多酚氧化酶的射频加热灭活效果及动力学分析

为了阐述多酚氧化酶(PPO)射频加热对灭活规律,本文研究了板间距和溶酶磷酸盐缓冲液电导率对加热速度及酶灭活效果的影响,进行了酶灭活动力学模型研究。结果表明:极板间距和电导率影响射频加热速度,极板间距越大升温越慢,电导率为0.1 S/m时,升温速度最快。在极板间距120 mm,电导率0.1 S/m时,射频加热105 s,PPO的灭活率为91.88%。对酶灭活的动力学曲线分别用一级动力学模型、Weibull模型和Log-Logistic模型进行拟合。通过对模型的拟合评价参数即精确因子Af、偏差因子Bf、根平均方差RMSE和决定系数R2进行比较,PPO的灭活动力学曲线不符合一级反应动力学模型,Weibull和Log-Logistic模型都能较好的拟合PPO失活曲线。通过衡量三种模型的预测值和实测值的一致性,Log-Logistic模型能最好地拟合射频加热条件下多酚氧化酶的失活曲线。     

大肠杆菌射频加热杀菌效果及动力学模型拟合

为研究射频加热杀菌效果及微生物致死动力学。选取大肠杆菌为试验菌株,研究极板间距和菌悬液电导率对加热速度及杀菌效果的影响。选取4种常见的模型,即一级动力学、Weibull、Dose-response和Log-Logistic模型对大肠杆菌射频加热失活曲线进行动力学模型拟合。结果表明:极板间距和电导率影响射频加热速度,极板间距越大升温速度越慢,杀灭微生物所需时间越长,当电导率接近1 000μs/cm时,升温速度最快。在极板间距115 mm,电导率1 083μs/cm的条件下,加热90 s,大肠杆菌致死率即可达100%。大肠杆菌的失活动力学曲线基本呈倒"S"型,不符合一级反应动力学模型。Weibull、Log-Logistic和Dose-response模型都能较好地拟合射频杀菌失活曲线。通过对模型的拟合评价参数,即精确因子Af、偏差因子Bf、根平均方差RMSE和决定系数R2的比较,确定Log-Logistic模型拟合效果最好。     

UV-C处理对采后草莓果实品质和活性氧代谢的影响

以"凤冠"草莓为材料,研究了0,1.5,3.0和4.5 k J/m2UV-C处理对草莓果实在10℃,12 d贮藏期间果实品质、呼吸强度、内源乙烯产生、可溶性糖、己糖激酶活性(HXK)、果糖激酶活性(FRK)和抗氧化酶活性的影响。结果表明,3.0 k J/m2UV-C处理能抑制草莓果实的呼吸速率和乙烯释放量,减少可滴定酸和维生素C的损失,维持果实较好的食用品质。同时发现,3.0 k J/m2UV-C处理果实中FRK和HXK活性显著高于其余3个处理果实,维持了整个贮藏期间较高的FRK和HXK活性,促进了贮藏后期果实中果糖和葡萄糖含量的下降。此外,3.0 k J/m2UV-C处理能维持草莓果实中较高的SOD,CAT,APX和POD的活性,降低超氧阴离子和MDA的积累。这表明,适宜剂量的UV-C处理(3.0 k J/m2)能维持草莓果实采后活性氧代谢通畅,抑制膜脂过氧化反应发生,延缓果实衰老的发生;3.0 k J/m2UV-C处理可能启动采后草莓果实己糖信号传导,调控草莓果实采后花色苷的合成和积累,进而促进了果实采后转色。     

微波杀灭玉米中烟曲霉工艺及动力学模型

为了探索微波处理对玉米霉菌的杀灭效果和机制,应用响应面法对微波杀灭玉米中烟曲霉的工艺进行优化研究,考查微波剂量和微波功率对烟曲霉孢子减少对数周期的影响,并建立相应的失活动力学模型。结果表明微波杀灭玉米烟曲霉的最佳工艺条件为:微波剂量2.5W/g、微波功率539W,通过五次验证实验,测得玉米烟曲霉孢子减少对数周期为1.0496;与线性模型相比,Weibull模型能更好的预测和拟合微波处理对烟曲霉孢子失活的动力学变化。     

