Valentina Prosapio*, Ian Norton

Transcription

1 Valentina Prosapio*, Ian Norton

2 Fruits and vegetables are perishable foods (Moisture content, MC > 90 % w/w) undergo degradation reactions by bacteria proliferation Drying: water removal Reduced water activity (a w ) longer product shelf-life Reduced volume lower transport and storage costs In order to avoid microbial spoilage, dried foods should have MC < % w/w and a w < 0.6 Undesired effects: Damage to the microstructure changes in colour, taste, texture Poor rehydration ability

3 Freeze drying (FD) Excellent quality and structural preservation High energy consuming Long processing time Osmotic dehydration pre-treatment (OD) Production of an intermediate moisture product, which can be completely dried using a conventional technique.

4 In a previous work*: - the application of the pre-treatment allowed reduced process time of OD+ FD up to 45% compared to FD for the same samples final a w and MC; - however, the rehydration capacity (RC) was lower than that obtained for freeze drying Process time [h] MC [g/100 g] a w FD ± ±0.007 OD+FD ± ±0.006 RC % Freeze drying Osmotic+freeze drying time, min Need to adopt a different strategy to better preserve the microstructure! *V. Prosapio and I. Norton, Influence of osmotic dehydration pre-treatment on oven drying and freeze drying performance. LWT - Food Science and Technology, : p

5 Ultrasounds (US) US are used to enhance the process mass transfer, but they induce the formation of micro-channels, which may cause the collapse of the microstructure. In order to strengthen the structure of dried foods, firming agents can be used Calcium lactate (E327) Calcium can interact with the free carboxyl groups of the pectin chains in the cell walls, forming the calcium pectate, a cross-bridge that reduces cell separation and, therefore, the tissue softening Many papers have been published on US-assisted OD and the use of firming agents in the osmotic solution, but no study has been carried out so far on the combination of them

6 Simultaneous use of ultrasounds and firming agents to optimise the drying process while improving the rehydration and retaining the properties of the fresh products Adversely affected by drying! Initial moisture content: MC = 91.00±0.05 g/100 g initial water activity: a w = 0.982±0.004 Osmotic dehydration: Fructose 50 Bx, 50 C, 3h Freeze drying only: 18h Freeze drying following OD: 7h Optimised in the previous work

7 Ultrasound-assisted osmotic dehydration + freeze-drying (USOD+FD) Water Heating Heating Ultrasounds RC = (w (t) w d) (w 0 w d ) w(t) is the sample weight at time t (g); - w d is the dried sample weight (g); - w 0 is the sample initial weight (g). Process Firming t OD [min] a w MC RC % agent [g/100g] FD ± ± ±1.32 OD+FD ± ± ±1.02 OD+FD Calcium lactate ± ± ±1.23 USOD+FD Calcium lactate ± ±1.93 n.p ± ± ± ± ± ± ± ± ±1.59 The use of US for 30 min during OD led the highest rehydration capacity (RC) and a processing time reduction of about 83% compared to classic OD (180 min) RC % FD OD+FD OD (with Calcium Lactate)+FD USOD (with Calcium Lactate)+FD time, min

8 Microstructure X-ray micro-computed tomography (sample cross section) Partial collapse and shape distortion FD OD+FD OD (CaLact)+FD USOD (CaLact)+FD Reduced heat exposure Reduced collapse of the structure

9 Colour Image analysis using Matlab L*: lightness coefficient a*: red colour coefficient b*: yellow colour coefficient C ab* :Chroma = a 2 + b 2 ΔE: relative colour difference index L 0 L 2 + a 0 a 2 + (b 0 b ) 2 (the subscript 0 is referred to fresh strawberry) Sample L* a* b* C ab * ΔE Fresh strawberry 57.54± ± ± ± FD 74.00± ± ± ± ±15.71 OD+FD 60.00± ± ± ± ±19.17 OD(Calcium Lactate)+FD 59.72± ± ± ± ±9.91 USOD(Calcium Lactate)+FD 55.57± ± ± ± ±2.00 Worst effect More colour retention No effect on the colour The colour indicators are the closest to the original colour of fresh strawberry

10 Texture Texture analyser: Puncture penetration test Force required to puncture sample (N) FD firmness reduction OD+FD more retention of the structure Calcium lactate addition textural properties further retained USOD+FD The highest firmness preservation 0.00 Fresh strawberries FD OD+FD OD (CaLact) +FD USOD (CaLact) +FD

11 The simultaneous application of ultrasounds and firming agent during osmotic dehydration prior to freeze-drying is a promising strategy to: Enhance the product rehydration ability Improve the product firmness Preserve the product colour Shorten the process time Reduce the energy costs

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