Critical functional properties of defatted peanut meal produced by aqueous extraction and conventional methods

Abstract

The functional properties of the defatted peanut meal produced by aqueous extraction, solvent extraction and cold screw pressing followed by solvent extraction were studied. Good gelling property, color and nitrogen dispersibility as well as the high protein content of the defatted peanut meals produced by these methods were verified to be the critical functional properties for their high value of application to the food industry (e.g. production of ham sausages or hotdog sausages) though they are not suggested to be applied to the foods needing good oil binding capacity, emulsifying activity or stability and foaming capacity or stability. These results should provide valuable reference data for the application of defatted peanut meal to the food industry, the correct selection of processing method of peanut kernels which should be pursued by manufacturers and the determination of research direction that peanut processing technologists should be interested in.

Introduction

Significance of processing of peanut kernels has been reported by scientists in literature (Stephens et al. 2010; Latif et al. 2013; FAOSTAT 2018a). For the production of oils, proportion of peanut kernels which was crushed in 2010–2013 accounted for about 41% of the world total production (Fletcher and Shi 2016). In 2017, the annual production of defatted peanut meal in the world is ca. 5.67 million metric tons, which results from the processing of ca. 10.70 million metric tons peanut kernels for ca. 5.03 million metric tons oils (FAOSTAT 2018b). This kind of defatted peanut meal usually contains ca. 50% proteins. Especially, the advantages of vegetable proteins over animal proteins have been well discussed (Asgar et al. 2010). Therefore, the utilization of defatted peanut meal is very important and should bring in great profit for any peanut processing plant.

Therefore, a method which is capable of simultaneously producing defatted meal and oil with high quality is preferable to others for processing peanuts. Solvent extraction, cold pressing followed by solvent extraction and hot pressing have disadvantages of (Stalker and Wilson 2016; Latif and Anwar 2009). A new aqueous method of successfully recovering 96% oil from peanuts has recently been published (Tu et al. 2017; Tu and Wu 2019), which overcomes the shortcomings of traditional aqueous extraction (with and without enzymatic assistance) using large quantities of water (e.g. formation of serious emulsion, low free oil yield, large quantities of waste water, high cost of drying the defatted residue) (Li et al. 2016; Ravber et al. 2015). The potential of applying the defatted peanut meal produced by this new aqueous method needs to be evaluated.

Functional properties are critical for the application of defatted peanut meal in foods or food products. They include those contributing to sensory properties and physicochemical properties and those contributing to biological functions (including nutritional properties). The former is usually associated with the consumer’s preference of foods (Stone 2018). Particularly, we should pay great attention to gelling property (GP) for application to production of for example ham sausages or hotdog sausages, oil binding capacity (OBC), foaming capacity (FC) or stability (FS), emulsifying activity index (EAI) or stability index (ESI) and nitrogen dispersibility (NDI). These functional properties are greatly affected by the production method of defatted peanut meal.

Therefore, the aim of this study is to compare the functional properties (including GP, OBC, FC, FS, EAI, ESI and NDI) of the defatted peanut meal produced by different methods, which has never been reported in the literature before. The results of this study should provide valuable reference data for the application of defatted peanut meal to the food industry, the correct selection of processing method of peanut kernels which should be pursued by manufacturers and the determination of research direction that peanut processing technologists should be interested in.

Materials and methods

Materials

Preparation of defatted peanut meal by aqueous extraction

The defatted peanut meal was prepared according to the method based on aqueous extraction that was developed by Tu et al. (2017) as well as Tu and Wu (2019). The only differences were: 1.0 kg instead of 10 g kernel slurry of peanut treated and the mixture of the free oil and aggregate hydrophilic group squeezed three times by a screw cold press instead of by centrifugation to separate oils and obtain defatted peanut meal. Peeled peanut seed kernels (baked at 110 °C for 90 min) were ground to slurry passed through a 300 mesh sieve. After the slurry (1 kg) was mixed with 1 g sodium chloride and 150 ml water in the container of agitator, the mixture was agitated at 64 °C until a continuous oil phase was observed. After the major part of free oil was removed by leaching, the hydrophilic aggregate was pressed three times to collect the minor part of free oil. The residue was collected and naturally dried in the air at room temperature (ca. 20 °C) in a dry dish put in a desiccator and milled through a 100 mesh screen. This kind of defatted peanut meal (523.8 g) was named as water-extracted peanut fraction (WPF).

