張 鵬,袁興鈴,薛友林,賈曉昱,李江闊
精準(zhǔn)溫度控制對枸杞鮮果貯藏品質(zhì)和香氣成分的影響
張 鵬1,2,袁興鈴3,薛友林3,賈曉昱1,2,李江闊1,2※
(1. 天津市農(nóng)業(yè)科學(xué)院農(nóng)產(chǎn)品保鮮與加工技術(shù)研究所,天津 300384;2. 國家農(nóng)產(chǎn)品保鮮工程技術(shù)研究中心(天津),農(nóng)業(yè)農(nóng)村部農(nóng)產(chǎn)品貯藏保鮮重點(diǎn)實(shí)驗(yàn)室,天津市農(nóng)產(chǎn)品采后生理與貯藏保鮮重點(diǎn)實(shí)驗(yàn)室,天津 300384;3. 遼寧大學(xué)輕型產(chǎn)業(yè)學(xué)院,沈陽 110036)
為研究精準(zhǔn)溫控技術(shù)在枸杞鮮果保鮮中的應(yīng)用效果,將枸杞分別裝入泡沫箱(Control Check,CK)、泡沫箱+蓄冷劑(冰溫)和精準(zhǔn)溫控箱+蓄冷劑(相溫)中,記錄箱內(nèi)溫度,結(jié)合貯藏期間枸杞的品質(zhì)變化,通過主成分分析法對3組枸杞進(jìn)行綜合評價(jià),并對不同處理香氣成分進(jìn)行分析。結(jié)果表明,精準(zhǔn)溫度控制后箱體內(nèi)部溫度更低,且溫度波動(dòng)保持在0.1 ℃以內(nèi),CK組、冰溫組、相溫組枸杞出現(xiàn)發(fā)霉腐爛現(xiàn)象的時(shí)間分別為10、20、40 d;貯藏40 d后,相溫組枸杞的腐爛率為4.11%,遠(yuǎn)低于CK組的14.85%,色差低于3,亮度達(dá)34.12,可溶性固形物含量、可滴定酸含量、維生素C含量、類胡蘿卜素含量分別較CK組高0.65個(gè)百分點(diǎn)、0.03個(gè)百分點(diǎn)、4.34 mg/100g、2.90 mg/g,相溫組枸杞有效抑制腐爛率和失重率的增加,延緩色差的升高和亮度的降低,保持較高的可溶性固形物、可滴定酸、維生素C和類胡蘿卜素含量,經(jīng)主成分得分分析后得到3組枸杞的品質(zhì)排序?yàn)橄鄿亟M、冰溫組、CK組,且精準(zhǔn)溫度控制貯藏的枸杞醛類、萜類等有利香氣成分相對含量更高。因此,精準(zhǔn)溫度控制技術(shù)有利于枸杞鮮果的冷藏保鮮,其中相溫貯藏效果更好。研究結(jié)果為枸杞鮮果精準(zhǔn)溫控保鮮提供參考。
貯藏;品質(zhì)控制;枸杞;精準(zhǔn)溫控;香氣成分
枸杞(L.),茄科枸杞屬,漿果呈紅色、卵狀,原產(chǎn)中國北部。枸杞內(nèi)含化學(xué)成分種類繁多,含有豐富的枸杞多糖、微量元素、維生素、脂肪醇類及無機(jī)鹽等[1],藥用價(jià)值和保健作用極高[2]。枸杞鮮果皮薄肉嫩,水分含量高,常溫條件下短時(shí)間內(nèi)就會(huì)變色、變味,失去食用價(jià)值[3],因此枸杞鮮果的貯藏保鮮技術(shù)是限制其產(chǎn)業(yè)發(fā)展的重要因素之一。
溫度在果蔬貯藏中對其品質(zhì)影響極大,基于此控制果蔬貯藏期間的環(huán)境溫度可有效延長果蔬的貯藏期[4]。在不同的控溫方式中,低溫冷藏是應(yīng)用最廣泛的一種保鮮方式。趙游麗等[5]對比常溫(20 ℃)和低溫(1、8 ℃)下枸杞果實(shí)維生素C含量和相對電導(dǎo)率的差異,表明低溫有利于枸杞保鮮,1 ℃保鮮效果優(yōu)于8 ℃。而精準(zhǔn)溫控技術(shù)即為基于低溫冷藏提出的一種新型保鮮技術(shù),其包括了冰溫貯藏和相溫貯藏。冰溫貯藏是指將貯藏溫度控制在0 ℃和果蔬冰點(diǎn)之間,在此溫度段內(nèi)果蔬組織細(xì)胞仍保持活性,但生理代謝將受到抑制,從而達(dá)到保鮮的目的[6]。