Impact of Different Processing Methods on Physico-chemical Characteristics and Antioxidant Activity of Peanut Milk

Peanut (Arachis hypogaea L.) is a leguminous crop which belongs to fabaceae family and it was considered as the most important oil seed crop among the world (Campos-Mondragon et al., 2009). Peanut does not contain any lactose, so it can be more suitable for lactose intolerance people and cow’s milk allergy people. It also contains high protein, vitamins,minerals, essential fatty acids like oleic and linoleic acid, phenolic compounds, phytosterols, because of their high nutritional value it can be suitable for all age groups (Kouane et al., 2005). 
       
The price of the peanut based products are comparatively low and can be afforadable for low income people also. Protein content of the peanut helps to treat the protein malnutritious children (Kouane et al., 2005). Peanut based milk beverages has been renowned as a most important substitutes in culture where the cow’s milk is too expensive and indigestible (Isanga and Zhang, 2009). Production of peanut in India was nearly 45% of oil seed area and 55% of oilseed production in the country and India was the third largest producer of peanut production of over 5-6 million tons (USDA Foreign Agricultural Service, 2008-2009) among the world peanut production (www. commodityonline.com).
       
Antioxidant is a substance it can ability to trap free radicals. Phenolic acids, polyphenols and flavonoids are some of the antioxidant compounds and facilitate the scavenging of peroxide, hydroperoxide or lipid peroxyl and other free radicals it will inhibit the oxidative mechanisms and helps to prevent degenerative diseases (Wu et al., 2011).
    
In this study peanut milk was prepared from fresh, blanching, roasting, soaking and germination method from local and CO 6 peanut variety. Effects of processing methods on physico chemical properties and antioxidant activity were studied to determine the best method for retention of higher antioxidant activity with good  physico chemical properties  in local and CO 6 variety peanut milk.

The experiments were conducted in the Department of Food Science and Nutrition, Community Science College and Research Institute, Madurai during (2018-2021).
 
Local and TNAU CO 6 variety peanuts were selected for preparation of peanut milk and selected based on complete maturity, raw, free from dust, infectant and mould. Local variety peanut was purchased from a simmakal (local market) and TNAU CO 6 peanut variety was purchased from Department of Plant Breeding and Genetics, Tamil Nadu Agricultural University, Coimbatore.
 
Peanut milk processing techniques
 
Peanut milk sample was prepared by five different processing methods like fresh (control), blanching, soaking, roasting and germination methods.
       
In each method local variety (100 g) and CO 6 variety (100 g) was weighed and washed. In blanching (121°C 15 psi for 2 mins), soaking for 3 hrs, roasting( roasting for 5 mins followed by soaking 3 hrs) and germination (8 hrs) pretreatments were done. Then grinded (1:3 ratio of kernel to water), filtered by using muslin cloth, slurry was separated. Then the peanut milk was homogenized for 5 mins and double pasteurized (85°C for 15 mins) (Fig 1).
 

 
Analytical methods
 
a) Physico chemical and antioxidant activity
 
The pH of the peanut milk sample was determined by the method described by Buck et al., (2002) by using pH meter. Peanut milk samples total soluable solids were determined by using a hand refractometer (0 to 32°Brix). The viscosity of the peanut milk sample was analyzed at 25±0.1°C using spindle  No. 61, 1-1 coaxial cylinder Brooke field viscometer. Brook field viscometer instrument was calibrated and set the torque was zero and the viscosity (centipose) was measured at a minimum of 100 rpm and the viscosity range was noted down. Peanut milk samples acidity content was determined  by the method described by AOAC (1990).
       
Peanut milk samples from (local and CO 6 variety) containing radical scavenging activity was determined by (DPPH) 2, 2-diphenyl-1-picrylhydrazyl- radical scavenging assay method described by Lim et al., (2007). The data obtained from various characters under study were analyzed by the method of analysis of variance as described by (Gomez and Gomez, 1984).

Physico chemical analysis
 
Physico chemical analysis of the local and CO 6 variety prepared peanut milk from fresh, roasting, blanching, soaking and germination methods were carried out and like pH, TSS, acidity, viscosity and colour values were analyzed.
       
Peanut milk was  prepared  from selected (local and CO 6 peanut) varieties showed significant differences (p<0.05) in different processing methods like fresh, blanching, soaking, roasting and germination methods (Table 1). The pH of peanut milk from local peanut variety was higher in soaking method (6.9) and lower in blanching method (6.3). Yuan et al., (2008) reported that the pH of the soy milk was ranged from 6-6.6 and the pH values of the present study also in close agreement with the report. pH of the peanut milk substitute was 6.82 as per the report of Aydar et al., (2020). The pH value of soy milk was 7.39 and acidity was 0.09 was determined by Kundu et al., (2018).
 

       
TSS content of fresh peanut milk was higher (13°Brix) and lower (11°Brix) in blanching and soaking method. During blanching and soaking method the solid content in the peanut was leached out during processing. TSS content of the peanut milk can increased by addition of sweetner (palm sugar). Viscosity of the peanut milk was higher (4.98 cp/100 ml) in fresh peanut milk (control) and lower in germination (4.89cp/100 ml) and soaking method (4.92 cp/100 ml).
       
Viscosity content of the peanut milk from both the varieties was reduced during soaking and germination process because of absorption of water and indirectly decreased the viscosity content of the peanut milk. Total soluble solid content and fat content of the peanut milk directly influences the viscosity of the peanut milk. Adesola et al., (2013) reported that groundnut milk prepared from fresh, roasted (170°C, 25 min) and steeped (water, 20 min) groundnuts and reported the quality characteristics such as pH 6.82 to 6.85, acidity 0.10 to 0.14%, viscosity 7.33 and 7.56 cp in peanut milk from different processing.
       