Preparation of defatted peanut meal by cold pressing followed by solvent extraction

The peanut kernels baked at 110 °C for 90 min were peeled and cut into eight pieces. When their moisture was tempered at 7%, they were squeezed with a cold screw press. The partially defatted residue was crushed and milled through a 100 mesh screen, and then the residual oil in it was recovered by reflux extraction for 4 h with n-hexane. The defatted peanut kernel was finally dried by the same method as that for preparing WPF. This kind of defatted peanut meal was named cold-pressed and solvent-extracted peanut fraction (CSPF).

Preparation of defatted peanut meal by solvent extraction

The peanut kernels were ground through a 100-mesh screen. Then, oil was recovered by reflux extraction for 8 h with n-hexane. The defatted peanut meal was placed in a dry dish to completely remove the residual solvent. This kind of defatted peanut meal was named as solvent extracted peanut fraction (SPF).

Evaluation of GP of defatted peanut meal

GP of pure defatted peanut meal

WPF, CSPF or SPF was weighed and an appropriate amount of water was added to give the mixture containing 62.5% moisture. After the mixture was stirred evenly, every 30 g of it was poured into a casing which was then tighten to prevent leakage. After cooking for 45 min at 100 °C, the gel with casing was cooled and stored at room temperature for overnight before measuring TPA profile (CT-3, US) and carrying out the sensory test by the methods similar to those described in “GP of defatted peanut meal in production of pork ham sausage” section.

GP of defatted peanut meal in production of pork ham sausage

Preparation of pork ham sausages containing defatted peanut meal

Pork lean meat (1344 g) and pork back fat (336 g) were mixed and minced by a mechanical mincer, which was then mixed well with water (600 g), salt (30 g), composite phosphate (15 g), carrageenan (12 g), monosodium glutamate (12 g) and spice (12 g). The amount of raw materials and additives were controlled according to Chinese National Standards (GB/T 20712-2006 2006). The mixture was stirred, fully rolled for 120 min and then allowed to stand for 24 h at 4 °C. Then, appropriate amounts of WPF, CSPF or SPF were added to appropriate amounts of the mixture so that their final proportion in the ham sausage was 0%, 4%, 6%, 8% and 10%. After stirring for 10 min, the moisture was adjusted to 69%. Every 30 g of the mixture with all well mixed ingredients was poured into a casing which was then tighten. After cooking for 60 min at 80 °C, the gel with casing was cooled by cold-flowing water for 10 min and stored at 7 °C for overnight before measuring TPA profile and carrying out sensory test.

Texture profile measurement

The pork ham sausage samples were cut into 10 mm × 10 mm × 10 mm cubes for texture profile analysis (TPA). The parameters of the texture profile analyzer (CT-3, US) were as follows: test speed 1.00 mm/s, compression ratio of 70%, trigger point 5 g, and TA10 probe. All TPA tests were carried out at room temperature and each sample was assayed five times. The results were expressed as the average of the measurements.

Sensory test

The sensory test was carried out according to Hayes et al. (2014) with modifications. Fifty panelists, who sense normal and have good experience, participated in the sensory evaluation. They were trained and kept in a clean, comfortable, air-pure and isolated evaluation room (25 °C) for evaluating. The pork ham sausages were cut into same thickness (20 mm in diameter and 10 mm thick; about 8 g) and numbered with a code. About 50 mL of water was prepared for gargling at each evaluation interval. Five sensory attributes, namely, color, aroma, consistency, saltiness and springiness were evaluated. A 9-point hedonic scale was used to determine these five attributes: 1, dislike extremely; 2, dislike very much; 3, dislike moderately; 4, dislike slightly; 5, neither like nor dislike; 6, like slightly; 7, like moderately; 8, like very much; 9, like extremely.

For comparison, TPA profile analysis and sensory test of pork ham sausages commercially made by Shuanhui Enterprise Group Ltd., Henan, PRC and sold in the market were also carried out. The operating procedures were the same as that described in the two paragraphs just mentioned above.

