番石榴儲藏對彈性性質影響之研究

Abstract

水果經採收後在室溫中容易造成軟化喪失商業價值，若要長期存放勢必需要儲藏在較低溫的環境下，以保持其物理性質，利用彈性性質建立水果品質非破壞檢測指標是工程重要的研究課題。本論文探討珍珠拔品種番石榴在儲藏時彈性性質的改變，以ASABE標準平板壓縮試驗及敲擊試驗，檢測在5°C、10°C、15°C與室溫四種溫度下，儲藏期間果實的敲擊振動共振頻率、振動阻尼比和彈性模數的變化，並以COMSOL軟體模擬珍珠拔共振時的振形與共振頻率，再利用巢狀試驗設計來分析儲藏溫度與時間對以上彈性性質的影響。軟體模擬則先將由雷射掃描器掃出的果實外型匯入程式中，再以敲擊試驗測得的彈性模數作為其材料特性，利用COMSOL的三維結構模組計算樣本的振動特徵頻率，並由文獻中彈性球體的振形來判斷共振頻率。模擬的結果除了可以得到與敲擊試驗所得實驗值相近的S20共振頻率，還能找出實驗不易觀察到的S30與 S40共振頻率。實驗結果顯示珍珠拔敲擊振動的共振頻率與平板壓縮及敲擊兩種方法所測得的彈性模數皆隨著儲藏天數的增加而降低，且三者的趨勢相近。敲擊試驗所計算出的振動阻尼比則會隨著儲藏時間上升，並且上升的速率隨儲藏溫度降低而變為緩慢。將此振動阻尼比以巢狀試驗設計分析顯示儲藏溫度及時間的影響皆為顯著，相較於敲擊振動的共振頻率，振動阻尼比的變化較能不受樣本個體差異的影響，更適合做為珍珠拔物理性質的非破壞檢測的指標。

Since harvested fruit easily softens and loses its market value at room temperature, a low-temperature environment is required to preserve the fruit’s firmness if a longer shelf life is desired. Thus using elastic property to establish a nondestructive quality evaluation index is one of the important research issues of biological engineering. This study aims at investigating the variations of the elastic properties of stored “Jen-Ju Bar” a most popular variety of guava in Taiwan. The ASABE standard parallel plate compression test and the impulse response test were used to measure the variation of impulse vibrating resonant frequencies, vibrating damping ratios and elastic moduli of the fruit stored at 5°C, 10°C, 15°C and room temperature. Besides, COMSOL Multiphysics was used to simulate the fruit’s vibration modes and resonant frequencies during resonating. Nested experimental design was adopted to analyze the effects of storage duration and temperature on the above elastic properties. Model geometries of fruits for finite elements simulation were constructed using surface topology scanning. Then, the elastic moduli measured in the impulse response test were taken for the material properties. The 3-D structural mode was then used to calculate the samples’ vibrating eigenfrequencies, and the resonant frequencies were determined based on the vibration modes of elastic sphere in literature. The results of the simulation showed that the resonant frequencies of S20 mode resembled the values measured by impulse response test; in addition, the resonant frequencies of S30 mode and S40 mode, rarely observed in experiments, were found. The results of the experiments showed that the impulse vibrating resonant frequencies and the elastic moduli measured by parallel plate compression test and the impulse response test decreased as the storage duration lengthened, and these three showed a similar trend. Vibrating damping ratios measured by impulse response test increased as the storage duration lengthened, with a slower rate of rising at a low storage temperature than at a high storage temperature. The nested experimental design analyses of vibrating damping ratios showed significant effects of both storage temperature and duration. Compared with impulse vibrating resonant frequency, the variation of vibrating damping ratio was less affected by individual sample differences, thus more appropriate for a nondestructive index to evaluate the physical properties of “Jen-Ju Bar”.