李齊等[7]通過對枸杞品質(zhì)和生理變化分析得出,冰溫貯藏能夠提升果實(shí)品質(zhì)并抑制生理活動(dòng),與普通冷藏相比,差異性指標(biāo)為腐爛率、呼吸強(qiáng)度和相對電導(dǎo)率。相溫貯藏則是在冰溫貯藏的基礎(chǔ)上將溫度波動(dòng)對果蔬引起的外界刺激降到最低,使果蔬在貯藏期間品質(zhì)更加穩(wěn)定[8],但相溫貯藏在枸杞中的應(yīng)用尚未見報(bào)道。蓄冷劑是通過相變蓄冷技術(shù)采用蓄冷材料生產(chǎn)出來的既能高效儲存冷量,又符合各種物理、化學(xué)要求的特種物質(zhì)[9],已在農(nóng)產(chǎn)品保鮮等領(lǐng)域得到了廣泛應(yīng)用。鄧改革等[10]基于枸杞生物特性,設(shè)計(jì)了小型鹽水蓄冷式帶柄鮮枸杞真空預(yù)冷裝置,但蓄冷劑在枸杞中的應(yīng)用研究較少,目前蓄冷劑在櫻桃、葡萄、油菜等[11-15]果蔬保鮮上也取得了一定的作用效果。精準(zhǔn)溫控箱(可蓄冷型溫控箱)是以發(fā)泡聚丙烯(Expandabled Polypropylene,EPP)為材質(zhì),在具有良好的保溫性能外,還具有抗震減震、抗沖擊、強(qiáng)度高、可循環(huán)利用等特點(diǎn)[16]。目前,大多研究集中在精準(zhǔn)溫控箱(加入蓄冷劑)中溫度場的數(shù)據(jù)模擬或保溫性能研究[17-18],在果蔬中保鮮效果研究鮮有報(bào)道。
目前冰溫庫及相溫庫的運(yùn)行成本較高,較難將冰溫庫和相溫庫推廣運(yùn)行,因此本文設(shè)計(jì)將蓄冷劑與普通冷藏庫相結(jié)合營造出冰溫貯藏的環(huán)境,將蓄冷劑與精準(zhǔn)溫控箱同時(shí)使用并結(jié)合普通冷藏庫營造相溫貯藏的環(huán)境,并將此種精準(zhǔn)溫度控制技術(shù)應(yīng)用于枸杞的貯藏保鮮中,探究其對枸杞鮮果貯藏品質(zhì)和香氣成分的影響,以期為枸杞產(chǎn)業(yè)發(fā)展提供一定參考。
1.1.1 試材
“青龍”枸杞,九成熟,產(chǎn)自河北秦皇島,選擇大小均勻、無病蟲害、無機(jī)械損傷的果實(shí)作為試驗(yàn)用果。聚苯乙烯(Expandable Polystyrene,EPS)泡沫箱(600 mm×451 mm×230 mm,壁厚30 mm,四川包工坊電子商務(wù)有限責(zé)任公司),EPP精準(zhǔn)溫控箱(規(guī)格:595 mm×400 mm×250 mm,壁厚30 mm,上海佳寰實(shí)業(yè)有限公司),聚對苯二甲酸乙二醇酯(Polyethylene Terephthalate,PET)塑料盒(180 mm×140 mm×78 mm,果味衣塑料制品有限公司)。蓄冷劑,自制(原料為甘露醇、氯化鈉、四硼酸鈉、高吸水性樹脂和蒸餾水),裝入168 mm×80 mm×20 mm的冰盒(材質(zhì)為高密度聚乙烯)中凍結(jié)使用。
1.1.2 儀器與設(shè)備
差示掃描量熱儀(Q-1000),美國TA儀器公司;溫濕度記錄儀(179-UTH),艾普瑞(上海)精密光電有限公司;離心機(jī)(3-30K),德國SIGMA公司;便攜式手持折光儀(PAL-1),日本愛宕公司;電位滴定儀(916 Ti-Touch),瑞士萬通中國有限公司;電子稱(KF-568),中國·凱豐集團(tuán);色差計(jì)(CM-700d),日本柯尼卡美能達(dá)公司;多功能微孔板檢測儀(Synergy H1),美國Biotek Instrument公司;氣相色譜-質(zhì)譜聯(lián)用儀及SPME Fiber萃取手柄,美國Thermo公司;PDMS/CAR/DVB萃取頭,北京康林科技有限責(zé)任公司。
1.2.1 試驗(yàn)處理