Evaluation of OBC of defatted peanut meal

OBC was determined according to Zhang et al. (2013) with modifications. WPF, CSPF or SPF (1.0000 g) was put into a 20 mL pre-dried and weighed centrifuge tube and then 10 mL of soybean oil was added. The mixture in the centrifuge tube was shaken with a vortex mixer for 30 s. After kept for 30 min in a water bath at 40 °C, the mixture was centrifuged for 30 min at 4000 r/min. After upper soybean oil in the centrifuge tube was removed, the residue and centrifuge tube were weighed. OBC (i.e. g oil held by 1 g WPF, CSPF or SPF) was calculated as the following:

$$\text{OBC}=\left({\text{W}}_2-{\text{W}}_1\right)/{\text{W}}_0$$

where W₀ was the mass of WPF, CSPF or SPF (g); W₁ was the total mass (g) of the dried centrifuge tube plus the dried WPF, CSPF or SPF; W₂ was the total mass (g) of the dried centrifuge tube plus the dried WPF, CSPF or SPF and oil held after centrifugation.

Evaluation of FC and FS of defatted peanut meal

The FC and FS were determined according to Barac et al. (2010) with modifications. WPF, CSPF or SPF (0.3000 g) was put into a 100 mL beaker and 30 mL water was added. The mixture was adjusted to pH 7.4, stirred to mix well and then homogenized by using a mini whipper for 1 min at 8000 r/min. The height of the bubble foamed was recorded at 0 min, 10 min, 30 min, and 60 min after homogenization. The FC was calculated as follows:

$$\text{FC}\left(\%\right)=\text{k}\times \left(\text{H}-{\text{H}}_0\right)/{\text{V}}_0\times 100$$

where k is 20 mL/cm; H is the total height after foaming; H₀ is the height of original solution; V₀ is volume of original sample solution before foaming. The total height after foaming recorded at 0 min, 10 min, 30 min, and 60 min was used to indicate FS while the FC (%) after each standing time was calculated according to the formula just described above.

Evaluation of EAI and ESI of defatted peanut meal

The emulsifying activity index (EAI; m²/g⁾ and stability index (ESI; min) were determined according to the method of Liu et al. (2010).

Evaluation of NDI of defatted peanut meal

The NDI was determined according to Wu and Long (2011) with modifications. WPF, CSPF or SPF (0.4000 g) was put into a 50 mL centrifuge tube and 40 mL water was added. After adjusted to pH 2.0, 3.0, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 by using 1.0 N NaOH or HCl solution, the mixture was stirred at room temperature for 60 min with a magnetic stirrer (Yi Chen Instrument Manufacturing Co., Ltd.). The mixture was centrifuged at 10,000×g for 20 min. The nitrogen content in the supernatant or in the original WPF, CSPF or SPF was measured by Kjeldahl method. The NDI (%) was calculated as follows:

$$\text{NDI}(\%)=\frac{\text{Amount}(\text{g})\text{of}\text{nitrogen}\text{in}\text{supernatant}}{\text{Amount}\text{of}\text{nitrogen}(\text{g})\text{in}\text{the}\text{original}\text{WPF,}\text{CSPF}\text{or}\text{SPF}}\times 100\%$$

Proximate analysis of defatted peanut meal

Protein, crude fat, moisture and ash contents were measured according to Chinese National Standards GB 5009.5-2016, GB 5009.6-2016, GB5009.3-2016 and GB5009.4-2016, respectively. All experiments were carried out in triplicate and the mean and standard errors were calculated and reported for each decision.

Color analysis

The color of WPF, CSPF or SPF was analyzed using Colorimeter made by Hanter Lab Ultra Scan Pro. USA (Khattab et al. 2017).

Data analysis

Data were analyzed by using SPSS 19.0 for testing significant differences and variance analysis (p < 0.05).

Results and discussion

Protein, fat and ash contents of defatted peanut meal

Although the protein content (49.66 ± 0.47%) of WPF was significantly lower than that (52.58 ± 0.39% or 52.53 ± 0.38) of CSPF or SPF (p < 0.05), it was concentrated from 26.47% in peanut kernel to about 50%. The protein content of the defatted peanut meals is so high that they can be directly sold in the market as a protein concentrate. WPF, CSPF and SP should be good protein supplementing materials applicable in the production of many foods.

Although the crude fat content in WPF was significantly higher than that (2.06 ± 0.02% or 2.08 ± 0.01%) in CSPF or SPF (p < 0.05), it was as low as 5.77 ± 0.04%. The level of fat should not limit the application of WPF since many popular foods normally consist of appropriate amounts of fats. Although the crude fat content of CSPF and SPF was quite low, their production needs a long time of solvent extraction to reduce the residual oil content so that their production cost is quite high.

