Quality of minimally processed carrots as affected by warm water washing and chlorination

Different applications of cold and warm tap water (4 °C and 50 °C) with and without chlorination, respectively, in the washing of uncut peeled carrots (Daucus carota L.) were conducted, and their effects on sensory and microbiological properties during storage for 9 days at 4 °C were assessed. To minimise cross-contamination of almost sterile inner root parts with the highly contaminated outer cortex during processing the peeled carrots were washed prior to cutting. The washing treatments were carried out using a commercial processing line, thus facilitating the scale-up to industrial production. Populations of aerobic mesophilic bacteria, lactic acid bacteria and enterobacteria on these minimally processed carrots were determined, and the sensorial quality of shredded carrots was evaluated by a sensory panel throughout storage. Additionally, colour, texture, sugars, and trichloromethanes were analysed. Washing uncut carrots with cold chlorinated water (200 mg/l, 4 °C) and warm tap water (50 °C) ensured sugar retention and reduced aerobic mesophilic bacteria by 1.7 and 2.0 log₁₀ colony forming units per gram (cfu/g), respectively, while washing with warm chlorinated water (200 mg/l) resulted in a 2.3 log₁₀ cfu/g reduction. By-product formation due to chlorination was negligible. Sensorial properties of the latter samples were slightly affected. It was shown that both washing uncut knife-peeled carrots with cold chlorinated water (4 °C) and warm tap water (50 °C), respectively, provided good microbiological safety paired with improved sensorial properties. Moreover, fresh-like character of the products was retained, as indicated by the persisting respiration of the living tissues.Industrial relevanceMinimally processed vegetables are an increasing market. Shelf-life extension and consumer safety are of immense relevance for the fresh-cut industry; therefore, the application of antimicrobial agents such as chlorine is widespread. However, various consumer groups object to the use of chlorinated water. The present study aimed at comparing the efficacy of cold and warm tap water with and without chlorination, respectively, in washing uncut carrots during the production of shredded, packaged carrots while operating on pilot-plant scale under conditions of industrial practice. In view of microbial reduction and maintenance of sensory properties, the use of cold chlorinated water and warm tap water, respectively, proved to be effective for washing peeled carrots. By-product formation due to chlorination was negligible.     

Behavior of Listeria monocytogenes and Aeromonas spp. on fresh-cut produce packaged under equilibrium-modified atmosphere.

Storage experiments were conducted to follow the behavior of pathogens on fresh-cut vegetables (trimmed brussels sprouts, grated carrots, shredded iceberg lettuce, and shredded chicory endives) packaged under an equilibrium-modified atmosphere (EMA) (2 to 3% O2, 2 to 3% CO2, and 94 to 96% N2) and stored at 7 degrees C. As a comparison, fresh-cut vegetables were also packaged in a perforated high-barrier film (air conditions) and stored at 7 degrees C. In a first step, the shelf life of the vegetables in the two kinds of packages was determined by evaluating the microbiological quality as well as the sensorial quality (appearance, taste, and odor). In general, sensorial properties were faster in limiting the shelf life than microbiological criteria. The shelf life of the vegetables stored under an EMA was extended by 50% or more, compared with the air-stored vegetables. In a second storage experiment, the four fresh-cut vegetables were inoculated with a cocktail of psychrotrophic pathogens (Listeria monocytogenes, Aeromonas caviae [HG4]) and A. bestiarum (HG2) before packaging under an EMA and air at 7 degrees C. The inoculated pathogens were more influenced by the type of vegetable than by the type of atmosphere. No growth was detected on the brussels sprouts or on carrots (L. monocytogenes). Aeromonas spp. had a higher growth rate than L. monocytogenes on the shredded chicory endives and shredded iceberg lettuce at 7 degrees C.     