高压脉冲电场对酿酒酵母杀菌效果影响和模型分析研究

研究了高压脉冲电场(pulsed electric field,PEF)对接种于胡萝卜汁中酿酒酵母(Saccharomyces cerevisiae,CGMCC2.604)的杀菌效果并进行了模型分析。结果表明,随电场强度和脉冲时间增加,PEF对酿酒酵母的杀灭效果增强,25kV/cm、2173μs时酿酒酵母降低了4.5个对数。Weibull模型和Log-Logistic模型均能很好的拟合PEF处理酿酒酵母的失活曲线,模型评价参数(Af,Bf,SS,RMSE和R2)分析表明,Log-Logistic模型比Weibull模型能更好地拟合PEF处理下酿酒酵母失活动力学变化。     

UV-C处理对双孢蘑菇采后品质的影响

本文以双孢蘑菇为材料,研究了不同剂量的短波紫外线(UV-C)对双孢蘑菇低温贮藏期间的失重率、硬度、颜色、可溶性固形物含量、细胞膜透性、PPO活力和微生物的影响。双孢蘑菇子实体先用0、0.3、0.6、0.9、1.2 k J/m2剂量的UV-C处理,随后放入4℃冷库存放16 d。结果表明:适当的短波辐照(0.6 k J/m2)处理可以抑制细胞膜透性的增加,延缓可溶性固形物含量的降低,在一定程度上延缓软化和褐变。对嗜冷菌、大肠杆菌、酵母和霉菌有显著性抑制作用。较大剂量(0.9、1.2 k J/m2)会对双孢蘑菇的表皮细胞产生伤害,使得细胞膜透性增加,导致可溶性固形物含量的降低,失重率增加以及褐变的加剧。     

采后UV-C处理对软枣猕猴桃果实酚类物质及抗氧化活性的影响

研究了采后UV-C辐照对软枣猕猴桃贮藏期间果实品质、酚类物质含量和抗氧化活性的影响。将果实采摘后立即用不同剂量(1.05、2.1和4.2 k J/m~2)的UV-C辐照处理,随后跟踪测定其在室温贮藏和冷藏期间硬度、可溶性固形物、总酚、总黄酮、总花色苷含量和抗氧化活性的变化。结果显示,UV-C处理能够提高果实贮藏期间可溶性固形物含量,延缓果实硬度下降,并显著促进了果实总酚、总黄酮及总花色苷的合成与积累,有效提高果实贮藏期间的抗氧化活性。室温贮藏条件下UV-C处理果实酚类物质含量达到最大时的剂量及其效应时间为4.2 k J/m~2 UV-C辐照后贮藏5 d;冷藏条件下为1.05 k J/m~2 UV-C辐照后贮藏3 d。UV-C辐照处理可作为一种安全无毒的采后处理方式改善和提高软枣猕猴桃果实品质、酚类物质含量和抗氧化活性。     

营养肉汤中金黄色葡萄球菌致死模型的构建

采用营养肉汤培养基,研究不同温度(T)、pH、水分活度(Aw)和乳链菌肽(nisin)对金黄色葡萄球菌的致死作用,构建金黄色葡萄球菌致死量的初级致死模型和次级致死模型。研究结果表明:金黄色葡萄球菌致死量随着T、pH和Aw的降低而增加,随着nisin的增加而降低;Aw与pH,nisin与pH存在明显的交互作用。初级致死模型符合Weibull模型,包括pH-Weibull模型,nisin-Weibull模型,决定系数R~2分别为0.9278,0.9849。通过二次多项式回归拟合得到的次级致死模型,决定系数R~2为0.9746。经验证,模型拟合度较高,能准确描述金黄色葡萄球菌的致死规律,并预测建模范围内金黄色葡萄球菌的致死量。     

6328A He-Ne激光对黑曲霉的作用

本文报道了6328AHe-Ne激光对黑曲霉作用的对照实验研究。实验结果表明,照射剂量为2×10~(-3)～2×10~(-2)J/cm~2时,He-Ne激光对黑曲霉孢子有提高存活率的趋势;照射剂量为3.5×10~(-2)～8×10~(-2)J/cm~2时,He-Ne激光对黑曲霉孢子有抑制作用。     