The ash content (5.42 ± 0.16%, 5.43 ± 0.07% and 5.53 ± 0.15%, respectively) of WPF, CSPF and SPF was not significantly different (p < 0.05). The level of ash should vary depending upon their content in the soil that the raw peanuts are grown.

Good color of defatted peanut meal

The results (L*, a*, b* and ∆E) of color analysis were as follows: 94.09 ± 0.13, − 0.31 ± − 0.02, 10.05 ± 0.08 and 11.67 ± 0.06 for SPF, 90.42 ± 0.21, 0.20 ± 0.01, 13.55 ± 0.14 and 16.59 ± 0.15 for CSPF, and 86.70 ± 0.08, 0.12 ± 0.01, 14.50 ± 0.10 and 19.67 ± 0.07 for WPF, respectively. All values are the mean ± standard deviation of three determinations. The color difference between the standard model and the sample: ΔL, Δa and Δb, where “ΔL+” means more white, “ΔL −” means more black, “Δa+” indicates more red, “Δa−” indicates more green, “Δb+” means more yellow, and “Δb−” means more blue. Therefore, WPF, CSPF and SPF had white color. The color of WPF was the deepest while that of SPF was the lightest. The reason for this may be that solvent extracted some color compounds from peanut kernel slurry into oils. This is why the oil extracted by water does not need a de-coloration process and that extracted by solvent does.

Although the color of WPF is slightly deeper than that of CSPF or SPF, this does not limit the application of WPF in the production of many foods such as meat ham, all kinds of sausages and biscuits because its color is reasonably good.

Good GP of defatted peanut meal

The experimental results indicated that pure WPF, SPF and CSPF were able to form good quality gels and they were able to improve the gelling characteristics (measured by TPA profile) of pork ham sausage as compared with starches. Also the sensory score of the pork ham sausages containing one of them was significantly increased as compared with the commercial starch-containing pork ham sausage currently sold in the market. Furthermore, the GP of defatted peanut meal produced by aqueous extraction was found to be significantly better when it was applied in pork ham sausage as compared with that produced by solvent extraction or cold screw pressing followed by solvent extraction.

GP of WPF as compared with that of SPF and CSPF

The WPF, SPF or CSPF was able to form a gel containing 62.55% water. All the gels formed did not release water after cooling.

The hardness, springiness and cohesiveness of the WPF, SPF, or CSPF gel are shown in Table 1. Although the hardness and springiness of the WPF gel were significantly lower than that of SPF or CSPF gel, they were significantly higher than that of the SPF or CSPF gel with the addition of salt. The amount of salt added was the same as that in WPF. Therefore, the salt from the extracting aqueous solution caused the decrease in the hardness and springiness of the WPF gel. Similarly, Wang (2018) reported that an increase in concentration of sodium chloride can significantly reduce the strength of gels based on peanut protein powder and the same conclusion was given to the gel prepared by using other raw materials (Sun and Arntfield 2011). It should be noted that it is necessary to add proper amount of salts when many gelling foods such as pork ham sausages are produced though over addition of them may be harmful to health. The cohesiveness of the WPF, SPF, or CSPF gel was not significantly different. The addition of salt decreased the cohesiveness of the SPF or CSPF gel. Addition of salt may compete for water which is essential for forming hydrogen bonds which may play a very important role in the formation of a gel network so that the hardness and cohesiveness of the gel are adversely affected.

Table 1.

Comparison of TPA characteristics of different gels prepared by using WPF, CSPF and SPF with and without the addition of salt

Gel species	Hardness (g)	Springiness (%)	Cohesiveness
WPF without added salt	Not detected	Not detected	Not detected
WPF with added salt from extraction reagent	411 ± 11a	63 ± 5b	0.30 ± 0.03abc
CSPF without added salt	618 ± 10b	70 ± 3a	0.31 ± 0.03abc
CSPF with added salt	394 ± 11c	58 ± 6c	0.28 ± 0.02bc
SPF without added salt	684 ± 37d	67 ± 2ab	0.30 ± 0.02abc
SPF with added salt	394 ± 15c	53 ± 3c	0.28 ± 0.01c

The water content of the gel was 62.5% and the cooking time was 45 min

All values are the mean ± standard deviation of five determinations

The mean values in the different superscript letters (a, b, c, d, e) in the same row were significantly different (p < 0.05)