Quality attributes of shredded carrot (Daucus carota L. cv. Nantes) as affected by alternative decontamination processes to chlorine

The effects of alternative decontamination processes to chlorine: ozonated-water (1 ppm/5 min), hot water (100 °C/45 s) and ultrasonication (45 kHz/1 min), applied pre- or post-cut in the technological diagram of minimal processing of carrots were tested. Ultrasonication in chlorinated-water and thermo-ultrasonication as combined processes applied just in pre-cut carrot were also tested. The initial microbial load reduction, soluble solids content, pH and sensorial attributes of shredded carrot just after processing were evaluated. Decontamination processes applied on pre-cut carrot provided maintenance of fresh-like sensorial quality, regardless the type of treatment, due to diminished leaching phenomena which is critical for shredded carrot. Chlorination, ozonization and ultrasonication achieved ca. 1 Log₁₀ reduction of initial microbial load. No additional decontamination effect in combined processes was observed. The use of heat in pre-cut carrot proved to be the most efficient process regarding microbial reduction (3 Log₁₀ units) providing, as well, an acceptable fresh-like quality product.Industrial relevanceThe major constraint for marketability of minimally processed shredded carrot is its limited shelf-life due to rapid microbial growth and colour loss (decrease of orange intensity and/or whitening of the shreds). These questions arise from the practical experience of a fresh-cut industry directly involved in the R&D research project which supported this study. Chlorine solutions have been widely used to sanitize fruit and vegetables in the fresh-cut industry. However, reduced microbiological efficiency allied to the eventual formation of carcinogenic chlorinated compounds pointed out the need for alternative methodologies. The present work aimed the evaluation of clean alternative decontamination processes applied both to pre- and post-cut carrot for the production of shredded carrot, operating under conditions of industrial practice at pilot-plant scale.     

Evaluation of a pre-cut heat treatment as an alternative to chlorine in minimally processed shredded carrot

The effect of a pre-cut heat treatment (100 °C/45 s) as an alternative decontamination treatment to chlorinated-water (200 ppm active chlorine/1 min, 5 °C) was evaluated in minimally processed carrot (shredded). The quality of shredded carrots was studied just after minimal processing and during storage at 5 °C (10 days) by evaluating microbial (total mesophilic aerobic, yeast and moulds and lactic acid bacteria counts), physical–chemical (soluble solids content, pH, titratable acidity, whiteness index), physiological (peroxidase activity and headspace analysis) and sensorial attributes (colour, fresh-like appearance, aroma and general acceptance). The relationships between sensory perception of undesired changes, microbial contamination threshold, physico-chemical and physiological indices were investigated and compared between heat-treated and control samples. The use of heat in pre-cut carrot proved to be more efficient than chlorinated-water concerning microbial control (threshold concentration of 7 Log₁₀ cfu g^? 1), providing an acceptable fresh-like quality product during 10 days of storage (5 °C), which corresponds to a 3-day shelf-life extension compared to control samples. Heat-treated shredded carrot showed lower respiratory and POD activities than chlorinated samples suggesting that the use of heat provides a metabolic activity lowering effect besides the microbial effect which could be important to shelf-life extension of the fresh-cut product.Industrial relevanceIn minimally processed vegetables, namely in shredded carrot, chlorine solutions have been widely used by the industry for sanitization purposes. However, reduced microbiological efficiency allied to sensorial changes and eventual formation of carcinogenic chlorinated compounds pointed out the need for alternative decontamination methodologies. Also, the evermore conscious consumers are demanding minimization of the potentially negative impact of food processing on human health and the environment. From the practical experience of a fresh-cut industry directly involved in the R&D research project which supported this study, the marketability of minimally processed shredded carrot is limited due to rapid microbial growth and colour loss (decrease of orange intensity and/or whitening of the shreds). As a result, a pilot-plant scale study was performed, evaluating quality attributes of shredded carrot processed according to a clean pre-cut alternative decontamination process.     

Influence of Washing and Packaging on the Sensory and Microbiological Quality of Fresh Peeled White Asparagus

ABSTRACT: We evaluate the microbiological quality and physicochemical and sensorial characteristics of white asparagus washed with and without chlorine and packaged in 2 different types of film (perforated polyvinyl chloride [PVC] and P-Plus 160) when stored at 4 °C for up 15 d. Dehydration was the main cause of quality losses. Asparagus packaged in P-Plus maintained their fresh appearance better than those packaged in perforated PVC. The total sugar and ascorbic acid content decreased about 9% and 41% after 15 d, respectively. Peeled white asparagus packaged in perforated PVC or P-Plus kept a reasonable sensorial quality after storage at 4 °C for 15 d, although mesophile and psycrotroph counts of about 9 log CFU/g were reached in asparagus washed with water. The effect of washing and type of film on different microbial groups is discussed.     

