Study On Ultrasound Assisted Processing Technology Of Tomato Products And Its Working Mechanism

Tomato(Lycopersicon esculentum)is one of the most popular vegetables in the world,with important nutritional and economic value.The consumption or intake of fresh tomato or tomato products has been found to be associated with risks reduction of various chronic diseases such as breast cancer,prostate cancer,cardiovascular disease and obesity.Those health-promoting effects of tomato and its products have been definitely attributed to the rich presence of carotenoids(lycopene,β-carotene,and lutein),phenolics,organic acids and vitamins.Tomato is the second most important vegetable with a global production around 170 millon tons.Only a small percentage of tomatoes are consumed as fresh.About 80%of tomatoes are processed into different types of products,including peeled tomatoes,tomato paste,tomato juice,tomato sauce,tomato powder,and lycopene,etc.As reported by the FAO,the tomato production of China has jumped to the top in the world.However,the tomato products of China are mainly exported in the form of concentrated tomato paste,which was a single form with low added value.In addition,the traditional processing technology and low processing precision of tomato products,caused some serious problems such as large loss of raw materials,poor product quality,and environmental pollution.Recently,the research has focused on adopting emerging technologies to replace or assist traditional processing technologies.As an emerging non-thermal technology,ultrasound technology has broad application in food industry because its low cost,low energy consumption and environmental friendliness.Based on the above-mentioned facts,the present study focused on the effects of ultrassound treatments on the component structures and quality properties of three different tomato products(peeled tomatoes,tomato juice and tomato paste).The main sections of this study are as follows:the effects and mechanism of ultrasound on the processing technology of peeled tomatoes;the effects and mechanism of cold ultrasound on the processing technology of fresh-squeezed tomato juice;the effects and mechanism of ultrasound on the cold break technology of tomato paste.The main findings are as follows:(1)Ultrasound post-assisted lye peeling regime for tomatoes could reduce the usage of lye while improve product yield and quality.(1)The method of ultrasound assisted lye peeling of tomatoes was established.Three varieties of tomatoes with mass varying from 15.64 to 126.66 g were selected to establish the method of tomato peeling evaluation.It was found that there was a significant linear correlation(R~2=0.965)existed between the mass(m,g)and surface area(A,cm~2)of tomatoes,and a linear function was constructed between these two parameters as A=0.810m+11.5.This equation attested that the overall surface area of tomatoes could be predicted using their mass data.It must be noted that this method produced precise quantitative peeling results.Tomatoes were selected and graded into three groups with their mass in the range of 30-40 g,50-60 g,and 70-80 g,respectively.The peeling performance were assessed after being processed in 10%(w/v)lye at 97oC for 45 s.The results showed that the peeling performance including peelability and peeling yield had no significant difference between different mass groups(p>0.05).Supported by this conclusion,the tomatoes were not size-graded prior to peeling in the following experiments.Under the same treatments,the peelability of tomatoes treated by the ultrasound treatment,lye treatment,ultrasound prior to lye treatment and lye prior to ultrasound treatment were10.72-11.75,30.31-34.10,63.39 and 95.44%,respectively.The ultrasound post-assisted lye was the best peeling method for tomatoes.It ascertained the synergistic effect of lye and ultrasound treatment during such two-step ultrasound post-assisted lye peeling of tomatoes.(2)The working parameters of ultrasound post-treatment(19.13-51.92 W/L,30-50 s)were first optimized,and then lye processing parameters(4-12%,10-30 s)were obtained on this basis.In view of energy conservation and environmental protection,the parameters of this two-step ultrasound-assisted lye peeling technique were concluded as follows:tomatoes were first processed in 4%(w/v)lye at 97oC for 30 s,and then further processed in 70oC water bath for 50 s at a ultrasound power of 31.97 W/L.(3)All ultrasound-assisted lye peeling procedure with 100%peelability presented the comparable peeling yields(92.12-94.12%),which were significantly higher than that of the conventional lye peeling method(82.77%).The lycopene content of tomatoes peeled by the present ultrasound-assisted lye peeling procedure(15.52-16.78 mg/100 g wb)was significantly higher than that of the conventional lye method(8.70 mg/100 g wb).Apparently,with the conventional lye peeling method,the yellowish vascular bundles could be seen clearly on the surface of the peeled tomatoes,indicating that most of the red layer disappeared.(4)Based on the evidence of water-soluble dye penetration,structural analysis,cell wall degradation and biomechanical analysis,the present two-step peeling method was concluded with a mechanism of chemico-mechanical synergism,in which the hot lye functions mainly in a chemical way while the ultrasound is a mechanical process.The peeling mechanism of tomatoes by the present two-step lye treatment plus an ultrasound treatment is summarized as follows.First,the hot lye dissolves the waxy crust and the waxes from the cuticle;Second,the lye penetrates across the de-waxed cuticle via a pitting model;Third,the penetrating lye degrades the cell wall materials and the middle lamella between the epidermal cells and the hypodermal cells and produces fissures at the boundary of the epidermal cell and hypodermal cell layers;Finally,the ultrasound expands the fissures and cracks the skins and leads to the separation of the epidermal layer from the fruits.The findings reported in this paper not only provide a novel tomato peeling method with significant environmental benefits but also discover new clues to the peeling mechanism using hot lye.