The WPF was deffated by salt solution, and the WPF contained 1.35% salt could be calculated. For a comparison, an appropriate amount of salt was added to CSPF or SPF to give its salt content equal to that of WPF. The TPA characteristics of the gel prepared by using CSPF or SPF with and without salt added as well as that by using WPF were analyzed. The hardness and springiness of the gel prepared by CSPF (added salt) and SPF (added salt) were significantly lower than CSPF and SPF without the addition of salt as well as that prepared by using WPF. Therefore, the texture characteristic of the gel is largely affected by the presence of salt

These experimental results should indicate that the aqueous extraction of oil is able to produce defatted peanut meal having similar or even better GP, as compared to solvent extraction or cold pressing followed by solvent extraction.

Sensory test of WPF, SPF, or CSPF gel

The sensory test of WPF, SPF, or CSPF gel containing 62.55% water was also undertaken. The sensory test indicated that the texture of the WPF gel was highly acceptable by panelists. The hedonic score of the WPF, SPF, or CSPF gel was quite identical.

This test further approves that the aqueous extraction of oil is able to produce WPE with good GP. It is therefore concluded that pure WPF produced by aqueous extraction of peanut oil has the potential of being processed into ham-like sausages (peanut sausages) commercially or such gelling meat products as pork ham sausage or meat.

Application of defatted peanut meal in production of pork ham sausage

According to Chinese National Standard, < 10% starches are allowed to be included in animal meat ham sausages. Therefore, large quantities of meat ham sausages containing 10%, 8% and 6% starches are currently sold in the market in China. If the starches can be replaced by WPF without any negative effect on the sensory quality of ham sausages, this should be of great benefit to consumers since protein content can be largely increased. In this study, the hardness and springiness of the pork ham sausage containing 4%, 6%, 8% or 10% of WPF, SPF or CSPF were first compared. Then, the hardness, springiness and sensory score of the pork ham sausage containing 6%, 8% or 10% of WPF were compared with that of the pork ham sausage containing 6%, 8% or 10% starch commercially produced and currently sold in the market.

The results of hardness of the pork ham sausage with the addition of 4%, 6%, 8% and 10% of WPF, SPF or CSPF are compared in Fig. 1. At the level of 4% and 6%, the addition of WPF produced the pork ham sausage having higher or identical hardness, as compared to the addition of CSPF or SPF. At the level of 8%, although the addition of CSPF produced the pork ham sausage with a slightly higher hardness, as compared to the addition of WPF or SPF, the difference was not significant. At the level of 10%, the addition of WPF produced the pork ham sausage having identical or higher hardness, as compared to the addition of CSPF or SPF. As compared with the pork ham sausage containing starch commercially sold in the market, that containing WPF had significantly lower hardness at 6% or 8% of addition, but significantly higher hardness at 10% of addition (Table 2).

Fig. 1.

Comparison of hardness (mean ± SD; n = 3) of the pork ham sausage prepared by adding different amounts of WPF, CSPF or SPF

Table 2.

Comparison of hardness, springiness and hedonic score of the pork ham sausage with the addition of WPF and the pork ham sausage with the addition of starch which is commercially sold

Samples	Hardness (g)	Springiness (%)	Hedonic score
6%*
WPF**	1348 ± 25a	80 ± 2a	8.84 ± 0.12a
Starch***	1561 ± 38b	82 ± 3a	8.85 ± 0.11a
8%*
WPF**	1169 ± 35c	79 ± 2a	8.47 ± 0.13b
Starch***	1336 ± 31d	80 ± 2a	8.45 ± 0.10b
10%*
WPF**	954 ± 25e	78 ± 2b	7.20 ± 0.14c
Starch***	918 ± 10f	67 ± 1c	7.11 ± 0.13c

The mean values in the different superscript letters (a–f) in the same row were significantly different (p < 0.05)

*Content in the pork ham sausage

**Pork ham sausage with the addition of WPF self-made in laboratory

***Pork ham sausage with the addition of starch commercially made by Shuanhui Enterprise Group Ltd., Henan, PRC and sold in the market

The results of springiness of the pork ham sausage with the addition of 4%, 6%, 8% and 10% of WPF, SPF or CSPF are compared in Fig. 2. At the level of 4% or 6%, the addition of WPF produced the pork ham sausage having a similar springiness, as compared to the addition of CSPF or SPF. At the level of 8% and 10%, the addition of WPF produced the pork ham sausage having identical or higher springiness, as compared to the addition of CSPF or SPF. As compared with the pork ham sausage containing starch commercially sold in the market, that containing WPF had no significant springiness at 6% or 8% or 10% of addition (Table 2).

