Polymorphism and Microstructural Changes in Avocado Pulp (Persea americana Mill.) After Scraped-Surface Heat Exchanger Processing

Abstract

Avocado (Persea americana Mill.) is a fruit with a high content of unsaturated fatty acids and bioactive compounds, whose consumption has considerably increased in the USA and Europe. Thus, the conservation of the avocado mesocarp (pulp) has become more relevant. Avocado pulp was processed using a scraped-surface heat exchanger (SSHE) system to extend the shelf-life of the mesocarp. Through analysis with X-ray diffraction and HRTEM, it was possible to identify crystalline-type structures in the avocado pulp processed and stored at 4 °C. The 2θ-angles and d-spacing of the structures that reported the highest diffraction intensity are comparable to the polymorphs β' reported in the literature for fatty acid mixtures processed under similar conditions. Furthermore, the X-ray signals suggest the presence of polymorphs α and β in all samples processed and stored at different temperatures. Calorimetry analysis showed curves with first-order phase changes as indicative of crystallization-type transitions. The shelf-life evaluation of the avocado pulp showed that the crystallization process minimized the losses of antioxidant capacity and prevented color change, while the enzyme polyphenol oxidase remained inactivated. The changes induced by the SSHE continuous processing applied might represent an alternative to obtaining avocado products that preserve avocado's properties and extend its shelf-life.

Keywords: HRTEM; X-ray diffraction; avocado; polymorphism; scraped-surface heat exchanger.

Figure 1

Scraped-surface heat exchanger system, (1) inlet of SSHE heating, (2) outlet of SSHE heating, (3) inlet of SSHE pre-cooling, (4) outlet of SHHE pre-cooling, (5) inlet of SSHE cooling, (6) outlet of SSHE cooling, (7) blades, (8) rotor, (a) outlet of heating medium, (b) inlet of heating medium, (c) inlet of pre-cooling medium, (d) outlet of pre-cooling medium, (e) outlet of cooling medium, (f) inlet of cooling medium.

Figure 2

Avocado pulp (a), and avocado pulp processed and stored at 4 °C for 2 weeks (b).

Figure 3

Enzymatic activity of polyphenol oxidase of avocado pulp and avocado pulp after SSHE processing. Circles represent the enzymatic activity found in the avocado pulp before the process, whereas rhombi are for the avocado pulp after processing.

Figure 4

DSC profiles of avocado pulp immediately after processing (AP) (a), avocado processed and stored at 4 °C (AP_P4) (b), avocado processed and stored at −20 °C (AP_P-20) (c), and avocado without processing and heated and cooled for comparison (d).

Figure 5

XRD patterns: green plot: avocado pulp without process; blue plot: avocado processed and stored at 10 °C; red plot: avocado processed and stored at 4 °C; and gray plot: avocado processed and stored at −20 °C.

Figure 6

TEM micrographs (a,b) and SAED pattern of avocado pulp processed and stored at 4 °C. (a) Region enclosed by the red box in the micrograph (b). (c). Fast Fourier Transform (FFT) masking tool shows atomic columns as found in the region indicated by the gray square.

Figure 7

TEM micrographs (a,b) and SAED pattern of avocado pulp processed and stored at 10 °C (c). (b) Region enclosed by the red box in the micrograph (a).

Figure 8

TEM micrographs (a,b) and FFT pattern of avocado pulp processed and stored at −20 °C in the region enclosed by the red box (c). (b) Region enclosed by the red box in the micrograph (a). The region enclosed by the green box shows particles with similar morphology to those found in samples stored at −4 °C and 10 °C.