Elsevier

Journal of Food Engineering

Volume 256, September 2019, Pages 28-36
Journal of Food Engineering

High-power ultrasound pretreatment for efficient extraction of fractions enriched in pectins and antioxidants from discarded carrots (Daucus carota L.)

https://doi.org/10.1016/j.jfoodeng.2019.03.007Get rights and content

Highlights

  • Discarded carrots produced gelling pectin-enriched fractions (PEF) with antioxidants.

  • High-power ultrasound (HPUS) power intensity > 10 W/cm2 altered carrot powder matrix.

  • HPUS pre-treatment and 1 h-sodium carbonate extracted the whole powder pectin content.

  • HPUS increased PEF yield and decreased methylation degree and molecular weight of PEF.

  • HPUS-PEF produced higher Newtonian viscosity and calcium-gels of lower elastic G.

Abstract

A useful pectin-enriched fraction (PEF) extracted through high-power-ultrasound (HPUS) pretreatment and sodium carbonate was efficiently obtained from discarded carrots. Formerly, the effect of HPUS-power intensity and time (A: 40 min-2.08 W/cm2; B-E: 5–20 min ≈ 10 W/cm2) on carrot powder (CP) used for PEF-isolation, was investigated. Reducing carbohydrates-, cell-wall neutral sugars- (NS), uronic acids- (UA) of pectins, and protein releases increased with HPUS-energy. CP had antioxidant capacity, containing α- and β-carotene, lutein, α-tocopherol. Only HPUS B-E treatments (≈10 W/cm2) were capable to produce matrix disruption, promoting polymers' release. CP pretreated through selected E-treatment (20 min; ≈10 W/cm2) followed by 0.1M-Na2CO3 (1 h-stirring) extracted the whole pectin content of CP (UA = 14.0%). PEFs were orange, with co-extracted antioxidants. More substituted (1.27 NS/UA ratio) three lower molecular weights' components of HPUS-PEF produced higher Newtonian viscosity before shear-thinning, and calcium-crosslinked gels with lower elastic modulus (G = 12Pa). Sustainable HPUS/Na2CO3 method isolated efficiently an antioxidants-carrying PEF useful as functional food additive.

Introduction

Food wastes constitute a significant problem for economic, environmental and food security reasons. About one-third of all food produced globally for human consumption (approximately 1.3 billion tons per year) is lost or wasted. Moreover, the major contribution to the food waste comes from vegetables (FAO, 2014). Fruit and vegetable wastes are produced in large quantities in markets, and constitute a big problem in municipal landfills (Varzakas et al., 2016).

Carrot (Daucus carota L. var. sativus), an important vegetable of the Umbelliferae family, is cultivated throughout the world. It is usually chopped, and eaten raw, cooked, fried or steamed and cooked in soups, stews, salads, cakes, as well as prepared meals for babies and pets (Dansa et al., 2017). Carrot is extensively consumed and considered one of the healthier vegetables for being a rich source of bioactive compounds, dietary fiber, carotenoids, minerals, and vitamins (Idrovo Encalada et al., 2016). In Argentina, between 200,000 and 240,000 tons of carrot roots are produced annually (Gaviola, 2013). The highest percentage of production is destined to fresh consumption, including ready-to-eat salads prepared in small scale food industries and greengrocers. A small proportion is destined mainly to the dehydration industry. These processes generate significant volumes of residues (Dansa et al., 2017). In total, about 25–35% of carrots are usually discarded after harvesting or industrialization because of irregular sizes and forms, being in part used as animal feed, while still contains useful compounds like antioxidants and pectins (Chantaro et al., 2008). Therefore, the use of discarded carrots can be a good alternative for obtaining antioxidant carrying pectins with useful rheological properties.

Pectins are complex polysaccharides that are found in the middle lamella and cell wall of all higher plants and, hence, they are part of the dietary fiber. The structure of pectin is composed mainly by d-galacturonic acid units (GalA) of the homogalacturonan (HG) chains, partly esterified with methanol, and neutral sugars (NS) such as l-rhamnose, l-arabinose, and d-galactose of the rhamnogalacturonan I (RG-I) core, as well as other 13 different monosaccharides (Hosseini et al., 2016). Pectin composition and structure depends on the origin, developmental stages, and extraction conditions (Petkowicz et al., 2017). They are widely used as a functional ingredient in the food industry as gelling, thickening and stabilizing, and texturizing agent (do Nascimento et al., 2016), as well as in the pharmaceutical industry for their beneficial health properties as soluble dietary fiber, for reducing blood fat, gut processes, and reducing heart disease, among others (Bagherian et al., 2011).