Fig. 2.

Comparison of springiness (mean ± SD; n = 3) of the pork ham sausage prepared by adding different amounts of WPF, SPF or CSPF

The results of sensory test indicated that all panelists gave the hedonic scores (HS) of texture of the pork ham sausage with the addition of 4%, 6% and 8% of WPF, SPF or CSPF higher than 8 (like very much). However, the average HS of texture of the pork ham sausage with the addition of 10% WPF, SPF or CSPF was just over 7 (like moderately). At this level of WPF, SPF or CSPF added, the brittleness of the pork ham sausage was not very good and it was easy to be deformed in the mouth perhaps because of poor cohesiveness. The HS of color, aroma, consistency, saltiness and springiness of the pork ham sausage with the addition of 4%, 6% and 8% of WPF, SPF or CSPF was compared in Table 3. The HS of these five sensory attributes was higher than 7 (like moderately). At the level of 6%, the pork ham sausage with the addition of WPF had a significant higher HS of aroma, as compared with the addition of CSPF or SPF. At other levels, the pork ham sausage with the addition of WPF had no significant different HS of aroma, as compared with the addition of CSPF or SPF.

Table 3.

Comparison of hedonic score of the pork ham sausage with the addition of different amounts of WPF, CSPF or SPF

Treatments	Sensory attributes					Moisture content (%)
	Color	Consistency	Aroma	Saltiness	Springiness
4%
WFHS*	7.90 ± 0.32a	7.97 ± 0.18a	7.36 ± 0.08a	7.25 ± 0.35	7.89 ± 0.17a	67.83 ± 0.16b
CFHS*	7.85 ± 0.75a	7.95 ± 0.37ab	7.30 ± 0.08ab	7.21 ± 0.26	7.87 ± 0.19a	68.21 ± 0.10a
SFHS*	7.91 ± 0.30a	7.95 ± 0.30ab	7.32 ± 0.09ab	7.20 ± 0.19	7.88 ± 0.18a	67.93 ± 0.11ab
6%
WFHS*	7.81 ± 0.23ab	7.90 ± 0.16ab	7.33 ± 0.05ab	7.27 ± 0.22	7.81 ± 0.23a	66.97 ± 0.13d
CFHS*	7.78 ± 0.25ab	7.89 ± 0.19ab	7.21 ± 0.14bcd	7.25 ± 0.24	7.78 ± 0.25a	67.38 ± 0.09c
SFHS*	7.76 ± 0.24ab	7.88 ± 0.32ab	7.25 ± 0.16bcd	7.20 ± 0.17	7.80 ± 0.26a	66.96 ± 0.08d
8%
WFHS*	7.58 ± 0.20ab	7.87 ± 0.25ab	7.15 ± 0.13cd	7.22 ± 0.33	7.33 ± 0.28b	66.52 ± 0.03e
CFHS*	7.57 ± 0.48ab	7.86 ± 0.40ab	7.13 ± 0.16cd	7.20 ± 0.40	7.36 ± 0.25b	66.90 ± 0.05d
SFHS*	7.60 ± 0.35ab	7.88 ± 0.46ab	7.10 ± 0.10cd	7.24 ± 0.37	7.33 ± 0.45b	66.23 ± 0.46e
10%
WFHS*	7.40 ± 0.43b	7.68 ± 0.31ab	7.03 ± 0.10d	7.23 ± 0.49	7.18 ± 0.26b	65.77 ± 0.07f
CFHS*	7.43 ± 0.24b	7.60 ± 0.46b	7.01 ± 0.18d	7.26 ± 0.24	7.16 ± 0.22b	65.79 ± 0.06f
SFHS*	7.45 ± 0.44b	7.65 ± 0.41ab	7.05 ± 0.13cd	7.16 ± 0.25	7.15 ± 0.34b	65.54 ± 0.14f

The mean values in the different superscript letters (a–e) in the same row were significantly different (p < 0.05)

