Đặc điểm lạnh đông và sự thay đổi chất lượng sau tan giá của quả nhãn Idor (Euphoria longan Lam.)
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Abstract
Freezing is the process of crystallizing water in food, primarily used for preservation and also to support other techniques such as cold concentration or freeze-drying. In this study, the changes in the core temperature of whole Idor longan (Euphoria longan Lam.) were monitored over time at different freezing temperatures, decreasing from -40°C, -50°C, -60°C to -70°C. The recorded results indicated that the freezing and eutectic temperatures of whole Idor longan were -2.43°C and -29.13°C, respectively. Post-thawing quality monitoring showed that freezing at -60°C and -70°C produced thawed products of better quality, with higher humidity (79.06–79.17%) and vitamin C content (32.45 – 32.74 mg%), as well as lower drip loss (1.01–1.16%) and browning index (BI) (13.02–13.18). However, the percentage of pericarp cracking remained high (82.85 87.54%) at these freezing temperatures. Therefore, additional techniques should be applied to reduce pericarp cracking in longan during freezing to maintain fruit quality during storage.
Tóm tắt
Lạnh đông là quá trình làm kết tinh nước trong thực phẩm, được ứng dụng chủ yếu để bảo quản, đồng thời hỗ trợ các kỹ thuật khác như cô đặc lạnh và sấy thăng hoa. Trong nghiên cứu này, sự thay đổi nhiệt độ tâm của nhãn Idor (Euphoria longan Lam.) nguyên quả theo thời gian được theo dõi ở các mức nhiệt độ lạnh đông giảm dần từ -40°C, -50°C, -60°C đến -70°C. Kết quả ghi nhận cho thấy, nhiệt độ đóng băng và nhiệt độ eutectic của nhãn Idor nguyên quả lần lượt là -2,43°C và -29,13°C. Đánh giá chất lượng sau tan giá cho thấy, lạnh đông ở -60°C và -70°C giúp sản phẩm đạt chất lượng tốt hơn, thể hiện qua độ ẩm (79,06-79,17%) và hàm lượng vitamin C cao hơn (32,45 - 32,74 mg%), đồng thời giảm thiểu tỉ lệ rỉ dịch (1,01-1,16%) và chỉ số hóa nâu BI (13,02-13,18). Tuy nhiên, nhãn đông lạnh ở hai mức nhiệt độ này vẫn có tỉ lệ nứt vỏ cao (82,85-87,54%). Do đó, việc áp dụng thêm các biện pháp kỹ thuật để hạn chế nứt vỏ quả nhãn khi lạnh đông là cần thiết nhằm duy trì chất lượng của quả trong quá trình tồn trữ.
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