Microbial Population of Shredded Carrot in Modified Atmosphere Packaging as Related to Irradiation Treatment

Shredded carrots in modified atmosphere packaging were treated with low-dose irradiation of 0.5 kGy in order to determine whether additional reduction of microbial population would be achieved for carrots previously treated with chlorine. Commercially prepared shredded carrots treated with irradiation had a mean microbial population of 1300 CFU/g at the expiration date (9 days after irradiation) compared with 87,000 CFU/g for nonirradiated, chlorinated controls. Oxygen content of the headspace gas and ethanol content of the carrots were not significantly affected. Irradiation appears to be a suitable technology for shredded carrots.     

Mint and pomegranate extracts/oils as antibacterial agents against Escherichia coli O157:H7 and Listeria monocytogenes on shredded carrots

Among minimally processed vegetables, shredded carrots are particularly popular. The aim of this study was to evaluate the antimicrobial activity of different washing treatments with aqueous solutions of mint essential oil (EO) (1:1000), mint hydrosol (1:10), and pomegranate juice (1:10) against two major foodborne pathogens on shredded carrots as well as their effect on carrot's quality. The results of this study indicate that the tested washing treatments resulted in a small but significant decrease of the microbial load of Escherichia coli and Listeria monocytogenes on the sixth day of storage. Pomegranate juice and mint hydrosol were more effective against L.   monocytogenes, whereas mint EO was more effective against E.   coli. Furthermore, an increase in carrot's total phenolics and antioxidants was observed with the application of mint hydrosol and pomegranate juice, whereas mint EO resulted in a decrease on the sixth day of storage. Additionally, a decreased chroma of the shredded carrots were observed during the application of mint hydrosol on the sixth day. Total carotenoids, ascorbic acid, total soluble solids, and tritratable acidity were not differing after six days of storage of shredded carrots. These washing products can be used for the partial disinfection of shredded carrots.

Practical applications

Pomegranate and mint natural products solution can be used for the partial disinfection of minimally processed fresh produce as an alternative to chlorine, and these products can be a part of the sanitation process.     

Acidified sodium chlorite as an alternative to chlorine to control microbial growth on shredded carrots while maintaining quality

Shredded carrots are particularly susceptible to microbial growth and quality deterioration as a result of a large cut surface area to mass ratio. Acidified sodium chlorite (ASC) in the concentration range 500–1200 µL L^?1 has been shown to have stronger efficacy against pathogens and spoilage bacteria than chlorine and does not form carcinogenic products. However, ASC in this concentration range aggravates tissue damage. The objective of this study was to optimize ASC treatment parameters to balance antimicrobial activity with quality retention of shredded carrots. Shredded carrots were immersed for either 1 min in 100, 250 or 500 µL L^?1 ASC solutions or 2 min in 200 µL L^?1 chlorine or water (control). Treated samples were spin‐dried and packaged in polypropylene bags and stored at 5 °C for up to 21 days. Carrots were evaluated at 7‐day intervals for visual appearance, package atmosphere composition (O₂ and CO₂), product firmness, tissue electrolyte leakage and pH. The microbial growth, including total aerobic bacterial counts, total coliforms/Escherichia coli, yeast and mold counts and lactic acid bacterial counts on the products was also determined. Treatments with all concentrations of ASC reduced the aerobic bacterial counts, coliform/E. coli counts, yeast mold and counts and lactic acid bacterial populations by 1.2–2.0 log cfu g^?1 when compared with the water‐washed and unwashed samples. During storage, unwashed samples had a sharp increase in lactic acid bacterial populations accompanied by a sharp decline in pH readings and rapid loss in firmness and tissue integrity; samples washed with 100 µL L^?1 ASC maintained the best overall visual quality, accompanied by the retention of tissue integrity and firmness. Therefore, 100 µL L^?1 was determined as the optimum concentration of ASC for maintaining overall quality and firmness, inhibiting microbial growth and prolonging the shelf‐life of shredded carrots. Copyright © 2006 Society of Chemical Industry     

Quality of Shredded Carrots as Affected by Packaging Film and Storage Temperature

ABSTRACT: Different packaging films and storage temperatures were used to establish a range of equilibrium modified atmospheres for storage of shredded carrots. Quality and storage-life of the packaged shredded carrots were determined using sensory evaluation, microbial counts, and a range of physical tests. Minimal processing steps such as peeling and shredding caused physical damage, physiological stress, and enhanced microbial growth, leading to a reduced shelf life when compared to the whole vegetable. A P-plus microporous film (CO₂ permeability of 29 X 10³mL.m ^-2.d ^-1.atm ^-1) was the most suitable for the storage of shredded carrots. Findings indicated that deterioration in these products was triggered by the depletion of oxygen more than by the rise in carbon dioxide.     

