包裝咖哩雞調理食品用微波輔助電磁加熱殺菌之熱傳機制及對產品品質之影響

Abstract

調理食品有食用方便性的優勢，為賣場中不可或缺且消費者常常採購的產品。目前市面上各種調理食品均採用傳統殺菌釜或熱水浸泡及水淋式熱處理，為了確保食品安全性及延長產品保存期限，熱處理時間通常很長。微波輔助電磁加熱 (microwave-assisted induction heating, MAIH) 設備有迅速升溫的特點，或許可以解決上述加熱時間過長的問題。MAIH為 1 kW 之 2450 MHz 微波由上往下加熱及 2.5 kW電磁由下往上加熱之複合熱源型式。本研究選用咖哩雞調理食品作為模型食品，以探討不同加熱條件下熱傳機制及對產品品質的影響。控制組為使用批次式滅菌釜殺菌，其殺菌效力同於95oC 直接加熱18.32分鐘 (F95 =18.32 min)，殺菌後產品可於冷藏溫度放置 45 天。本研究之加工程序分為四段製程：預熱使樣品溫度達60oC、複合加熱、室溫冷卻3分鐘，及冷水 (20oC) 冷卻4.5分鐘。於MAIH加熱條件，電磁溫度固定為160oC，調控微波加熱程序與輸出功率，並量測靠近容器外圍之冷點溫度。首先，實驗證實單一電磁加熱無法快速加熱，單一微波加熱無法均勻升溫，且均無法達到與控制組相同殺菌值。在複合加熱程序方面，實驗發現一段式加熱 (電磁溫度160oC，微波功率750 W，時間3分鐘)、二段式加熱 (總微波輸出功率與一階段複合式加熱相同之前提下，調整兩階段不同微波功率，各1.5分鐘)、三段式間歇性加熱 (第一與第三階段開啟微波，第二階段微波與電磁電源均關閉，僅利用腔體餘熱)，都可能因為金屬腔體與塑膠盒緊貼時間太短，造成熱傳導效率不佳，底層食品之升溫幅度小，無法達到目標殺菌值。本研究發現，可行之加工程序為三段式連續性加熱，電磁爐於三段加熱中均需開啟，但僅須在第一與第三階段施予微波。如此，適當的控制第一階段與第三階段的微波功率，於容器中提早產生足夠蒸氣壓，可以使塑膠容器底部與電磁加熱板提早貼合，讓電磁加熱之熱傳導可以迅速發揮功效，達到目標殺菌效果。於安全性方面，先以溫度曲線間接判斷各加工程序之殺菌效力，結果發現第一加熱階段使用之微波功率超過 690 W之三段式連續性加熱即可超過目標殺菌值；再以生菌數實驗確定微生物失活程度，證實三段式連續性加熱處理之樣品於各稀釋倍數均未檢出菌落。於食品品質方面，以相近冷點殺菌值之殺菌條件下做比較，實驗組的青豆顏色、青豆質地、醬汁 pH 值及肉品脂質氧化的變化程度較控制組大，反映MAIH提供較多熱效應至熱點。於皮爾森積差相關分析，樣品系統之最高溫度與青豆紅綠度及肉品脂質氧化程度呈現中度相關。本研究所採取的三段式連續性加熱製程，可以縮短約86.1% 控制組所需之製程時間，且具有與控制組類似殺菌值；但是於品質方面之變化程度較高。

The prepared and packaged food products are important items on the exhibition shelves in supermarkets or convenience store because of their convenience for eating. To ensure food safety and prolonged shelf life, conventional thermal pasteurization has been extensively used in the food industry. However, conventional thermal processing, such as retorting or hot water immersion/spraying, often requires very long processing time, which may decrease processing efficiency. Microwave-assisted induction heating, abbreviated as MAIH, provides quicker heat transfer rate and shorter heating time, which may have advantages over the conventional thermal processing. The MAIH equipment used in this study combines 2450 MHz microwave (MW) and induction heating (IH) from top-down (MW) and bottom-up (IH), respectively. We used curry chicken as a model system to investigate the heat transfer mechanism and changes of food properties during MAIH process. The proper processing procedures and conditions were established to achieve the target lethality, which used commercial product, processed by batch retort for 90 mins, with the lethality of F95 = 18.32 min and shelf life of 45 days at 4oC, as a reference. The whole process can be divided into four sections: preheating, MAIH heating, 3 min room temperature cooling, and 4.5 min cold water (20 oC) cooling. During MAIH heating, the IH temperature was set at 160oC, while the MW was operated at different output percentages (100% as 1000 W). The time-temperature profile revealed that the coldest spot of MAIH-processed curry chicken was at the circumference of the container. Neither IH nor MW alone could not achieve the target lethality due to low conductive heat transfer rate by IH and differential heating by MW. With regard to processing procedure, no matter one-stage MAIH (IH 160oC, MW 750 W, 3min)、two-stage MAIH (the total MW output energy is the same as the one-stage MAIH, various MW outputs in the two 1.5-minute stages), and three-stage intermittent MAIH (MW on during the 1st and the 3rd stages while both MW and IH were power off during the 2nd stage). Possibly due to the tight contact between the plastic container and the IH heating plate was not established soon enough, thermal conduction between plastic container and IH heating plate was not efficient, the 3-stage intermittent MAIH heating could not result in the expected lethality. On the other hand, we found that the continuous three-stage MAIH process (IH on in all three stages, MW on in the 1st and the 3rd stage with adjusted power output) could satisfy our pasteurization purpose. This processing procedure increased heat transfer efficiency of IH and facilitated temperature rise, which induced enough steam generation at an earlier time. When the MW power in the first stage was larger than 690 W, the target lethality could be reached. Aerobic plate count of the control group and the continuous 3-stage MAIH process showed nondetectable (< 1.00 log CFU/g). In terms of food quality, changes of pea color, pea texture, sauce pH and lipid oxidation of the MAIH group were larger than the control, revealing that the hot spot of MAIH groups received more thermal energy than the control group based on similar lethality at cold spot. Pearson correlation coefficient showed moderate correlation between the highest temperature and peas a* value along with lipid oxidation. In summary, MAIH could shorten 86.1% process time as compared with conventional thermal processing, however, changes of food qualities were greater than the control.