Infusion of walnut (Juglans regia L.) shell tea: multi response optimization and antioxidant potential
Graphical abstract
Introduction
Walnut (Juglans regia L.) is one of the nuts which contains high amounts of protein, lipids and as well as minerals (Chatrabnous et al., 2018). The shell of a walnut constitutes nearly 50 % of its whole body and it is commonly discarded into environment as a waste which can be utilized for producing value-added products (Hemmati et al., 2018) in the pharmaceutical and cosmetic industries (Stampar et al., 2006). Walnut shells are good sources of bioactive molecules which have the ability of decreasing the risks of cancer, cardiovascular diseases, allergy and molecular oxidation. Lignin is known its strong antioxidant potential and approximately 52.3 % of shell is lignin (Han et al., 2018). Shelled walnut is commonly considered as safe product due to its low-moisture content (Zhang et al., 2018). However, several studies reported that the microorganisms may present on the shell in case the surrounding conditions are suitable for their growth (Blessington et al., 2014; Frelka and Harris, 2015; Frelka et al., 2016).
The most popular drink consumed by people is (black) tea (Camellia sinensis) and Turkey is the fifth largest manufacturer in worldwide (Kraujalytė et al., 2016). Although, the black tea has potential benefits on human health such as being anticarcinogen, antioxidant and preventing atherosclerosis and obesity (He et al., 2018), it contains caffeine. This alkaloid may cause psychological disorders, gastrointestinal diseases and palpitation (Sun et al., 2010). Hence, undesirable health effects of caffeine encourage researchers and producers develop caffeine-free beverages (Kraujalytė et al., 2016). Herbal teas which include no detectable amount of caffeine are made of fruit or other herbs and can be prepared by infusion or decoction (Chandrasekara and Shahidi, 2018). The extraction of bioactives from tea to medium depends on several factors such as temperature and time (Pérez-Burillo et al., 2018). Previous studies about the infusion of byproducts focused on onion peel (Amariei and Mancas, 2015), pecan nut shell (Pinheiro do Prado et al., 2009) and peanut skin (Francisco and Resurreccion, 2012) have been declared as novel teas.
Response Surface Methodology (RSM) enables us to observe interactions between responses to optimize experimental conditions and this procedure is helpful in reducing the number of experimental trials (Nabet et al., 2019). RSM which provides successful results in the multifactorial processes has been used for optimizing the extraction parameters (Ayyildiz et al., 2018; Eruygur et al., 2018) and investigating the effects of brewing conditions of green tea (Liu et al., 2018) and rosehip tea (İlyasoğlu and Arpa, 2017). Furthermore, it was utilized in formulation and characterization of a novel drink made from green tea and hibiscus (Preciado-Saldaňa et al., 2019).
Thus far, there is no data regarding the cumulative effect of temperature, time and solvent to sample ratio on the infusion of tea made from any kind of nut shell in literature. Hence, the purpose of this research was to perform a deep study on optimizing both individual and interactive impacts of operating conditions and solvent to sample ratio on color properties (a* and b*), total phenolic and tannin content of tea from walnut (Juglans regia L.) shell with the aid of RSM. Antioxidant studies were carried out and the phenolic and mineral profiles of tea produced at optimum conditions were also specified.
Section snippets
Materials
The walnuts (2018 crop year, Maraş 18 cv.) were obtained from Çağlayancerit, Kahramanmaraş, Turkey. The walnut shells were removed manually after using a nutcracker and crushed into small particles using a grinder (Premier, PRG 277, Turkey). The ground powder was sieved into five different sizes using a sieve shaker and average diameter of particles (D50) was found 10.20 μm. The moisture content of ground walnut shells was 10.58 % ± 0.24. All chemicals were purchased from Sigma-Aldrich (St.
Fitting the models
As shown in Table 2, the suggested models for all responses were remarkably significant (p < 0.0001) with high R2 and adjusted R2 values, and no significant lack of fit (p > 0.05). This proved the strong relationship between variables and responses (Chen et al., 2018). The R2 values for a*, b*, TPC and TTC were 0.894, 0.966, 0.988 and 0.901, respectively. R2 levels were higher than 0.8 which was an indicator of a good fit (Ganesan et al., 2018).
Effect of infusion variables on color
ANOVA showed significant linear (A (solvent to
Conclusion
Multi response optimization of WST infusion parameters considering color and bioactives was accomplished using RSM. The experimental results indicated that all independent factors; solvent to sample ratio, temperature and time had significantly affected a*, b* values and TPC. TTC was influenced by linear and quadratic terms of solvent to sample ratio and temperature. Optimum infusion conditions for WST were determined as 4:1 of solvent to sample ratio, 77.28 °C of temperature and 30 min of time
Declaration of Competing Interest
The authors declared no conflict of interest for this work.
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