Sustainable Exploitation of Apple By-Products: A Retrospective Analysis of Pilot-Scale Extraction Tests Using Hydrodynamic Cavitation

Abstract

Apple by-products (APs) consist of whole defective fruits discarded from the market and pomace resulting from juice squeezing and puree production, which are currently underutilized or disposed of due to the lack of effective and scalable extraction methods. Bioactive compounds in APs, especially phlorizin, which is practically exclusive to the apple tree, are endowed with preventive and therapeutic potential concerning chronic diseases such as cardiovascular diseases, metabolic diseases, and specific types of cancer. This study investigated the exploitation of APs using hydrodynamic cavitation (HC) for the extraction step and water as the only solvent. High-temperature extraction (>80 °C) was needed to inactivate the polyphenol oxidase; a strict range of the cavitation number (around 0.07) was identified for extraction optimization; less than 20 min were sufficient for the extraction of macro- and micro-nutrients up to nearly their potential level, irrespective of the concentration of fresh biomass up to 50% of the water mass. The energy required to produce 30 to 100 g of dry extract containing 100 mg of phlorizin was predicted at around or less than 1 kWh, with HC contributing for less than 2.5% to the overall energy balance due to the efficient extraction process.

Keywords: apple by-products; bioactive compounds; bioeconomy; green extraction; hydrodynamic cavitation; sustainability.

Figure 1

HC devices and reactors used in the experiments: (a) general layout, with numbers indicating 1—centrifugal pump; 2—electronic control panel with inverter (HC50 and HC300); 3—inline tank; 4—Venturi-shaped reactor; (b) reactor with throat area of 24 mm (HC200); (c) reactor with throat area of 7 mm (HC50); (d) reactor with throat area of 20 mm (HC300).

Figure 2

Temperature, cavitation number, target quantity as a percentage of its potential level and peak process yield for the samples collected in each test: (a) TPC; (b) ORAC.

Figure 3

Temperature, cavitation number, target quantity as a percentage of its potential level and peak process yield for the samples collected in each test: (a) Chlorogenic acid; (b) Phlorizin; (c) Epicatechin; (d) Procyanidin B2; (e) Total sugars.

Figure 3

Temperature, cavitation number, target quantity as a percentage of its potential level and peak process yield for the samples collected in each test: (a) Chlorogenic acid; (b) Phlorizin; (c) Epicatechin; (d) Procyanidin B2; (e) Total sugars.

Figure 4

Linear regression plots. The relationship between the predicted yield (%) (X-axis) and the actual measured values (Y-axis) is represented by blue dots, while the curves indicate the confidence intervals of the regression: (a) ORAC for whole apple; (b) TPC for whole apple; (c) ORAC for AP; (d) TPC for AP.

Figure 4

Linear regression plots. The relationship between the predicted yield (%) (X-axis) and the actual measured values (Y-axis) is represented by blue dots, while the curves indicate the confidence intervals of the regression: (a) ORAC for whole apple; (b) TPC for whole apple; (c) ORAC for AP; (d) TPC for AP.

Figure 5

General scheme of the steps involved in the production of dry extracts from HC-based processing of AP. Black arrows refer to water–biomass mixture or water; brown arrows refer to insoluble wet residues; grey arrow refers to sugars. Dashed steps are not considered in the quantitative assessment.

Figure 6

Scenario of the chain of process steps leading to the production of dry extracts from AP, with total energy consumption and energy consumption per unit mass of dry extract represented as a function of the water to dry biomass ratio, assuming biomass moisture of 83% and process time of 20 min.