將枸杞分裝于小盒中,每盒500 g果實(shí),然后放置于(0±1)℃冷庫中,打開頂蓋預(yù)冷24 h后將蓋子蓋上,置于泡沫箱或精準(zhǔn)溫控箱中貯藏,同時(shí)放入溫濕度記錄儀進(jìn)行溫度監(jiān)測,分為CK組、冰溫組、相溫組。CK組:置于泡沫箱中;冰溫組:置于泡沫箱中,加入果實(shí)與蓄冷劑質(zhì)量比為3:8的蓄冷劑;相溫組:置于精準(zhǔn)溫控箱中,加入果實(shí)與蓄冷劑質(zhì)量比為3:8的蓄冷劑。預(yù)冷結(jié)束后選取3盒果實(shí)進(jìn)行測定,另外冷庫中有3個(gè)處理共計(jì)36盒果實(shí)(每個(gè)處理重復(fù)3次),每個(gè)處理每隔10 d取出3盒果實(shí)進(jìn)行指標(biāo)測定。
1.2.2 指標(biāo)測定
蓄冷劑制備后采用差示掃描量熱儀(Differential Scanning Calorimeter,DSC)進(jìn)行熱物性測定,參照傅一波等[19]的方法,得到蓄冷劑的凝固曲線、融化曲線、過冷點(diǎn)及共晶點(diǎn),使用儀器自帶軟件進(jìn)行處理,融化曲線與基線所構(gòu)成的峰面積即為相變潛熱,融化曲線中吸熱峰的最大斜率與基線相交得到溫度即為onset溫度(相變溫度),共晶點(diǎn)減去過冷點(diǎn)為過冷度。將自制蓄冷劑與水凍結(jié)后放入裝有等量枸杞的泡沫箱中密封監(jiān)測溫度,進(jìn)行蓄冷劑實(shí)載控溫效果測定。
每個(gè)處理隨機(jī)稱取400 g左右果實(shí)進(jìn)行感官評定,腐爛率(%)= 爛果質(zhì)量/總果質(zhì)量×100;失重率(%)=(貯藏后果實(shí)質(zhì)量-貯藏前果實(shí)質(zhì)量)/貯藏前果實(shí)質(zhì)量×100。
果皮色澤采用手持式色差計(jì)進(jìn)行測定[20];可溶性固形物含量采用手持折光儀測定[21];可滴定酸含量采用電位滴定儀進(jìn)行測定[22];維生素C含量采用鉬藍(lán)比色法[23]進(jìn)行測定;類胡蘿卜素含量參照李合生[24]的方法進(jìn)行測定。
香氣成分分析采用頂空固相微萃取-氣相色譜-質(zhì)譜(Head Space Solid-Phase Microextraction with Gas Chromatography-Mass Spectrometry,HS-SPME-GC-MS)法測定,將枸杞打漿后離心10 min(10 000 r/min),取8 mL上清液于15 mL頂空瓶中,向頂空瓶中加入2.5 g NaCl,采用有孔蓋蓋上。采用Thermo Triplus RSH自動(dòng)進(jìn)樣裝置進(jìn)行固相微萃取及進(jìn)樣操作,選用50/30m PDMS/CAR/DVB萃取頭。氣相色譜條件:HP-INNOWAX色譜柱(長30 m,內(nèi)徑0.25 mm,液膜厚度0.25m);程序升溫:40 ℃保留2 min,然后以3 ℃/min升至160 ℃,接著以10 ℃/min升至210 ℃并保持2 min;傳輸線溫度250 ℃,載氣為He,流速為1.0 mL/min,不分流。質(zhì)譜條件:連接桿280 ℃,采用電子離子源,離子源200 ℃,掃描范圍質(zhì)荷比(/)為45~450。
香氣成分結(jié)果通過NIST/Wiley標(biāo)準(zhǔn)譜庫檢索,進(jìn)行定性分析,并用峰面積歸一法測算各化學(xué)成分的相對含量;圖表制作通過Excel 2010進(jìn)行;差異顯著性分析通過DPS 7.5軟件LSD法進(jìn)行(<0.05代表差異顯著);主成分得分分析通過SPSS 25軟件進(jìn)行。
