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Advances in spectral techniques for fruit quality evaluation: case of ULF-NMR and NIRS

Isingizwe Nturambirwe, Jean Frederic (2017-03)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Industrial application of non-destructive analytical techniques still faces a challenge of lack of general and specialty models for quality evaluation. Current developments strive to alleviate this problem by the development of new cost effective equipment. In the food industry, and especially the horticultural industry, two spectroscopic methods seem to lead the way in terms of analytical variety, advancement in software for data handling and analysis, and relevance. The techniques in question are nuclear magnetic resonance (NMR) and near-infrared (NIR) spectroscopy. This project used practical experimental studies of fruit quality, using both techniques, to further research towards their non-destructive and online application, especially for horticultural products. A SQUID-NMR (Bm = 1Gauss) system was used to study the ripening of banana and predict its ripening index. Measurements of the NMR spin-lattice (T1) and the spin-spin (T2) relaxation times were acquired prior to destructive measurements. Various physico-chemical attributes were monitored for changes during the ripening process. Four out of six measurements, taken over a period of 10 days of storage (at 15 oC and 85%RH), were significantly different. Average T2 gave less promising results than average T1, that was highly correlated to attributes that changed during ripening, namely,lightness, L* (r=0.61), chromaticity coefficient, b* (r=0.65), totoal soluble solids, TSS (r= 0.72), sugar:acid ratio, TSS/TA (r=0.82), chromaticity coefficient, a* (r=0.84) and hue angle, h (r=-0.85). Correlations with T2 were found for TSS (r=-0.53), TSS/TA (-0.54) b* (-0.58) and pH (r=0.70), all significant at p<0.05. The ripening index was defined subjectively, based on the visual standards of the ripening index in banana. Average T1 distinctively explained the variance in ripening index, together with TSS, TSS/TA color parameters a* and h and total color difference. Calculation of the multicomponent distribution of T1 resulted in two components, one slow and another fast. Improvements in consistency of the transform is still required before it can be used for further analysis and accurate peak assignment. The results above show that there are opportunities of using SQUID detected NMR spectral data in the T1 domain for further studies of banana quality, and very likely other fruits as well. It is apparent that issues of temperature dependence of T1 should be taken into account in building more robust models. Fourier transform NIR (FT-NIR) was used in studying internal quality and mechanical damage in apples. Using three cultivars from two sources and three spectrometer modes from two FT-NIR spectrometers, we were able to account for the need of variability in building robust models. Different levels of predictability for each attribute were obtained for different cultivars, using PLS regression methods. The predictive ability was different between distinct spectral acquisition modes as well, but also depended on attributes. It was noticed that, in all scenarios considered (single exclusive and all inclusive cultivar or source), the emission head (EH) of the Matrix-F spectrometer led to similar model performances as for the integrating sphere (IS) of the multipurpose analyzer (MPA) FT-NIR spectrometer, in models predicting TSS. Model optimization was done successfully using both pre-processing methods and genetic algorithms applied on PLS of non-processed spectra. The influence of either cultivar or instrument on models predicting TA was different than that obtained for TSS and for TSS/TA, and overall, with lower model performance. Results revealed aspects that are likely to favor calibration transfers between the EH and IS acquisition modes. Bruising in apples is very common and quite intricate to detect if it is internal or not showing on the outside yet. NIR, mostly multispectral, hyperspectral imaging and visible spectrum (VIS) combined with NIR (VIS/NIR), have been used frequently to distinguish between bruised and sound tissue of apple fruits. It has been customary, as seen from many reports, that bruise studies by NIR calls for variable selection. The study carried out on bruise damage in this project involved variable selection by genetic algorithm, influenced by cultivar, and validated by a variable importance in projection (VIP) method, that used a different approach to the filter method. Favored wavebands were brought to light. Both methods were compared to the literature. This may serve as a good basis for further development towards online applications in the horticulture industry. Overall, advanced prospects in applications of the two most highly developed spectroscopic techniques to non-destructive fruit quality evaluation were identified and recommendations were given in light of possibilities for future industrial application.