Sensory, Microbiological and Chemical Quality of Mini-Peeled Carrots as Affected by Edible Coating Treatment

The effect of three application rates of an edible cellulose-based coating on sensory, microbiological and chemical quality of mini-peeled carrots was studied during storage at 2°C. Carrots treated with edible coating had reduced levels of white surface discoloration, and higher sensory scores for orange color intensity, fresh carrot aroma, fresh carrot flavor, and overall acceptability than non-coated carrots. Edible coating treatment did not affect levels of total aerobic microorganisms, yeast, mold or lactobacilli. Oxygen, carbon dioxide and ethanol levels in packages, and sugar and soluble phenolic content of mini-peeled carrots were not affected by coating treatment. An edible coating application rate of 0.23–0.49 L/min protected against surface discoloration and retained quality during storage at 2°C.     

Changes in Phenolic Content in Fresh Ready-to-use Shredded Carrots during Storage

Phenolic compounds of shredded carrots were characterized and quantified by HPLC and their concentrations were measured during storage in air at 4°C. Trans 5′-caffeoylquinic acid amounted to 60% of total phenolic content and accumulated rapidly. Para-hydroxybenzoic acid and p-hydroxybenzoic esters were not found in freshly prepared shredded carrots and their content increased after the first day. Patterns of accumulation varied, between samples from the same carrot cultivar grown in different geographical areas. When shredded carrots were stored in polypropylene film pouches or in controlled atmospheres containing 30% CO₂ and/or 0% 0₂, phenolic compounds accumulated very slowly. The increase in phenylalanine ammonialyase activity was consistent with accumulation of phenolic compounds and may relate to microbial spoilage.     

Effect of Minimal Processing Conditions on Respiration Rate of Carrots

ABSTRACT: Measurement, analysis, and modeling of respiration rate (RR) of fresh produce are fundamental for the engineering design of MAP. This study investigates the effect of type of cutting (sliced in circular shape, batons in rectangular shape, and shredded into thin strips) on the respiration rate of carrots at different temperatures (4, 8, 12, 16, and 20 °C) with ambient atmosphere. The higher respiration rates were found for shredded carrots and the lowest for whole carrots at all the temperatures tested. The RR increased from 9 ± 1 to 26 ± 3, 10 ± 1 to 53 ± 2, 11 ± 1 to 55 ± 2, and 18 ± 1 to 75 ± 3 mL of O₂ per kilogram hour for whole, sliced, baton, and shredded carrots, respectively, as the temperature increased from 4 to 20 °C. On an average, RO₂ and RCO₂ were increased by 85% and 64%, 100% and 72%, and 151% and 124% for sliced, baton, and shredded carrots, respectively, compared to the RR values of whole carrots. The temperature dependence of RR followed an Arrhenius‐type relationship for all types of carrots, with activation energies in the range of 35 ± 3 to 62 ± 2 kJ/mol. Type of cut was the major factor for both RO₂ and RCO₂, with shredded carrots having the highest rates and whole carrots the lowest. RR of whole carrots did not vary during storage, whereas it increased for cut carrots and especially for shredded carrots. The respiratory quotient (RQ) increased with temperature and did not vary during storage in all types of carrots, except the RQ of whole carrots decreased significantly over time.     

Impact of plant essential oils on microbiological,organoleptic and quality markers of minimally processed vegetables

