High-Pressure Pasteurization of Soy Okara

Abstract

Okara is a by-product from the production of soy beverages, which has a high content of protein and fiber. Even though it has a high nutritional value, it is generally discarded or used as animal feed or compost. The problem is its short shelf life due to its high water content and high water activity. The aim of this study was to investigate the effect of high-pressure pasteurization at 200 MPa, 400 MPa, and 600 MPa on the shelf life of soy okara. Microbiological growth, as well as thermal properties, viscosity, water holding capacity, and oil holding capacity, was evaluated after the pressure treatments. Treatment at 600 MPa significantly reduced (p < 0.05) the growth of total aerobic count, yeast and mold, and lactic acid bacteria for up to four weeks of storage at 4 °C. The pasting properties were increased while the water and oil holding capacities of the soy okara did not significantly change (p > 0.05) after high-pressure pasteurization at 400 MPa and 600 MPa. High-pressure pasteurization is therefore a potential application technique for soy okara to produce a microbiologically safe product with maintained functional properties. However, more research is needed to optimize the process and to further investigate the microbiological species present in untreated soy okara to exclude any potential food safety risks.

Keywords: food waste; high-pressure pasteurization; okara; shelf life.

Figure 1

The experimental design of the microbiological storage study of HPP-treated soy okara, reference sample, and crude sample (decanter image adapted with permission from Ref. [37] 2023, Alveteg).

Figure 2

The microbiological growth of crude soy okara (cross), reference (triangle), and HPP-treated soy okara (200 MPa (circle), 400 MPa (square), and 600 MPa (diamond)) stored for 4 weeks (4 °C, vacuum-packed) on TSA (A), MA (B), and MRS (C). Statistical comparisons were made for crude soy okara, reference soy okara, 200 MPa, 400 MPa, and 600 MPa at each storage time for each agar type. Data with different letters are significantly different, p < 0.05, n = 6.

Figure 2

The microbiological growth of crude soy okara (cross), reference (triangle), and HPP-treated soy okara (200 MPa (circle), 400 MPa (square), and 600 MPa (diamond)) stored for 4 weeks (4 °C, vacuum-packed) on TSA (A), MA (B), and MRS (C). Statistical comparisons were made for crude soy okara, reference soy okara, 200 MPa, 400 MPa, and 600 MPa at each storage time for each agar type. Data with different letters are significantly different, p < 0.05, n = 6.

Figure 3

DSC curves for reference (green) and HPP-treated soy okara at 200 MPa (light gray), 400 MPa (gray), and 600 MPa (black).

Figure 4

RVA graphs presenting average viscosity for reference (green) and HPP-treated soy okara: 200 MPa (light gray), 400 MPa (gray), and 600 MPa (black). Peaks are marked with arrows.

Figure 5

WHC (solid) and OHC (striped) for reference (green) and HPP-treated soy okara: 200 MPa (light gray), 400 MPa (gray), and 600 MPa (black). Data with different letters are significantly different, p < 0.05, n = 3 (small letters for WHC, and capital letters for OHC).

Figure 6

SEM micrographs of okara. (a,b) Reference, and (c,d) HPP-treated oat okara at 200 MPa, (e,f) 400 MPa, and (g,h) 600 MPa. The magnification of micrographs in the first column was 500× (a,c,e,g), whereas the second column had a magnification of 1000× (b,d,f,h).

Figure 6

SEM micrographs of okara. (a,b) Reference, and (c,d) HPP-treated oat okara at 200 MPa, (e,f) 400 MPa, and (g,h) 600 MPa. The magnification of micrographs in the first column was 500× (a,c,e,g), whereas the second column had a magnification of 1000× (b,d,f,h).