Innovative food processing technologies to enhance the biosynthesis and accumulation of free and bound phenolics in carrots (Daucus carota)

Abstract

Nowadays, chronic degenerative diseases constitute a major challenge for the health system around the world. These diseases are closely related with a poor-quality diet, which are high in calories, and low in dietary fiber and essential micronutrients. In this scenario, horticultural crops are a well-recognized source of bioactive compounds such as provitamin A carotenoids, antioxidant phenolic compounds and dietary fiber that can prevent chronic degenerative diseases. The application of postharvest abiotic stresses (i.e. wounding stress) to fruits and vegetables can promote the biosynthesis and accumulation of bioactive compounds. Different approaches have been evaluated to use stressed plant tissues as raw material to produce nutraceutical food ingredients. These ingredients could be used to produce next generation foods with enhanced nutraceutical properties. Among secondary metabolites induced by abiotic stresses, antioxidant phenolic compounds are highly can prevent different diseases. However, around half of the phenolic compounds present in plants are bound to cell wall components, limiting their absorption in the gastrointestinal tract. Thus, it is relevant to study food processing technologies that can induce the biosynthesis and increase the extractability of phenolics compounds. Therefore, in the present thesis, innovative food processing technologies such as wounding stress, extrusion and high hydrostatic pressure applied alone and combined were evaluated as tools to increase the biosynthesis and extractability of phenolic compounds in carrots. In the first part of this thesis (chapter 1 and 2) wounding stress was studied as a tool to modify the cell wall constituents of carrots and to increase its free and bound phenolic content. Likewise, the stressed tissue was dehydrated and then subjected to vi extrusion cooking to further increase the extractability of bound phenolics induced by wounding and to obtain a carrot powder with modified functional properties [water absorption index (WAI), oil absorption index (OAI), water solubility index (WSI), and water retention capacity (WRC)]. To apply wounding stress, whole carrots were shredded (wounding stress carrots, WSC) and stored for 48 h at 15 °C. Thereafter, the tissue was dehydrated to obtain a final moister of 25%, and further processed with extrusion under different temperatures (63 and 109 °C) and screw configuration (continuous or expansion). Cellulose content increased (112%) as a response to wounding. Furthermore, extrudates obtained from WSC showed higher content of cell-wall components. For instance, insoluble and total lignin content increased (54–84%) with extrusion conditions. Furthermore, WSC showed higher oil absorption index and lower water solubility index (WSI); whereas extrudates showed the highest WSI. Furthermore, wounding stress increased total free (288.1%) and bound (407.6%) phenolic content, whereas the carotenoid content was unaltered. The free and bound phenolics that showed the highest increase due to wounding were the chlorogenic (579.8%) and p-coumaric (390.9%) acids. Extrusion, at 109 °C under expansion screw configuration, further increased the wound- extractability of total free (296.6%) and bound (22.1%) phenolics. In the second part of this thesis the application of high hydrostatic pressure (HHP) was evaluated as a tool to induce the biosynthesis of free and bound phenolic compounds in carrots. HHP change the cell membrane permeability of plant cells, thus herein was hypothesized that HHP could elicit stress responses similar to wounding stress. In the specific case of HHP, it has been proposed that mild treatments (<150 MPa) could act as abiotic elicitor in different plant cell cultures and it is still unknown if elicitation of plant metabolism occurs during the pressure rising stage (come-up time, CUT), or if the holding time is relevant. For this study, the effect CUT at 60 and 100 MPa on the biosynthesis of free and bound phenolics in whole carrots were evaluated. Variables such as the phenylalanine ammonia-lyase (PAL) activity as well as the respiration rate and volatile organic compounds (VOCs) production was evaluated to gain insights on the physiological response of carrots to HHP. As an immediate response to HHP, samples treated at 100 MPa showed increases in the content of free [5-O-caffeoylquinic acid (63.9%) and 3,4-di-O-feruloylquinic acid (228.6%)] and bound [p-coumaric acid (82.6%)] vii

phenolics. Furthermore at 1 d, samples treated at 60 MPa showed accumulation of free phenolics [4,5-di-O-caffeoylquinic acid (60.2%), and isocoumarin (98.9%)]; whereas samples treated at 100 MPa showed increases of 5-O-caffeoylquinic acid (291.2%) and 3,4-di-O-feruloylquinic acid (466.1%). At 2 d of storage, whole carrots treated at 60 MPa showed accumulation of bound phenolics [rutin (85.5%) and p-coumaric acid (214.7%)], whereas at 3 d, 100 MPa samples showed higher quercetin (371.2%). During storage, samples treated at 60 and 100 MPa showed higher respiration rate, and ethylene production, respectively. Results allowed the generation of a potential mechanism summarizing the main physiological changes induced by HHP in whole carrots. In conclusion, in the present study it was reported for the first time the effect of wounding stress on cell wall constituents and bound phenolic content of carrot tissue. Wounding induced modifications in cell wall constituents that resulted on the obtention of carrot powders with modified functional properties. Likewise, carrot powder obtained from stressed tissue contained high levels of bound phenolics. The application of extrusion cooking to carrots treated with wounding stress, resulted on further modifications of cell wall constituents, and increased the extractability of bound phenolics produced by wounding stress. Thus, the application of wounding stress followed by extrusion cooking resulted an interesting approach to modify the functional properties and increase the extractability of phenolics in carrots, making it an attractive ingredient to generate foods with enhanced nutraceutical properties. Finally, the application of HHP at CUT 60 and 100 MPa, increased the extractability and induced the biosynthesis of free and bound phenolics in carrots. Whole carrots with enhanced phenolic content could be used as food or as raw material to produce processed foods. The results obtained in the present study allows the elucidation of strategies to enhance the nutraceutical properties of carrots, which can be further used as an ingredient to produce next generation functional foods that prevents chronic and degenerative diseases.