and cholesterol contents of sausages, but the water- and fat-binding properties, moisture stability, and texture were poor and the acceptability of sensory properties were reduced. To further improve these properties of reduced-fat sausages, various amounts of EP were added (1, 2, and 3%). With increasing EP contents, the sausages had increased water- and fat-binding properties, moisture stability, texture, and sensory properties. In particular, the low-fat sausages with 2% EP addition had the best acceptability in terms of appearance, flavour, texture, and mouthfeel. However, EP had a negative effect on the colour of the low-fat sausages, as it significantly decreased the L*- and a*-value. Moreover, the sensory evaluation was carried out by panellist instead of consumer. Overall, EP showed potential as a means of enhancing water- and fat-binding properties, moisture stability, texture, and sensory properties of sausages. As a future investigation, the mechanism underlying the physicochemical influence of EP on reduced-fat sausages would be assessed.