丁香鱼薄层干燥数学模型研究

为船上实际生产提供理论指导,对丁香鱼薄层干燥特性进行研究。通过单因素分析,对决定系数(R2),秩检验(χ2),均方根误差(RMSE)及平均相对百分误差(PE%)进行检验,确定了Midilli模型为丁香鱼热风干燥的最适模型。同时本实验采用二次正交旋转回归设计,以拟合Midilli模型各系数k、a、b、n与温度和风速之间的函数关系,建立丁香鱼热风干燥数学模型,并用该模型进行预测,其预测值与实验值拟合程度高。结果表明:Midilli模型可以对温度40～70℃,风速0.2～1.4m/s范围中丁香鱼水分变化进行较精确的预测。     

共培养体系下米曲霉与产毒黄曲霉的相互影响

研究了在共培养体系中,产毒黄曲霉对米曲霉产蛋白酶、淀粉酶的影响及米曲霉对黄曲霉产黄曲霉毒素B₁（AFB₁）的影响,还分析了米曲霉降解AFB₁的能力。结果发现黄曲霉对米曲霉产蛋白酶和α-淀粉酶有明显的抑制作用,而对β-淀粉酶的分泌无影响,培养10 d后,酸性蛋白酶活性、中性蛋白酶活性、碱性蛋白酶活性和α-淀粉酶活性分别降低了3.1、3.2、4.1和6.9倍;同时发现在整个培养过程中,米曲霉能有效抑制黄曲霉合成AFB₁的能力,抑制率达75～90.91%,而对AFB₁无降解作用。     

Inhibitory effect of Aspergillus niger on the growth of Aspergillus ochraceus and Aspergillus flavus,and on aflatoxin formation

Aspergillus ochraceus and A flavus were grown on synthetic media (SM) supplemented with 50 or 200 ml litre^?1 SM on which A niger had been grown previously ( ‘A niger medium’ = ANM). Controls included SM acidified to pH 6.0 or 4.4, SM diluted with 50 or 200 ml litre^?1 water, and diluted-acidified SM. For both fungi, higher growth inhibition was recorded on ANM-containing SM than in the controls. Aflatoxin formation was markedly inhibited on SM to which 20 ml litre^?1 ANM extract (in methanol/chloroform, 2:1 v) had been added, although the growth of A flavus on that medium was almost the same as that in the control. It is concluded that the inhibitory effect of A niger on the growth of fungi should not be attributed merely to pH reduction, but also, mainly, to metabolites produced by the fungus in the growth medium, even at early stages of its growth.     

黑曲霉、米曲霉和解脂假丝酵母协同处理竹片

在相同发酵工艺条件下,探索黑曲霉、米曲霉、解脂假丝酵母、黑曲霉 米曲霉、黑曲霉 解脂假丝酵母、米曲霉 解脂假丝酵母、黑曲霉 米曲霉 解脂假丝酵母七组中的最优处理竹片茵种,然后确定其最佳发酵工艺,最后进一步研究接种量和接种比例的影响。研究结果表明,黑曲霉 米曲霉 解脂假丝酵母是最优处理竹片菌种,其最佳处理工艺为温度35℃,pH7.0,定时通氧,添加营养液,处理时间8d,接种量影响不大,接种比例以2:1:1(黑曲霉:米曲霉:解脂假丝酵母)为佳。     

Cladal relatedness among Aspergillus oryzae isolates and Aspergillus flavus S and L morphotype isolates

Aspergillus flavus is the main etiological agent for aflatoxin contamination of crops. Its close relative, A. oryzae, does not produce aflatoxins and has been widely used to produce fermented foods. We compared the phylogeny of A. oryzae isolates and L- and S-type sclerotial isolates of A. flavus using single nucleotide polymorphisms in the omtA gene in the aflatoxin biosynthesis gene cluster and deletions in and distal to the norB-cypA intergenic region as phylogenetic signals. Aflatoxin-producing ability and sclerotial size also were weighted in the analysis. Like A. flavus, the A. oryzae isolates form a polyphyletic assemblage. A. oryzae isolates in one clade strikingly resemble an A. flavus subgroup of atoxigenic L-type isolates. All toxigenic S-type isolates closely resemble another subgroup of atoxigenic L-type isolates. Because atoxigenic S-type isolates are extremely rare, we hypothesize that loss of aflatoxin production in S-type isolates may occur concomitantly with a change to L-type sclerotia. All toxigenic L-type isolates, unlike A. oryzae, have a 1.0 kb deletion in the norB-cypA region. Although A. oryzae isolates, like S-type, have a 1.5 kb deletion in the norB-cypA region, none were cladally related to S-type A. flavus isolates. Our results show that A. flavus populations are genetically diverse. A. oryzae isolates may descend from certain atoxigenic L-type A. flavus isolates.     