The objectives of this study were to evaluate the efficacy of plant essential oils (EOs) for control of the natural spoilage microflora on ready-to-eat (RTE) lettuce and carrots whilst also considering their impact on organoleptic properties. Initial decontamination effects achieved using EOs were comparable to that observed with chlorine and solution containing oregano recorded a significantly lower initial TVC level than the water treatment on carrots (p < 0.05). No significant differences were found between the EO treatments and chlorine considering gas composition, color, texture and water activity of samples. The sensory panel found EO treatments acceptable for carrots throughout storage, while lettuce washed with the EO solutions were rejected for overall appreciation by Day 7. Correlating microbial and sensory changes with volatile emissions identified 12 volatile quality markers. Oregano might be a suitable decontamination alternative to chlorine for RTE carrots, while the identification of volatile quality markers is a useful complement to sensory and microbiological assessments in the monitoring of organoleptic property changes and shelf-life of fresh vegetables.Industrial relevanceThere is industrial demand for natural alternatives to chlorine, which is commonly used for decontamination of fresh produce but which has limitations with respect to antimicrobial efficacy and possible formation of carcinogenic compounds in water. Plant essential oils have proven antimicrobial and other bioactive properties, however their usefulness in foods can be mitigated by their high sensory impact. This study examined the application of EOs for fresh produce decontamination addressing control of spoilage microflora and improving shelf-life characteristics whilst also considering the impact on organoleptic properties. The effectiveness of oregano as a decontamination treatment was comparable with that of chlorine. Carrot discs treated with the EO regimes were acceptable in terms of sensory quality and appreciation, therefore oregano could offer a natural alternative for the washing and preservation of fresh produce. Combining EOs with other natural preservatives might minimize doses and reduce the impact on organoleptic properties of fresh vegetables.     

Microbiological, physicochemical and organoleptic changes of shredded carrots stored under modified storage

The effect of initial head spaces of air, 4.9% CO₂/2.1% O₂/93% N₂ and 5% CO₂/95% N₂ on the microbial flora of shredded carrots was studied at 4 and 10°C. The microbial flora of shredded carrots comprised lactic acid bacteria, pseudomonads and yeasts. Lactic acid bacteria were the predominant organisms in all samples. The pH dropped during the storage of carrots and this was more pronounced at 10°C. The concentration of different organic acids such as lactic, acetic, tartaric, citric and succinic increased in all samples stored under modified atmosphere packaging conditions at both temperatures. The spoilage of carrots stored under 5% CO₂/95% N₂ was delayed, as indicated by the changes in their texture, colour and odour, compared with those samples stored under air or 4.9% CO₂/2.1% O₂/93% N₂.     

Shelf-life of minimally processed cabbage treated with neutral electrolysed oxidising water and stored under equilibrium modified atmosphere

Minimally processed vegetables (MPV) have a short shelf-life. Neutral electrolysed oxidising water (NEW) is a novel decontamination method. The objective of this study was to test the potential of NEW to extend the shelf-life of a MPV, namely shredded cabbage. Samples of shredded cabbage were immersed in NEW containing 40 mg/L of free chlorine or tap water (control) up to 5 min, and then stored under equilibrium modified atmosphere at 4 degrees C and 7 degrees C. Proliferation of aerobic mesophilic bacteria, psychrotrophic bacteria, lactic acid bacteria and yeasts were studied during the shelf-life. Also pH and sensorial quality of the samples as well as O(2) and CO(2) composition of the headspace of the bags was evaluated. From the microbial groups, only psychrotrophic counts decreased significantly (P<0.05) due to the effect of NEW, but the counts in treated samples and controls were similar after 3 days of storage at 4 degrees C and 7 degrees C. Packaging configurations kept O(2) concentration around 5% and prevented CO(2) accumulation. pH increased from 6.1-6.2 to 6.4 during the shelf-life. No microbial parameter reached unacceptable counts after 14 days at 4 degrees C and 8 days of storage at 7 degrees C. The shelf-life of controls stored at 4 degrees C was limited to 9 days by overall visual quality (OVQ), while samples treated with NEW remained acceptable during the 14 days of the experiment. The shelf-life of controls stored at 7 degrees C was limited to 6 days by OVQ and browning, while that of samples treated with NEW were limited to 9 days by OVQ, browning and dryness. According to these results, a shelf-life extension of at least 5 days and 3 days in samples stored respectively at 4 degrees C and 7 degrees C can be achieved by treating shredded cabbage with NEW. NEW seems to be a promising method to prolong the shelf-life of MPV.     