圖1所示為DSC測定所導(dǎo)出的水及蓄冷劑的熱物性圖像,從圖中可以得到水及蓄冷劑的熱物性參數(shù),包括相變潛熱、onset溫度、過冷點(diǎn)和共晶點(diǎn)。經(jīng)測定,水及蓄冷劑的相變潛熱分別為305.5、241.1 J/g,所制蓄冷劑的相變潛熱低于水,可能是因?yàn)樗菩罾鋭榛旌衔矬w系,且其屬于潛熱蓄冷材料,而水屬于顯熱蓄冷材料,且是一種純凈物,其融化過程為突變的固液轉(zhuǎn)化過程[25]。水及蓄冷劑的onset溫度分別為2.07、-4.81 ℃,而物料往往在onset溫度以上才會(huì)開始大量吸熱[19],因此相比而言所制蓄冷劑的onset溫度更適用于果蔬貯藏。通過過冷點(diǎn)和共晶點(diǎn)計(jì)算得到水和蓄冷劑的過冷度分別為3.19、4.09 ℃,差距較小,說明蓄冷劑中不同成分對其過冷現(xiàn)象的影響較小。圖2所示為水和蓄冷劑的實(shí)載控溫曲線,從圖中可以看出監(jiān)測期間自制蓄冷劑所在的泡沫箱中環(huán)境溫度均低于冰所在的泡沫箱,在第20 h時(shí),冰所在泡沫箱的環(huán)境溫度已達(dá)室溫,而自制蓄冷劑所在泡沫箱環(huán)境溫度仍低于室溫,說明自制蓄冷劑的控溫效果優(yōu)于冰。
注:CK組為普通冷藏組,下同。
圖3為枸杞貯藏期間不同貯藏環(huán)境的內(nèi)部溫度變化曲線,從曲線中可以看出,冰溫組和相溫組的溫度波動(dòng)弱于CK組,且溫度更低。在整個(gè)貯藏期間CK組、冰溫組和相溫組箱內(nèi)溫度范圍分別為(0.12±0.17)、(-0.04±0.07)、(-0.05±0.04) ℃,精準(zhǔn)溫度控制后箱體溫度波動(dòng)低于0.1 ℃,說明蓄冷劑的加入為箱內(nèi)引入了冷源,使箱內(nèi)溫度更低,且冷源的存在使外界溫度的變化對箱內(nèi)溫度的影響變??;精準(zhǔn)溫控箱質(zhì)地緊密,可緩沖外界溫度變化對箱內(nèi)溫度的影響,維持箱內(nèi)低溫狀態(tài),精準(zhǔn)溫控箱中加入蓄冷劑后,雙重作用下使得箱內(nèi)溫度控制更加精準(zhǔn)。因此,基于冰溫貯藏和相溫貯藏降低溫度及減緩溫度波動(dòng)的理論,蓄冷劑和精準(zhǔn)溫控箱的使用可有效達(dá)到枸杞貯藏所需的冰溫貯藏及相溫貯藏環(huán)境。
2.3.1 感官圖片
從圖4可以看出,在貯藏初期果實(shí)的外觀良好,未出現(xiàn)發(fā)霉情況。隨著貯藏期的延長,果實(shí)的感官狀態(tài)逐漸變差,其中CK組果實(shí)在10 d時(shí)開始出現(xiàn)發(fā)霉現(xiàn)象,到40 d時(shí)腐爛發(fā)霉現(xiàn)象嚴(yán)重,冰溫組的果實(shí)從20 d起開始出現(xiàn)果實(shí)腐爛發(fā)紅的現(xiàn)象,40 d時(shí)有較多霉變果,相溫組果實(shí)在40 d時(shí)才出現(xiàn)了個(gè)別發(fā)霉現(xiàn)象,說明冰溫貯藏和相溫貯藏均有利于枸杞冷藏期間感官品質(zhì)的保持,且相溫貯藏后效果更佳,可有效延長枸杞貯藏期至40 d。
2.3.2 不同貯藏環(huán)境對枸杞感官品質(zhì)的影響
圖5a、5b顯示的是3種貯藏環(huán)境下枸杞的感官品質(zhì)指標(biāo)(腐爛率、失重率)在40 d貯藏期內(nèi)的變化曲線。從圖中可以看出隨著貯藏期的延長,3組枸杞果實(shí)的腐爛率、失重率均呈顯著(<0.05)增加趨勢,表明枸杞貯藏期間感官品質(zhì)在下降,但冰溫和相溫貯藏的枸杞感官品質(zhì)下降較為緩慢,且冰溫組在貯藏前20 d內(nèi)枸杞感官品質(zhì)尚好,3組枸杞的腐爛率在5%以下,到第30天時(shí)CK組開始上升至5%以上,到第40天時(shí)CK組果實(shí)腐爛率甚至上升到了14.85%,而冰溫組和相溫組果實(shí)腐爛率分別為7.80%、4.11%。3組處理枸杞腐爛率差異顯著(<0.05),說明冰溫貯藏及相溫貯藏對于枸杞貯藏期間感官品質(zhì)的保持均發(fā)揮了有效作用,且相溫貯藏效果顯著優(yōu)于冰溫貯藏(<0.05)。相溫組枸杞貯藏期間腐爛率均低于5%,具有良好的商品性,有效延長枸杞貯藏期至40 d。