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為何真空包裝煙三文魚會受到李斯特菌污染?
曾耀源
(廠商會檢定中心)
上週食物安全中心(中心)表示,從一家本地食品加工廠抽取的一個即食煙三文魚樣本驗出李斯特菌。中心呼籲市民不要食用由該生產商製造的煙三文魚產品(不論品牌、批次及包裝)。
甚麼是李斯特菌?
李斯特菌是在泥土和水中常見的細菌。冷存的即食食物較易受到該菌污染,包括生吃或煙燻的海產、未經殺菌處理的牛奶、軟芝士、雪糕及未經烹煮的蔬菜等。
李斯特菌的獨特之處在於它能在冷凍低溫下繁殖,是一種重要的食品危害。
大部分身體健康的人在感染這種細菌後不會出現病徵或只出現輕微病徵。但對孕婦、初生嬰兒、長者和免疫力較低的人,則可能出現嚴重的併發症。在美國,每年因進食含李斯特菌的食物而死亡的個案平均有255宗。
食品加工廠與細菌污染
Tocmo et al., 2014 曾研究真空包裝煙三文魚受到李斯特菌污染的可能原因。

原材料
由於李斯特菌天然存在於環境中,魚類可受到該菌污染。雖然從魚身檢出李斯特菌的個案不多,但亦不能完全排除其可能性。
去頭、去內臟及切魚肉
即使加工用具經過消毒(例如次氯酸鈉 /過氧乙酸),其接觸食物的表面亦曾被檢出李斯特菌。這反映該菌的抗性頗強,並可在漁產品加工廠的某些地點存活下來。
因此,要加大清潔和消毒方面的力度(包括方法和次數),以及定期檢測食品接觸表面有否受到李斯特菌的污染。
加鹽
由於李斯特菌有耐鹽性,它可在鹽水及其儲存容器存活下來,並成為潛在污染源。
因此,以鹽水處理魚肉的區域須加強清潔和消毒。
此外,在該區域工作的人員可能帶有李斯特菌,這會增加污染食品的機會。
冷煙燻
由於冷煙燻的溫度(≤30 ◦C)不足以殺滅李斯特菌,細菌在煙燻過後能存活下來和繁殖的機會頗大。
在煙燻過程中,不潔的食物接觸表面和個人安全用品亦可造成污染。有調查曾檢出煙燻時使用的盤子、個人手套和保護衣著含李斯特菌,可造成交叉污染。
因此,要避免煙燻區域受到李斯特菌污染,員工衛生和嚴格清潔和消毒食物接觸面是非常重要的。
去皮及切片
有調查發現一家煙三文魚加工廠的切片機器(例如運輸帶、蓋等)和工作枱含李斯特菌。
為減少污染,可考慮拆開刀片工具,以便有效和定期清潔和消毒,減低李斯特菌依附的可能性。此外,應馬上丟棄可能含有細菌的魚肉片。
真空包裝
真空包裝並不能壓抑李斯特菌,因為該菌能在缺乏氧氣的情況下生長。由於以真空包裝的即食食物的保質期一般較長,或會令李斯特菌有足夠時間大量繁殖,足以致病。
透過縮短保質期和嚴格遵守儲存溫度,可避免零售產品中李斯特菌大量繁殖達到致病的劑量。
食品檢測不足
根據一項關於蘇格蘭煙三文魚生產的研究,天花冷凝水滴落食品常造成李斯特菌污染,尤其常見於小型食品加工廠。
然而,該研究發現很多小型生產商一般不會檢測其食品是否含有李斯特菌。究其原因是他們認為細菌檢測費用昂貴和缺乏技術支援。