Antimicrobial silver-montmorillonite nanoparticles to prolong the shelf life of fresh fruit salad

In this work, silver-montmorillonite (Ag-MMT) antimicrobial nanoparticles have been obtained by allowing silver ions from nitrate solutions to replace the Na⁺ of natural montmorillonite and then to be reduced by a thermal treatment. Ag-MMT were used as active antimicrobial compounds to improve the shelf life of fresh fruit salad. In order to assess their influence on product shelf life, sensorial and microbiological quality has been monitored during the storage. The microbiological quality was determined by monitoring the principal spoilage microorganisms (mesophilic and psychrotrophic bacteria, coliforms, lactic acid bacteria, yeasts and molds). Additionally, the evolution of sensorial quality was assessed by monitoring color, odor, firmness and product overall quality. The Ag-MMT nanoparticles seemed to be effective in inhibiting microbial growth, above all at the highest tested concentration. Consequently, the sensorial quality of samples stored in the active packaging appeared to be better preserved. Thus, experimental results showed that a significant shelf life prolongation of fresh fruit salad can be obtained by a straightforward new packaging system.     

Hot water treatment and peracetic acid to maintain fresh-cut Galia melon quality

The aim of the present study was to investigate if the use of hot water immersion dipping (HWD) alone or combined with other ecofriendly methods, could replace the use of chlorine in fresh-cut fruits such as melon. Melon pieces were subjected to hot (60 °C) or cold (5 °C) water dipping (60, 90, 120 s or 60 s, respectively) followed by immersion in 80 mg L^{− 1} peracetic acid (PAA) for 60 s at 5 °C or in water, packed in polypropylene trays under passive modified atmosphere (7.4 kPa O₂ and 7.4 kPa CO₂ at steady state), and stored up to 10 days at 5 °C. Respiration rate, ethylene emission, microbial load, flesh firmness, polyamine content and sensorial quality were determined. As main conclusions the longer HWD treatment times (90 and 120 s) followed by PAA dip, provided the lowest metabolic activity and helped to control microbial load without affecting the sensorial quality. In addition, both treatments increased the polyamine content helping to maintain the cell membranes integrity.

Industrial relevance

Maintaining quality and microbial safety are the most important concerns of the fresh-cut fruit and vegetables industry. The present study focused on assessing the effect of HWD treatments alone or in combination with PAA, on the respiration rate, ethylene emission, microbial load, flesh firmness, polyamines content and quality retention of fresh-cut Galia melon. According to our results, the use of a heat treatment alone or combined with PAA could replace the use of chlorine, and could be a feasible alternative for fresh-cut industry as a sanitizing method, as or more effective as chlorine.     

Accumulation of Chlorogenic Acid in Shredded Carrots During Storage in an Oriented Polypropylene Film

Chlorogenic acid content of four samples of shredded carrots was measured and related to their flavor stability during storage wrapped in oriented polypropylene film (OPP). Chlorogenic acid was quantified by HPLC/UV: Shredded carrots accumulated chlorogenic acid faster in OPP film. This appeared related to higher sensory quality retention during storage.     

Reduction of chlorine concentration and microbial load during washing-disinfection of shredded lettuce

Disinfection of shredded lettuce by washing is an essential operation to eliminate foreign matter and cellular fluids produced by cutting, and to reduce the initial microbial contamination. The response surface methodology was used to evaluate the effect of initial chlorine concentration (50–150 ppm), water‐to‐produce ratio (20–50 L kg^?1) and time (2–8 min) on the depletion of total and available chlorine and also the microbial count reduction on shredded lettuce. The predictive models obtained, validated through additional confirmatory experiments, were shown to be adequate and could be used as a way of improving the operation.     

Vitamin C content and sensorial properties of dehydrated carrots blanched conventionally or by ultrasound

Vitamin C content and sensorial properties have been evaluated in air-dried carrots previously subjected to different ultrasound (US) or conventional blanching pretreatments. In addition, mass spectral fingerprints obtained by the Headspace ChemSensor System have been evaluated for the first time for classification of carrots according to their processing. Conventional blanching treatments at high temperature gave rise to carrots with retention of vitamin C in the range 37.5–85%, whereas carrots blanched conventionally at 60 °C and by US-probe at temperatures up to 60 and 70 °C showed vitamin C retention values lower than 4%. Regarding sensorial analysis of rehydrated carrots, US-pretreated samples presented acceptable quality, and no statistically significant differences with respect to conventionally blanched carrots, were detected. In spite of this, differentiation of samples processed under comparable intensity conditions and/or with similar composition was possible from their mass spectral fingerprints after chemometric data analysis.