枸杞鮮果作為一種小漿果,貯藏期間易發(fā)生質(zhì)量損失[26]。從圖5b可以看出,CK組和冰溫組在貯藏第20天開始有質(zhì)量損失,失重率分別為0.26%和0.10%,而相溫組枸杞在貯藏第30天時(shí)開始有質(zhì)量損失,失重率為0.06%,到第40天時(shí)3組枸杞失重率分別為0.92%、0.58%、0.15%,差異顯著(<0.05),相溫組失重率顯著低于其他處理(<0.05),由此可見使用精準(zhǔn)溫控箱+蓄冷劑貯藏對維持枸杞質(zhì)量有較好效果,可能是由于精準(zhǔn)溫控箱質(zhì)地緊密,能有效防止水分的流失,蓄冷劑進(jìn)一步減少環(huán)境溫度的波動(dòng),進(jìn)而體現(xiàn)為失重率較低,但3組枸杞40 d貯藏期間失重率均低于1%,說明低溫貯藏條件下枸杞的失重現(xiàn)象不明顯。
在色澤數(shù)據(jù)中,Δ代表顏色變化,值越大代表顏色變化越大;值代表光澤明亮度,值越大,亮度越高。從圖5c、5d可以看出隨著貯藏期的延長,枸杞果實(shí)的Δ值呈顯著(<0.05)上升趨勢,其中相溫組枸杞Δ值在40 d貯藏期間低于3,而CK組和冰溫組果實(shí)Δ值在30 d時(shí)分別為3.08和3.22,均高于3,而相溫組Δ值低于3,與CK組和冰溫組差異顯著(<0.05)。值整體呈顯著(<0.05)下降趨勢,40 d貯藏期間CK組、冰溫組及相溫組枸杞的值分別下降了2.24、1.69、1.56,貯藏40 d時(shí)CK組與另外2組的下降幅度之間差異顯著(<0.05),但冰溫組及相溫組枸杞值分別為33.99、34.12,差異不明顯(>0.05),總體來說,冰溫貯藏和相溫貯藏均有利于枸杞貯藏期間色澤的保持,且相溫貯藏效果更佳。
可溶性固形物(Total Soluble Solids,TSS)含量與果實(shí)的口感和營養(yǎng)品質(zhì)密切相關(guān)。圖6顯示枸杞貯藏期間TSS含量呈下降趨勢,且3組枸杞貯藏前30 d的TSS含量下降緩慢,到第40天時(shí)才有較明顯的變化,貯藏40 d時(shí)CK組、冰溫組及相溫組枸杞的TSS含量分別為13.83%、14.32%、14.48%,其中冰溫組和相溫組TSS含量分別比CK組高0.49個(gè)百分點(diǎn)、0.65個(gè)百分點(diǎn),CK組與另外2組間有顯著差異(<0.05),而冰溫組及相溫組枸杞的TSS含量下降幅度雖呈遞減趨勢,但差異不顯著(>0.05)。可滴定酸(Titratable Acid,TA)含量呈下降趨勢,貯藏20 d時(shí),CK組、冰溫組和相溫組枸杞可滴定酸含量沒有顯著差異(>0.05),貯藏30 d時(shí)相溫組枸杞可滴定酸含量差異顯著高于CK組和冰溫組(<0.05),冰溫組枸杞可滴定酸含量高于CK組(>0.05),貯藏40 d時(shí)CK組、冰溫組及相溫組枸杞的可滴定酸含量分別為0.27%、0.28%、0.30%,其中冰溫組和相溫組TA含量分別比CK組高0.01個(gè)百分點(diǎn)、0.03 個(gè)百分點(diǎn),相溫組枸杞TA含量最高,與其他2組差異顯著(<0.05),表明冰溫貯藏和相溫貯藏均有利于枸杞TA含量的保持,相溫組抑制枸杞TA含量下降效果更好。貯藏期間維生素C(Vitamin C,Vc)含量呈下降趨勢,且相溫組枸杞Vc含量下降最為緩慢,在貯藏20 天時(shí),相溫組枸杞的VC含量是CK組的2.55 倍、冰溫組的1.98倍,達(dá)顯著性水平(<0.05);貯藏40 天時(shí),CK組、冰溫組及相溫組枸杞的VC含量分別為8.24、8.57、12.58 mg/100 g,其中冰溫組和相溫組VC含量分別比CK組高0.33、4.34 mg/100 g,這與Zhang等[27]的結(jié)果類似,可能是因?yàn)殍坭街械目箟难嵫趸富钚栽诘蜏丨h(huán)境中受到了抑制,且相溫條件下酶活性更穩(wěn)定,從而在一定程度上減緩枸杞中抗壞血酸含量的損失[28]。