Overexpression of Aspergillus aculeatus cellobiohydrolase I in Aspergillus oryzae

To express the cbhI gene, encoding Aspergillus aculeatus cellobiohydrolase I (CBHI), in Aspergillus oryzae, a plasmid was constructed. The strain that displayed the strongest CBHI activity among the transformants produced about 941 mg/l in liquid culture. It was confirmed by a PCR method that the plasmid was integrated at the niaD locus.     

Identification and toxigenic potential of the industrially important fungi, Aspergillus oryzae and Aspergillus sojae

Mold strains belonging to the species Aspergillus oryzae and Aspergillus sojae are highly valued as koji molds in the traditional preparation of fermented foods, such as miso, sake, and shoyu, and as protein production hosts in modern industrial processes. A. oryzae and A. sojae are relatives of the wild molds Aspergillus flavus and Aspergillus parasiticus. All four species are classified to the A. flavus group. Strains of the A. flavus group are characterized by a high degree of morphological similarity. Koji mold species are generally perceived of as being nontoxigenic, whereas wild molds are associated with the carcinogenic aflatoxins. Thus, reliable identification of individual strains is very important for application purposes. This review considers the pheno- and genotypic markers used in the classification of A. flavus group strains and specifically in the identification of A. oryzae and A. sojae strains. Separation of A. oryzae and A. sojae from A. flavus and A. parasiticus, respectively, is inconsistent, and both morphologic and molecular evidence support conspecificity. The high degree of identity is reflected by the divergent identification of reference cultures maintained in culture collections. As close relatives of aflatoxin-producing wild molds, koji molds possess an aflatoxin gene homolog cluster. Some strains identified as A. oryzae and A. sojae have been implicated in aflatoxin production. Identification of a strain as A. oryzae or A. sojae is no guarantee of its inability to produce aflatoxins or other toxic metabolites. Toxigenic potential must be determined specifically for individual strains. The species taxa, A. oryzae and A. sojae, are currently conserved by societal issues.     

Nannocystis exedens: a potential biocompetitive agent against Aspergillus flavus and Aspergillus parasiticus.

This study examined the potential for controlling toxigenic Aspergillus flavus and Aspergillus parasiticus by biological means using a myxobacterium commonly found in soil. The ability of Nannocystis exedens to antagonize A. flavus ATCC 16875, A. flavus ATCC 26946, and A. parasiticus NRRL 3145 was discovered. Cultures of aflatoxigenic fungi were grown on 0.3% Trypticase peptone yeast extract agar for 14 days at 28 degrees C. When N. exedens was grown in close proximity with an aflatoxigenic mold, zones of inhibition (10 to 20 mm) developed between the bacterium and mold colony. A flattening of the mold colony on the sides nearest N. exedens and general stunting of growth of the mold colony were also observed. When N. exedens was added to the center of the cross-streak of a mold colony, lysis of the colony by the bacterium was observed after 24 h. Microscopic observations revealed that N. exedens grew on spores, germinating spores, hyphae, and sclerotia of the molds. These results indicate that N. exedens may be a potential biocontrol agent against A. flavus and A. parasiticus.     

Aflatoxin-producing potential of Aspergillus flavus and Aspergillus parasiticus isolated from samples of smoked-dried meat

Over a period of three years 420 samples of various smoke-dried meat products, collected from individual households in different region of Croatia were analysed for the presence of aflatoxigenic strains of the Aspergillus flavus group. Strains of A. flavus and A. parasiticus were present in 17,8% of the samples, and aflatoxin-producing ability was tested in 75 strains. In relation to sequential method of aflatoxin detection, 5 of 8 isolates were found in the first step (fluorescence in aflatoxin-producing ability medium - APA) and all of them in the second step (extraction method from syntheses on moist shredded wheat - SW). A. flavus strains produced mainly aflatoxin B₁, and had various levels of toxigenicity (1.4–3.12 mg/kg). Some strains of A. parasiticus produced all four aflatoxins B₁ B₂ G₁ G₂, while the other ones produced AF B₁ + G₁ only, with concentrations of aflatoxins from 0.1 to 450 mg/kg.