(2)The cold ultrasound treatment at 10 ~oC could improve the physical stability,nutritional value and microbial safety of raw tomato juice.(1)The cold ultrasound treatment(CUT,87.52 W/cm~2,10 ~oC)was implemented to a raw tomato juice up to 30min.With the extension of CUT,the cloud stability of the juice substantially improved in the first 15 min,but finally remained stable within the remaining time of 30 min.However,upon the extension of CUT,the apparent viscosity of the juice firstly increased to a peak at 15 min and then decreased gradually.(2)The results of microscopic observations and particle analysis confirmed that CUT could induce a significant homogenizing effect by breaking down the cell structure of tomato juice upon CUT within 15 min.However,the CUT has no significant influence on the particles of tomato juice with the time above 15 min.The results of rheological properties showed that upon CUT for 20 min and 30 min,the higher physical stability of the viscosity-decreased juices should be a consequence of their higher resistance to shear thinning.Based on the results of physico-chemical properties of the pectin fractions from the serum of raw fruit juice,it could be conclude that the prolonged CUT(≥20 min)induced shear thinning resistance of the juice should be chiefly attributed to the degrations in the backbone of pectin molecules.(3)On a CUT time scale(0-30min),the total phenolic content of raw tomato juice continuously increased;the total plate count was adversely altered;the total carotenoids obtained shared a parabolic changing pattern but peaked at 10 min.Finally,the ascorbic acid sharply increased at an earlier stage(5 min),and then remained stable throughout the whole process.(4)Notably,the occurrences of these improvements are of spatial-temporal nature and resulted from different cavitation induced stress fields.At the initial stage(0-15min),CUT chiefly worked via the mechanical field with the particles in pulp phase,making them smaller and releasing the soluble materials into serum phase.However,the CUT has no significant influence on the particles of tomato juice with the time above 15 min.The CUT entered its second stage and mainly functioned in the serum phase via both mechanical and chemical fields.As a result,the serum pectin and carotenoids were depolymerized and degraded,respectively.Upon the accumulation of the radicals in the first stage,carotenoids were degraded more rapidly in the second stage.Therefore,the sacrifice of carotenoids was certainly accompanied with the depletion of radicals,thereby protecting the ascorbic acid,phenolics and polysaccharides from the oxidative degradation.This was partially supported by maintaining of ascorbic acid in the second stage.(5)More important,the current data attested preliminary that the transition of two stages is controlled by the largest size of particles in the juice and it was determined as approximately 160μm.(3)Ultrasound assisted cold break treatments could improve the viscosity and nutritional properties of tomato paste.(1)The activities of PME in tomato juice decreased by 37.26,100,8.28 and 76.70%after cold break,hot break,ultrasound break and ultrasound assisted cold break treatment,respectively.Meanwhile,the activities of PG in tomato juice decreased by 22.44,100,5.16 and 63.96%,respectively.This ascertained that ultrasound assisted cold break treatments was a more efficient way of enzyme inactivation than cold break treatments.(2)The apparent viscosity of cold break paste(CBP),hot break paste(HBP),ultrasound break paste(UBP)and ultrasound assisted cold break paste(CUBP)were 1295.13,2906.34,3788.57 and 5287.62 mPa·s.(3)The yield stress,consistency coefficient,linear viscoelastic behavior,nonlinear viscoelastic behavior and solid-like nature of the pastes shared the same trend,i.e.CUBP>UBP>HBP>CBP.While the ability of structural recovery had no significant difference(p>0.05)between different tomato pastes.(4)To uncover the truth governing the development of the viscosity and rheological properties of tomato paste,the changes of the particles phase and serum phase of the tomato paste were explored.For the side of particle phase,the particle size of four tomato paste was CUBP<UBP<HBP<CBP.This indicated that ultrasound treatment could result in a greater reduction in particle size than heat treatment,and the heat and ultrasound have a synergistic effect.For the side of serum phase,the content of water-soluble pectin in the serum of tomato paste was CUBP>UBP>HBP>CBP.The degree of methoxylation,weight-average molecular weight and the molar ratio of galacturonic acid which represented the linearity of pectin shared a same trends,ie HBP>CUBP>CBP≈UBP.Overall,the apparent viscosity,yield stress,consistency coefficient,linear viscoelastic behavior,nonlinear viscoelastic behavior and solid-like nature of the tomato paste were gradually increasing along with the particle size decreasing,the content and the backbone length of the pectin increasing.The effects of the particle size reduction and pectin content increase on the apparent viscosity and rheological properties of tomato pastes were larger than the pectin degradation by the enzyme.(5)Ultrasound-assisted cold break treatments could improve the nutritional value of tomato paste compared with traditional heat treating.The content of ascorbic acids of the CBP,HBP,UBP and CUBP was 113.93,83.52,83.25 and 133.98 mg/100 g DW,respectively.The content of phenolic of the CBP,HBP,UBP and CUBP was 419.76,444.3,494.23 and 456.55μg/g DW,respectively.The content of carotenoids of the CBP,HBP,UBP and CUBP was138.08,132.28,153.13,153.38 mg/100 g DW,respectively.The content of cis-carotenoids of the CBP,HBP,UBP and CUBP was 21.15,23.64,25.31 and 25.73mg/100 g DW,respectively.The antioxidant activities of the CBP,HBP,UBP and CUBP was UBP>CUBP>HBP≈CBP.The findings reported in this our study provide a novel ultrasound assisted cold break technology in tomato paste production with significant increasing viscosity and nutritional value.