類胡蘿卜素是構(gòu)成枸杞色澤的重要營養(yǎng)成分[29],從圖6 d可以看出,枸杞貯藏期間類胡蘿卜素含量呈下降趨勢,在貯藏前20 d內(nèi),相溫組枸杞的類胡蘿卜素含量為CK組和冰溫組的2倍以上,貯藏30 d時(shí),相溫組枸杞的類胡蘿卜素含量顯著高于CK組和冰溫組(<0.05),貯藏40 d時(shí),CK組、冰溫組及相溫組枸杞的類胡蘿卜素含量分別為3.20、5.44、6.10 mg/g,其中冰溫組和相溫組類胡蘿卜素含量分別比CK組高2.24、2.90 mg/g,說明精準(zhǔn)溫控條件下貯藏的枸杞類胡蘿卜素含量的下降將在較短時(shí)間內(nèi)得到控制,且相溫貯藏的抑制效果最好。
表1 主成分的特征值及貢獻(xiàn)率
表2 主成分得分表
表3為不同貯藏環(huán)境下枸杞香氣成分的變化情況。從表3中可以看出枸杞香氣成分中醛類、萜類和醇類物質(zhì)占較大比例,在整個(gè)貯藏期內(nèi)相對含量分別為11.66%~47.70%、19.19%~34.75%及17.90%~22.60%。醛類物質(zhì)相對含量呈先升高后降低的趨勢,其中CK組果實(shí)中的醛類相對含量低于另外2組,貯藏40 d時(shí),CK組、冰溫組、相溫組枸杞中醛類物質(zhì)的相對含量分別為11.66%、19.68%、27.42%,相溫組醛類物質(zhì)是CK組的2.35倍、冰溫組的1.39倍,說明冰溫貯藏及相溫貯藏均可促進(jìn)枸杞香氣成分中醛類物質(zhì)的釋放,且相溫貯藏效果更佳。CK組枸杞萜類物質(zhì)相對含量呈下降趨勢,冰溫組枸杞萜類物質(zhì)相對含量變化較小,相溫組枸杞萜類物質(zhì)相對含量整體呈上升趨勢,貯藏40 d時(shí),CK組、冰溫組、相溫組枸杞中萜類物質(zhì)分別為22.16%、24.64%、34.75%,相溫組萜類物質(zhì)是CK組的1.57倍、冰溫組的1.41倍,說明降低溫度有利于維持枸杞貯藏期間萜類物質(zhì)相對含量,而相對恒溫環(huán)境則可促進(jìn)枸杞萜類物質(zhì)的釋放。枸杞香氣成分中醇類物質(zhì)貯藏期間變化較小,不同處理貯藏期間枸杞醇類物質(zhì)相對含量變化規(guī)律不強(qiáng)。
從表3還可以看出,貯藏期間共檢出23種物質(zhì),包括醛類(6種)、萜類(3種)、醇類(8種)等,CK組、冰溫組、相溫組各檢出22、22、19種物質(zhì),使用精準(zhǔn)溫控箱貯藏的枸杞香氣成分更集中,主要香氣成分按相對含量排序?yàn)槟⒐酱迹?9.19%~34.75%)、3,4-二甲基-1-庚烷(4.83%~31.75%)、2-己烯醛(4.85%~31.04%)、苯甲醇(11.05%~13.19%,甜的果香和花香香氣)、苯乙醛(4.17%~6.84%,風(fēng)信子香味)。其中,蘑菇醇是一種具有蘑菇香味的萜類物質(zhì)[30],從表3可以看出,貯藏期間冰溫組和相溫組枸杞中蘑菇醇相對含量高于CK組,在第40天時(shí),CK組、冰溫組、相溫組枸杞中蘑菇醇的相對含量分別為21.76%、24.64%、34.75%,相溫組枸杞中蘑菇醇相對含量最高,為CK組的1.60倍、冰溫組的1.41倍。2-己烯醛具有濃郁的綠葉清香和果香[31],貯藏期間精準(zhǔn)溫控貯藏的枸杞中2-己烯醛相對含量總體上高于CK組,在第20 天時(shí),CK組、冰溫組、相溫組枸杞中2-己烯醛的相對含量為9.50%、16.77%、27.75%,分別為CK組2.90倍、冰溫組的1.66倍。此外,本次香氣成分分析還檢出了3-甲硫基丙醛,這是一種含硫化合物,可能是由于其存在于枸杞籽中,打漿時(shí)枸杞籽被破壞,使3-甲硫基丙醛釋放在枸杞漿液中[32]。
表3 不同貯藏環(huán)境下枸杞的香氣成分
注:“-”代表未檢測到。 Note: “-” means not detected.