*WFHS, pork ham sausage with the addition of WPF; CFHS, pork ham sausage with the addition of CSPF; SFHS, pork ham sausage with the addition of SPF; SEM, standard error of mean

Table 2 shows that the HS of pork ham sausage with the addition of WPF appears to compare identically with the pork ham sausage containing starch commercially made by Shuanhui Enterprise Group Ltd., Henan, PRC and currently sold in the market at the level of 6% and 8%, which was higher than 8. At the level of 10% addition, the average HS of the pork ham sausage containing WPF or starch was just over 7 (like moderately), but that containing WPF scored slightly higher as compared to that containing starch. At this level of starch added, the brittleness of the pork ham sausage commercially sold in the market was not very good and it was easy to be deformed in the mouth perhaps because of poor cohesiveness.

It is concluded that WPF can be a good material applicable in the production of pork ham sausage or meat and other kinds of meat ham sausage or meat and pure peanut sausage with high protein content.

Good NDI of defatted peanut meal

The NDI of WPF, CSPF and SPF is shown in Fig. 3. This figure obviously indicates that WPF had significantly higher NDI at pH value above 8, as compared with CSPF and SPF. At pH 10.0, the NDI of WPF, CSPF and SPF were 85.09%, 70.04% and 80.37%, respectively. Therefore, the aqueous extraction of oils produced defatted peanut meal having significantly higher NDI, as compared with solvent extraction and cold pressing followed by solvent extraction.

Fig. 3.

Comparison of NDI of WPF, SPF or CSPF at different pH values

High NDI is essential for the extraction and further purification of peanut proteins. It is therefore concluded that WPF can be a good material of producing peanut protein isolate (containing > 90% proteins). This may also indicate that the defatted peanut meal produced by aqueous extraction of oils can be a good material for making high quality peanut protein drink.

OBC, EAI and ESI of defatted peanut meal

The OBC (g/g), EAI (m²/g) and ESI (min) were as follows: 1.07 ± 0.04, 2.61 ± 0.05 and 70.83 ± 1.44 for WPF, 1.11 ± 0.05, 2.47 ± 0.05 and 75.50 ± 7.09 for CSPF, and 1.01 ± 0.03, 2.76 ± 0.09 and 90.00 ± 3.00 for SPF, respectively. It was found that WPF had a slightly lower OBC, as compared with CSPF, but the difference was not statistically significant. WPF had a significantly higher OBC, as compared with SPF. The EAI of WPF, CSPF and SPF was not significantly different. Furthermore, the absolute value of EAI of all these three defatted peanut meal was not high. This result should indicate that the application of WPF, CSPF or SPF in foods as an emulsifier is quite limited.

However, heat treatment or denaturation of proteins may improve their OBC, EAI or ESI. This may mean that the application of further properly treated WPE to the production of foods as an oil binding reagent or emulsifier should not be ruled out.

FC and FS of defatted peanut meal

The FC (23.0 at 0 min, 11.5 at 10 min, 9.8 at 30 min and 4.9 at 60 min) of WPF was slightly lower than that (23.5 at 0 min, 15.1 at 10 min, 9.9 at 30 min and 8.1 at 60 min) of CSPF, but the difference was not statistically significant. The FC of WPF was significantly higher than that (15.5 at 0 min, 4.9 at 10 min, 2.5 at 30 min and 1.3 at 60 min) of SPF. The values of measured FC of WPF, CSPF and SPF at 0 min were 15.0–23.7%, which was quite low as compared with soy protein and egg protein. Also, all the foaming formed was not stable. This means that the application of WPF, CSPF or SPF in foods as a foaming reagent is quite limited. However, this should facilitate their application in some food products such as meat ham or other kinds of sausages.

It should be noted that hydrolyzation of proteins by alkali or enzyme can greatly improve their foaming capacity or stability. Therefore, the application of further properly treated WPE to the production of foods as a foaming reagent should not be ruled out.

Reasons of excluding the investigation of defatted peanut meal produced by other methods

Because the defatted peanut meal produced by high temperature pressing has grey dark color with proteins highly denatured and can only be used as feed stuffs and fertilizers, it is not investigated in this study though this method currently appears to be the major method of producing peanut oils on an industrial scale (List 2016). Solvent extraction of peanut is used in only one processing plant out of 4 in the USA (List 2016).

