Abstract
The preparation and testing of a colour-based prototype indicator for high-pressure processing (HPP) are described. The indicator is a layered structure comprising a pressed disc of a mixture of silica gel, which has been previously loaded with a set wt% of acidified water, and polytetrafluoroethylene, PTFE, powders, a water-permeable barrier layer, and a Congo-Red-based pH indicator layer, all vacuum-sealed in a water impermeable plastic film. The value of the wt% is calculated from the ratio of the mass of acidified water added to the mass of originally dry silica gel. The high pressures associated with HPP drive the release of the acidified water from the silica gel and its subsequent transport through the water-permeable barrier layer to the pH indicator, thereby producing a striking red-to-blue colour change. The response of the HPP indicator can be tuned to different HPP conditions by varying the wt% of acidified water used to load the silica gel powder. Indicators, with 61, 63, and 65 wt% acidified water loaded silica gel, are prepared and found to require, respectively, the application of at least, 600, 400, and 300 MPa pressure for 3 min to effect a change colour. To our knowledge, this is the first reported example of a prototype HPP indicator that can be tuned to respond to the very different pressure and time conditions used in HPP to sterilise such very different products as milk, apple and orange juice, and aloe vera gel.
Highlights
Pasteurisation is a common thermal processing technique used in the food industry, which, in many cases, induces undesired changes in product quality, such as loss of smell, colour, flavour, texture, and nutritional value [1]
The waterimpermeable, clear plastic polymer film (110 μm thick), used to encapsulate the indicator components, was a laminate of polyethylene terephthalate (PET) with an aluminium coating, low-density polyethylene (LDPE), and a peelable co-extruded film of proprietary composition; this film was provided by Sensor Indicator Products (Seattle, WA, USA) [19]
A brief examination of these results reveals that the high-pressure processing (HPP) indicator loaded with 61 wt% water only gave the expected colour change due to acidification of the pH indicator film, from red to blue, when exposed to the highest pressure and longest time used in this work, namely, 600 MPa for 3 min
Summary
Pasteurisation is a common thermal processing technique used in the food industry, which, in many cases, induces undesired changes in product quality, such as loss of smell, colour, flavour, texture, and nutritional value [1]. Consumer demands for fresh, safe, and nutritional food products are driving the development of alternative food pasteurisation technologies, such as high-pressure processing (i.e., HPP) [2,3]. It is often claimed that, in comparison with thermal-processed foods, HPP foods have “better texture, fresher taste, improved appearance and increased retention of nutrients” [1], and it is reported that “HPP usually results in minimal or no permanent change in textural characteristics” [7]. In HPP, the pressure is transmitted to food products from all sides and, as a result, the foods are not crushed during treatment and many foodstuffs can be treated using HPP [1]
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