溫度是影響果蔬保鮮效果的重要因素之一,適宜的低溫可以有效延長其貯藏期[33]。近年來,隨著冷藏設(shè)施的不斷完善,在考慮貯藏溫度高低的同時(shí),而更加重視貯藏期間溫度的波動(dòng)。研究表明,貯藏環(huán)境中溫度的波動(dòng)會(huì)引起果蔬的內(nèi)部冰晶發(fā)生變化并再結(jié)晶,容易引起冷害,另外還會(huì)加速其生理代謝,致使貯藏品質(zhì)劣變[34]。賈曉昱等[35]考察了(14±0.05)、(14±0.5)、(14±1) ℃ 3個(gè)溫度波動(dòng)下四川仔姜生理變化和中篡改品質(zhì)的影響,研究表明波動(dòng)越小其貯藏品質(zhì)越好,能夠顯著減少自由基的積累并抑制丙二醛升高。本文使用自制蓄冷劑和精準(zhǔn)溫控箱模擬了枸杞鮮果冰溫貯藏和相溫貯藏的環(huán)境,結(jié)果表明CK組、冰溫組和相溫組箱內(nèi)溫度分別為(0.12±0.17)、(-0.04±0.07)、(-0.05±0.04) ℃,精準(zhǔn)溫度控制后箱體溫度波動(dòng)低于0.1 ℃,自制蓄冷劑結(jié)合精準(zhǔn)溫控箱營造的相溫溫度環(huán)境更加精準(zhǔn)。
相溫貯藏(-0.4±0.1)℃可以抑制蘭州百合的呼吸強(qiáng)度和乙烯生成速率,延緩果實(shí)硬度的下降同時(shí),保留了較高的營養(yǎng)成分,抑制醇類的揮發(fā)、促進(jìn)醛類等香氣成分的釋放[8]。本文研究結(jié)果表明,與對照組相比,冰溫和相溫貯藏均有效抑制了枸杞鮮果貯藏期間感官品質(zhì)和營養(yǎng)品質(zhì)的下降,有效延長枸杞鮮果的貯藏期。與冰溫貯藏相比,相溫貯藏顯著抑制腐爛率和失重率的增加(<0.05),顯著延緩貯藏40 d時(shí)色差的增加、可滴定酸和維生素C含量的下降,顯著維持貯藏10~30 d時(shí)枸杞類胡蘿卜素含量,可有效延長枸杞貯藏期至40 d;貯藏40 d時(shí),醛類物質(zhì)、萜類物質(zhì)相對含量分別是冰溫組的1.39、1.41倍,蘑菇醇、2-己烯醛相對含量分別是冰溫組的1.41、1.66倍,維持枸杞較高的醛類、萜類等有利香氣成分相對含量,促進(jìn)蘑菇醇、2-己烯醛等特征香氣成分的揮發(fā)。另外,隨著貯藏時(shí)間的延長,枸杞由于生理代謝消耗和衰老導(dǎo)致失重率不斷增加、果實(shí)表面顏色逐漸轉(zhuǎn)色,而相溫貯藏能夠延緩失重率的升高,抑制果實(shí)和表面色差的增加,這與真空預(yù)冷對鮮枸杞失重率和色差影響基本一致[36]。在冷庫中,通過精準(zhǔn)溫控箱結(jié)合蓄冷劑,能夠避免由于除霜而引起的溫度波動(dòng),維持貯藏環(huán)境溫度的穩(wěn)定,這樣可以減少冰溫庫、相溫庫的運(yùn)行成本,也能滿足冷鏈物流智能包裝的需求,具有很好的應(yīng)用前景。
1)精準(zhǔn)溫控箱結(jié)合蓄冷劑(相變潛熱241.1 J/g、onset溫度-4.81 ℃)形成的精準(zhǔn)溫度控制(相溫)環(huán)境,貯藏溫度為(-0.05±0.04) ℃,比(0.12±0.17) ℃普通冷藏和(-0.04±0.07) ℃冰溫貯藏環(huán)境的溫度波動(dòng)更小。
2)與冰溫貯藏相比,相溫貯藏顯著延緩枸杞鮮果腐爛率和失重率的增加(<0.05),顯著抑制貯藏40 d時(shí)色差的增加以及可滴定酸、維生素C含量的下降,保持貯藏10~30 d時(shí)枸杞類胡蘿卜素含量,提高貯藏期間果實(shí)的品質(zhì),貯藏期延長至40 d。
3)枸杞主要香氣成分由蘑菇醇(蘑菇香味)、2-己烯醛(濃郁的綠葉清香和果香)、苯甲醇(甜的果香和花香香氣)、苯乙醛(風(fēng)信子香味)等物質(zhì)構(gòu)成。相溫貯藏維持枸杞較高的醛類、萜類等有利香氣成分相對含量,同時(shí)促進(jìn)蘑菇醇、2-己烯醛等特征香氣成分的揮發(fā)。
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Effects of precise temperature control on the storage quality and aroma components of fresh goji fruit
Zhang Peng1,2, Yuan Xingling3, Xue Youlin3, Jia Xiaoyu1,2, Li Jiangkuo1,2※
(1.,,300384,; 2.,,,(),300384,; 3.,,110036,)
Goji (L.) is one of the most popular medicinal plants, particularly as a typical nutritional fruit supplement rich with bioactive compounds. However, fruits browning, taste loss, and storage intolerance often occur during postharvest, because of tender and juicy tissue. Among them, the temperature is one of the most significant factors to maintain the quality and extend the shelf life of fruits. It is necessary to place fruits in a constant temperature environment after harvest since temperature fluctuations and variation tend to induce product quality deterioration. This study aims to investigate the effect of precise temperature control on storage quality and aroma components of fresh goji fruits. Three treatments of storage temperature at 0℃ were also set, including foam box (CK), foam box + cool storage agent (ice temperature), as well as the precise temperature control box and cool storage agent (phase temperature). The temperature was real-time recorded in different cabinets to evaluate the storage