Acidity content of the peanut milk from both the varieties was higher in roasting method (0.08%) and lower in blanching method (0.04%). The pH of the peanut milk from blanching method was high when compared to other treatments. Acidity of the peanut milk is indirectly influenced by the pH value of the peanut milk. pH value of the peanut milk can be influenced by nutritional composition and biochemical changes and also pH value influences the flavor perception of the peanut milk. Hassan et al., (2010) determined that peanut milk total acidity yoghurt content  was 0.852 g/100 ml was significantly lower than cows milk yoghurt 1.173 g/100 ml. Specific gravity of soy milk was ranged from  0.95 - 2.35 as per the report of Krishna et al., (2003).
       
Colour value of fresh peanut milk (control) was higher (L*- 91.02, a*- 0.97, b- 12.47) when compared to other processing methods like blanching, roasting, germination. Roasting method had influences and reduce the colour value. Walkowiak-Tomczak et al., (2016) reported that L*a*b* values were mostly depends on pH values of the product. Colour value of the peanut milk had higher lightness value because of the white colour of the peanut milk.
       
Peanut milk from Co-6 variety had a pH value of 6.9 in soaking method and lower (5.8) in blanching method. TSS content was higher (13°Brix) in control (fresh peanut) and lower (11°Brix) in blanching and soaking method. Viscosity of the peanut milk was higher 5.00 (cp/100 ml) in fresh peanut milk (control) and lower (4.87 and 4.92 cp/100 ml) in blanching and soaking method. Roasting process reduces the acidity content, total soluable solids of the peanut milk, offlavour and bitterness taste of the peanut milk, antinutritional factors and also improves the nutritional value, stability of the milk.
 
Acidity content of the peanut milk from both the varieties was higher (0.06%) in roasting method and lower in blanching and germination (0.04%) method. Acidity content of the peanut milk will influences the pH value of the peanut milk. Colour value of fresh peanut milk (control) was higher L* -91.02, a* -0.97, b -12.47) when compared with other processing methods like blanching, roasting, soaking and germination. Yadachi et al., (2012) quoted the physico chemical properties of peanut milk like pH, TSS, acidity, viscosity and its value was 6.4, 8.6°Brix, 0.24 (g%), 5.62 cp/100 ml respectively. Albuquerque et al., (2015) reported the physico chemical properties of peanut milk such as pH (6.70), acidity (0.30%), moisture (90%) respectively.
 
Antioxidant activity
 
Table 2 shows the results of antioxidant activity of peanut milk from fresh, blanching, soaking, roasting and germination method in both the peanut varieties.
 

       
Table 2 shows the results of antioxidant activity in peanut milk prepared from different processing methods in local and CO 6 variety. Antioxidant activity of peanut milk of CO 6 variety from fresh, blanching, soaking, roasting and germination method was 61, 62, 62, 64 and 63 (% RSA) respectively. Antioxidant activity was significantly higher in roasting method (60% RSA) and followed by germination method (59% RSA), soaking and blanching method (58% RSA) in peanut milk from local variety.
       
The antioxidant activity of Local and CO 6 variety peanut milk antioxidant activity was siginificantly higher in Roasting method (60 and 64% RSA respectively) and followed by germination method (59 and 63% RSA respectively), soaking (58 and 63% RSA respectively) when compared to control peanut milk (57 and 61% RSA respectively). During processing the antioxidant activity of the peanut milk was increased due to enzyme activity and among these five different processing methods roasting method had highest antioxidant activity, CO 6 variety peanut milk had highest antioxidant activity when compared to local variety. Duarte et al., (2005) reported that during roasting process the antioxidant activity was increased because of maillard reaction formed roasting process and it produces antioxidant compounds and polyphenolic compounds were reduced. Karunasiri et al., (2020) reported that radical scavenging activity by DPPH method in liquid coconut milk, powder coconut milk, Domestic coconut milk and found that antioxidant activity was siginificantly higher in Domestic coconut milk compared to liquid and powder coconut milk and also determined that reduction of total phenolic compounds in the milk during processing can increases the antioxidant activity of the milk.
       
Acar et al., (2009) found that total phenolic content in the peanut was reduced during roasting process (150°C for 60 mins)  and it can directly increases the antioxidant activity content. Abou-Dobara et al., (2018) reported that peanut milk rayeb possessed higher antioxidant activity than those detected in Rayeb (50% cow’s milk and 50% peanut milk) prepared from cow milk. Nuts and oilseed based milk had higher amount of antioxidants it can reduces CVD, cancer and diabetes Aydar et al., (2020).

Different processing methods such as blanching, soaking, roasting and germination methods have varying impact on the physico chemical, nutritional properties and antioxidant property. Peanut milk from roasting method (roasting 5 mins followed by 3 hrs soaking) had retained higher antioxidant activity from local and CO 6 peanut variety. From the findings of the research work, peanut milk from roasting treatment (roasting 5 mins followed by soaking 3 hrs) had more therapeutic value when compared to soaking,blanching, fresh and germination methods in both local and CO 6 variety peanut milk because it significantly does not affect the physico chemical properties and antioxidant activity and helps for scavenging of  free radicals and prevent oxidization.  Among the  five processing methods peanut milk from both the varieties  roasting method had secured higher sensory score (overall acceptability : 9). Hence, roasting process  is a suitable for  increasing the  taste, colour value and in the  reduction of  offlavour, beany taste and  it indirectly increases the antioxidant activity of the peanut milk.