The commercially available pectin is obtained using conventional extraction by means of a mineral acid (hydrochloric, nitric, and sulfuric acid) and it is recovered by precipitation with ethanol (Chan and Choo, 2013). Some innovative pectin extraction techniques such as ultrasound, microwave, and enzymatic extraction, have been developed to improve the yield and the product quality (Marić et al., 2018). Ultrasound-assisted extraction (UAE) uses high-frequency sounds and solvents to enhance the release and diffusion of cell material. The increase of the mass transfer is produced by the acoustic cavitation induced in a liquid medium, which is one of the beneficial effects of this technology (Wang et al., 2015). There are significant advantages of UAE such as the reduced extraction time, low energy consumption, yield increase, and use of lower volumes of solvent when compared to conventional extractive methods (Tao et al., 2014).

By sequential extraction of the polymers from the isolated cell wall material (alcohol insoluble residue), at low temperatures (18-22 °C), it is possible to ascertain the chemical composition and polymer interactions within cell walls (Fry, 1986; Koh and Melton, 2002; Basanta et al., 2013). This sequential scheme begins with isolation for at least 4 h of the loosely (water-soluble) bound pectins, followed by the extraction for 24 h of the CDTA soluble fraction (calcium crosslinked pectins). The third extractive step is performed for 24 h to obtain the 0.1M Na2CO3 soluble fraction, composed by the remaining pectins that are anchored in the cell wall matrix through covalent bonds like diester bridges of ferulate and galacturonate, as reported by Basanta et al. (2013).

The present study proposes HPUS as a pretreatment to facilitate the subsequent extraction of a PEF from misshapen carrots with 0.1M Na2CO3 aqueous solution at room temperature, with the aim of using a sustainable method for the valorization of vegetable residues as food additives or ingredients. As a prelude to PEF-isolation, the effect of HPUS-power intensity and sonication time on water soaked CP used for PEF extraction was investigated. Thus, the releases of reducing carbohydrates, UA, NS, and proteins were determined in the water solvent after performing HPUS treatments at 20 kHz, and either 20% of constant amplitude for 40 min (A-treatment) or 80% of amplitude for 5–20 min (B-E treatments). The best conditions for the PEF extraction were then selected.

Section snippets

Chemicals

Chemicals were of analytical grade. α-carotene, β-carotene, lutein, α-, β- and γ-tocopherols, retinol, bovine serum albumin, and d-galacturonic acid standards were of Sigma-Aldrich, while the rest of the chemicals were of Merck Química (Argentina). Deionized water (Milli-Q™, USA) was used.

Ultrasonic treatments in CP

Carrots (Daucus carota L. var. Nantes) harvested in Valle de Uco (Mendoza province, Argentina), discarded after harvesting or industrialization because of irregular sizes and forms, were used in the present

HPUS energy and power

Table 1 summarizes the amplitude and treatment times, as well as the energy and power values displayed by the HPUS equipment (20 kHz constant frequency) while performing the assays in open systems constituted by the dispersion of CP in a volume of water contained into a glass beaker of determined dimensions. On the other hand, adiabatic experiments were performed only to calculate the acoustic energy (eq. (1)) and power (eq. (2)) actually provided by the ultrasound equipment through the

Conclusions

Smaller, twisted, and misshapen carrots discarded at harvesting were used to produce a sugar-exhausted blanched freeze-dried powder (CP; water activity = 0.300) enriched in cell wall polymers, whose whole pectin content (UA: 14.0% w/w) was successfully extracted at room temperature in a short period (1 h) by stirring in 0.1M Na2CO3, when CP was HPUS pretreated in water (1 g:40 mL) for 20 min net time (E treatment: power intensity of ≈10 W/cm2). This PEF had low DM and was co-extracted with

Acknowledgements

The authors are grateful to the University of Buenos Aires [UBACyT 2014-2017 20020130100553BA], ANPCyT [PICT 2013-2088; PICT 2015-2109], and INTA [9.2013.5.39-2189398, PNPA 1126044] for their financial support.

References (32)

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