quality. Principal component analysis (PCA) was used to evaluate the aroma components of goji.Results showed that the foam box containing the self-made refrigerant performed better at ambient temperature, compared with that with the ice. Temperatures in the box of CK, ice, and phase temperature group were (0.12±0.17), (-0.04±0.07) and (-0.05±0.04)℃, respectively, during the whole storage period. A cold accumulator provided a cold source in the box to maintain a low-temperature environment.The temperature distribution was more uniform in the precision temperature control box, while the maintenance time of low temperature was longer than before, due to the excellent performance of heat preservation. The time of mildew and rot in the CK group, ice, and phase temperature group were 10, 20, and 40 d, respectively. At the storage of 40 d, the rot rate in the phase temperature group were 4.11%, much lower than 14.85% in the CK group. Chromatic aberration Δwas lower than 3, while Brightnessvalue reached 34.12. The content of soluble solid, titratable acid, Vitamin C conten, carotenoid were 0.65 percentage, 0.03 percentage, 4.34 mg/100g, 2.90 mg/g higher than CK group, respectively. Correspondingly, the treatment in the phase temperature group inhibited the increase of decay rate and weight loss rate, indicating the delay of Δand, as well as a higher soluble solid, titratable acid, VC, and carotenoid content. The principal component analysis demonstrated that the goji quality in three groups was in the order of phase temperature group, ice temperature group and CK group. Moreover, the contents of favorable aroma components, such as lycium aldehydes and terpenoids, were relatively higher in the storage under precise temperature control. Consequently, the ice and phase temperature storage was the most effective in the long-term preservation of goji, where the color change of goji was delayed to facilitate the release of volatile substances, while effectively prolonging the storage period of goji to 30 days. The finding can be greatly beneficial to guide the preservation of goji fruits.
storage; quality control; Goji; precise temperature control; aroma components
張鵬,袁興鈴,薛友林,等. 精準(zhǔn)溫度控制對枸杞鮮果貯藏品質(zhì)和香氣成分的影響[J]. 農(nóng)業(yè)工程學(xué)報(bào),2021,37(18):322-330.doi:10.11975/j.issn.1002-6819.2021.18.037 http://www.tcsae.org
Zhang Peng, Yuan Xingling, Xue Youlin, et al. Effects of precise temperature control on the storage quality and aroma components of fresh goji fruit[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(18): 322-330. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2021.18.037 http://www.tcsae.org
2021-05-13
2021-07-13
兵團(tuán)重點(diǎn)領(lǐng)域科技攻關(guān)項(xiàng)目(2019AB024);國家重點(diǎn)研發(fā)計(jì)劃資助項(xiàng)目(2018YFD0401303)
張鵬,博士,副研究員,研究方向?yàn)楣哔A運(yùn)保鮮和無損檢測技術(shù)。Email:zhangpeng811202@163.com
李江闊,博士,研究員,研究方向?yàn)檗r(nóng)產(chǎn)品安全與果蔬貯運(yùn)保鮮新技術(shù)。Email:lijkuo@sina.com
10.11975/j.issn.1002-6819.2021.18.037
TS255.3
A
1002-6819(2021)-18-0322-09