Effect of minimal processing steps and operation modes on quality of leafy vegetables

Minimal processing and storage of leafy vegetables are limited by several quality challenges despite numerous research interventions. Some studies have assessed pre-harvest and postharvest effects on quality of raw material and sometimes also on quality and shelf-life of the resulting processed product, however, the information on effect of the processing steps on its quality is lacking. This study, assessed the effect of operation modes, processing steps and storage on the nutritional, sensorial and microbial quality of minimally processed rocket leaves and cut lettuce. Rocket leaves and Lettuce (Iceberg and Romaine lettuce) samples were collected during processing from 4 and 2 processors respectively, by sampling the product flowing after each main processing steps (from raw material to washing drying and packaging). In addition processors belonged to Italy (Puglia) and USA (California and Arizona), and particularly 2 processors for rocket leaves and one for cut lettuce (Iceberg lettuce for Italy and Romaine lettuce for USA). Processors differed in their mode of operation and mainly by washing plant and design, type of sanitizer used and drying equipment. The effect of operation modes were evaluated on different quality attributes. For experiments in Italy, Total soluble solids (TSS), Titratable acidity (%TA), pH, Total phenols (TP), Total antioxidant activity (TAA), Ascorbic acid (AA), Dehydroascorbic acid (DHAA), Total vitamin C (Vit. C), Mesophilic count (MC), Psychrophilic count (PC), Yeast and molds (Y&M) and aroma volatiles of products after each processing step, including storage at 5 oC, were evaluated. For the experiments conducted in the USA, the effect of operation modes was evaluated on TP, TAA, MC, PC, Y&M, glucose, sinapic acid and ferulic acid (rocket leaves) as well as chlorogenic acid and p-Coumaric acid (romaine lettuce), with RT-PCR confirmatory tests for screening total bacteria. Results obtained from common steps and quality attribute were evaluated with a 2-Way ANOVA design and random effect, and by applying a multivariate clustering technique in order to detect variation due to different processors and processing steps. Moreover results of each processor were individually analyzed for each processing step. Results showed that raw material quality influenced final product quality. For rocket leaves irrespective of location, the washing and sanitization steps increased phytonutrients properties of leaves (TP, TAA, AA, Total Vit. C sinapic and ferulic acids), inducing a reduction of microbial counts (MC, PC, Y&M), except for one case (processor C) where cross-contamination occurred. The washing steps that significantly increased TP also influenced the release of stress related volatiles (2-Methyl furan, Benzaldehyde, Dimethyl sulphide and (Z)-2-Penten-1-ol), for- Processor A whereas for Processor B, the stress related volatiles decreased (mostly aldehyde and sulphur compounds) when the increase in TP was not substantial during washing. Generally this increase was not affecting the overall nutrient content of the final product since a further oxidation was then induced by the drying steps (spin or tunnel drying). Passive MAP storage caused increase (p≤0.05) in DHAA, Y&M with high level ketone, sulphur compounds and aldehyde indicating early senescence where more stress was induced during processing, but did not significantly affect other phytonutrient, an increase of antioxidants was observed, most probably as a defence response to the growth of PC and Y&M which also induced an increase of the production of benzaldehyde, methyl thiocyanate and dimethyl sulphide. For lettuces, operation modes and processing steps of processors of iceberg and romaine caused a decrease in both phytonutrients and microbial counts (MC, PC, Y&M), but with some differences. The cutting of iceberg lettuce induced the release of alcohols, aldehydes and dimethyl sulphide while the cut surface of romaine had increased levels of all microbial counts probably from a contamination of the cutting equipment. In both cases nonetheless, the washing steps reduced the emission of volatiles and microbial counts respectively. Active MAP storage for 3 days in the case of iceberg lettuce led to a decrease in AA, DHAA, but a rise in TP, TAA, MC, PC, Y&M and volatiles already identified after drying. Moreover, microbial counts remained lower than that of the raw material prior to processing. Glucose content increased in Romaine lettuce after cutting but decreased during further processing. TSS, %TA and pH were not considerably affected by the processing steps. A further study on the effect of the drying conditions on quality of rocket leaves after storage was also carried out. Temperature, belt speed and time were regulated to achieve two air-drying treatments leading to a residual surface water reduction below 2%, namely T-A (33oC, 0.022m/s and 5.37mins) and T-B (40oC, 0.026m/s and 4.45mins). Air ventilation was maintained at 100% in both treatment combinations. Raw material prior to processing was sampled and tested for sensory and microbial quality, while rocket leaves soon after the drying (achieving residual surface moisture of 1.9%-T-A and 0.4%-T-B), were directly packaged in polypropylene film bag (passive modified atmosphere) and stored for 5, 9 and 15 days at 5oC. The stored samples were analyzed in triplicates for colour changes, TSS, pH, %TA, Total chlorophyll (TC), Total Vit. C (AA and DHAA), TP, TAA, sensory quality, MC, PC and Y&M and aroma volatiles. Two-way anova results showed that although L*, b* and chroma angle showed differences (p≤0.05) in T-A and T-B , the overall change in colour ΔE was due to the storage period (15days), but was not visually perceptible. %TA increased with decrease pH after 5days, while TSS increased for both treatments during storage. Raw material sensory quality decreased after minimal processing which also was effective in reducing microbial load; no major significant differences between T-A and T-B drying treatment during the storage period were observed. Passive MAP packaging and cold storage temperature minimized quality changes over time. Slight higher water surface of rocket leaves from the treatment at the lowest temperature (T-A), may have affected microbial and tissue disintegration in storage inducing the production of 2-Ethyl furan, but without inducing sensible variation in sensorial quality. Phytonutrients were also maintained during the storage period, with the exception of TC which reduced by 9.43% and 13.27% for T-A and T-B and TP which reduced by 4.32% for T-A. MC and PC reached the limit for spoilage after 15 days of storage while yeasts and molds growth exceeded the limit for spoilage after 9 days. These results suggested that the treatment at 40 °C and shortest time should be preferred in order to increase process productivity, allowing to maintain lower residual water without inducing any thermal degradation. It can be concluded that minimal processing steps, the mode of operation of different processors, the type of product and storage conditions influenced the final product quality at varying degrees. Generally the washing reduced microbial counts, inducing a stress that in whole rocket leaves favoured an increase phytonutrient, while in cut lettuce as also in consequence of cutting a more pronounced oxidation was observed. All these finding contributed to fill the existing gap between physiology of cut tissues and general knowledge about the impact of processing operations on quality of the final product, by increasing the amount of detailed information which can be available for processors.