AFRIKAANSE OPSOMMING: Industriële toepassings van nie-destruktiewe analitiese tegnieke is tans skaars a.g.v. ’n gebrek aan spesialiteitsmodelle vir kwaliteitevaluering. Huidige ontwikkelinge op hierdie gebied streef daarna om hierdie probleem op te los deur die ontwikkeling van koste effektiewe nuwe toerusting. In die kosbedryf, en spesifiek die vrugtebedryf, is daar twee spektroskopiese tegnieke wat tans die voortou neem t.o.v. analitiese verskeidenheid, die vooruitgang in programmatuur vir dataverwerking, en relevansie.Hierdie tegnieke is kern magnetiese resonasie (KMR) en naby-infrarooi (NIR) spektroskopie. In hierdie projek is praktiese eksperimentele studies op vrugte gedoen deur beide tegnieke te gebruik om hul toepaslikeid vir nie-destruktiewe toetsing en aanlyn toepassings op vrugte te bepaal. ’n SQUID-KMR (Bm=1Gauss) stelsel is gebruik om die rypwordingsproses van piesangs te bestudeer en om die rypwordingsindeks te bepaal. ’n Verskeidenheid van fisio-chemiese eienskappe is waargeneem gedurende die rypwordingsproses. Vier uit ses metings oor ’n tydperk van 10 dae van berging (15 C, 85%RH) was beduidend verskillend. Metings van die KMR uitsterftyd T1 (spin-latwerk) en T2 (spin-spin) is geneem voordat destruktiewe metings gedoen is. Gemiddelde T2-metings het minder belowende resultate gegee as T1-metings, wat hoogs gekorreleerd was t.o.v. eienskappe wat verander gedurende die rypwordingsproses, soos TA(r=-0.52), L* (r=0.61), b* (r=0.65), TSS (r= 0.72), TSS/TA (r=0.82), a* (r=0.84) and h (r=-0.85). Korrelasie met T2 is gevind vir TSS (r=-0.53), TSS/TA (-0.54) b* (-0.58) en pH (r=0.70), almal beduidend met p<0.05. Die rypwordingsindeks is subjektief bepaal, gebaseer op visuele standaarde van die rypwordingsineks van piesangs. Gemiddelde T1-metings kon die variansie in die rypwordingsindeks, saam met TSS, TSS/TA, kleurparameters a* en h en die totale kleurverskil, verklaar. Die bepaling van die multikomponent verspreiding van T1 het twee komponente tot gevolg gehad, een stadig en die die ander vinnig. Verbeteringe in die herhaalbaarhied van die transform is egter nodig voordat verdere analise en die betroubare indentifisering van pieke gedoen kan word. Die verkreë resultate wys egter dat daat geleenthede is vir die gebruik van SQUID-gemete KMR spektrale data in die T1-gebied vir verdere studies van die gehalte van piesangs, en waarskynlik ook ander vrugte. Dit is egter belangrik dat die temperatuurafhanklikheid van T1 in berekening gebring moet word wanneer meer robuuste modelle ontwikkel moet word. Fourier transform NIR metings is gebruik om die interne gehalte en meganiese skade in appels te bestudeer. Drie kultivars van twee verskillende bronne, en drie spektrometer modusse van twee FT-NIR spektrometers is gebruik om robuuste modelle te bou. Verskillende vlakke van voorspelbaarheid vir elke eienskap is verkry vir verskillende kultivars, deur van PLS regressiemetodes gebruik te maak. Die voorspelbaarheid was ook verskillend tussen die spesifieke verkrygingsmetodes, en was ook afhanklik van die eienskappe. Dit is waargeneem in al die scenario’s (enkel-inklusief en alles-inklusief, asook bron) dat die emissie kop (EK) van die Matrix-F spektrometer tot dieselfde model gedrag gelei het as vir die integrerende sfeer (IS) van die veeldoelige analiseerder (VDA) FT-NIR spektrometer, in alle modelle wat TSS voorspel het. Modeloptimisering is suksesvol gedoen deur beide voorafprosseringsmetodes en genetiese algoritmes toe te pas op PLS van niegeprosesseerde spektra. Die invloed van kultivar of die tipe instrument op modelle wat TA voorspel, was verkillend van modelle wat TSS en TSS/TA voorpspel, en in die geheel met slegter modelprestasie. Die resultate het getoon dat sekere aspekte waarskynlik voorkeur sal toon vir kalibrasie oordrag tussen EK en IS verkrygingsmodusse. Kneusing in appels kom dikwels voor en is moeilik om waar te neem, veral as die skade nog nie ekstern sigbaar is nie. NIR, meestal multispektraal, hiperspektraal en VIS/NIR, word dikwels gebruik om tussen beskadigde en nie-beskadigde weefsel in appels te onderskei. Dit blyk uit talle navorsingsverslae dat dit gebruiklik is om veranderlike seleksie te doen met NIR. In hierdie studie op kneusingskade is veranderlike seleksie deur genetiese algoritmes, soos beïnvloed deur kultivar, gedoen en gevalideer deur die veranderlike belangrikheid in projeksie (VBP) metode, wat ’n ander benadering is as die filtermetode. Voorkeur golflengtebande is hieruit geïdentifiseer. Beide metodes is met die literatuur vergelyk. Die navorsing is ’n goeie basis vir toekomstige ontwikkelinge in aanlyntoepassings in die vrugtebedryf. Addisionele gevorderde toepassings van twee hoogsontwikkelde spektroskopiese tegnieke om nie-destruktiewe evaluasie van vrugtegehalte te doen, is geïdentifiseer. Aanbevelings vir toekomstige industrietoepassings is gemaak aan die hand van die belowende nuwe toepassings.

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