The traditional aqueous extraction method of oils by using large quantities of water (including the use of enzyme; liquid:solid ratio being > 2:1) has not yet been successfully applied in the commercial processing of peanut seeds because of its disadvantages (Li et al. 2016; Ravber et al. 2015; Zhang et al. 2011; Jiang et al. 2010), Therefore, the functionality of defatted peanut meal produced by this method was also not investigated in this study.

Although solvent extraction has the disadvantages (Sánchez et al. 2018; Liu et al. 2017; Yusoff et al. 2015; de Moura et al. 2009), it is well known that it usually produces defatted peanut meal with quite good functionality. Therefore, the defatted peanut meal produced by it was used as a reference in this study for comparison.

Analysis of dietary safety of defatted peanut meal

Defatted peanut meal produced by aqueous method should be safe for consumption because peanut seeds are edible and no harmful chemical is added during processing. Although an allergenity to some people (mainly baby and children) was stated, the morbidity was reported to be only 0.4–2% (Bunyavanich et al. 2014). This should mean that 99% of world population can consume peanut proteins safely. Particularly, the treatment of peanut proteins by pulse UV radiation significantly reduced allergenity (Yang et al. 2012). Furthermore, children who experienced peanut allergy became to be not susceptible to this disease after they take an appropriate amount of peanut powder for a certain time (Nowak-Wegrzyn and Chatchatee 2017). Also, the allergenity of peanut proteins can be significantly reduced by thermal processing (Cabanillas et al. 2015). Ham sausage is a kind of cooked (a thermal processing) ready-to-eat foods. In fact, many foods have allergenity, for example the morbidity of cow’s milk allergy in children is 1.9–4.9% (Fiocchi et al. 2010). The morbidity of shellfish allergy can be as high as 2% of the general world population (Lopata et al. 2016) while that of fish allergy can be as high as 0.2–3% (Kalic et al. 2019). The morbidity of wheat allergy can be as high as 0.4–4% both in adults and children (Czaja-Bulsa and Bulsa 2017). Such foods as eggs [affecting 0.5–2.5% of young children (Caubet and Wang 2011)] and soybeans (Yoshimitsu et al. 2019) also have allergenity. These are currently produced on a large scale and consumed in a large quantity daily in the word. Therefore, it may not be wise to discard peanut or defatted peanut meal and its protein product as a valuable food material. Furthermore, the presence of peanut products in their product label can be indicated clearly for eliminating the risk of causing any health problem because of allergy.

Another health risk might be caused by aflatoxin. However, this kind of compound is the contamination of growth of microorganisms. If raw peanuts are properly handled, the avoidance of health problem caused by aflatoxins is possible. Furthermore, further proper treatment of WPE with the presence of nucleophiles can reduce aflatoxins (Saalia and Phillips 2011).

Analysis of nutritional quality of defatted peanut meal

Proteins in all the defatted meals unlikely underwent adverse changes since they were obtained under mild conditions. The nutritional quality of proteins in WPE since they were reported to have digestibility comparable with that of animal proteins and their amino acid pattern superior to that of whole wheat proteins (Arya et al. 2016).

Water soluble vitamins such as niacin, folate, thiamin, riboflavin, pantothenic acid and pyridoxine were also reported to be rich in peanuts (Arya et al. 2016). These vitamins should not be damaged during the aqueous extraction of oil and therefore they should be present in WPE.

Polyphenols (including flavonoids) were also reported to be rich in peanuts (Watson et al. 2018). These functional compounds should not be eliminated during the aqueous extraction of oil and therefore they should exist in WPE.

Conclusion

It is concluded that the good GP, NDI, high nutritional value and color of the defatted peanut meal produced by aqueous extraction, solvent extraction or cold pressing followed by solvent extraction of oils are the critical functional properties for its high value of application to the food industry. WPF, CSPF and SPF had good GP so that they can be applied in the production of pork ham sausage or other meat ham sausages, meat ham as well as pure peanut gelling foods. The color of WPF, CSPF and SPF was white so that they can be used in many foods without adverse effect on their color. Good NDI of WPF, CSPF and SPF indicates that it is a good material of producing further purified peanut protein products such as isolate or making protein drink. They can be valuable supplements in many foods for enhancing nutritional quality because they have high protein content.

The defatted peanut meal was found to be not a material having other good functional properties including OBC, EAI, ESI, FC and FS. It is therefore not suggested to be applied to the foods need good OBC, EAI, ESI, FC and FS.