Freeze-drying of beef. II. A calorimetric method for comparing theoretical with actual drying rates (measurement of freeze-drying rates of beef)

Summary. A calorimetric method is described by which some of the predictions of the theory described in Part I (Bralsford, 1967) are compared with the actual freeze-drying behaviour of beef. In particular, the method permits the continuous measurement of heat transfer into the sample as it dries. From this the sublimation rate at any time may be deduced and the end of sublimation is readily determined. For the most part the agreement between experiment and theory is good; the prediction that, at normal surface temperatures, the sublimation rate should increase with chamber pressure is confirmed. It is suggested that such discrepancies that do exist can be explained as the effects of liquid diffusion into the dried shell and that the consequences of this are that product quality must be expected to depend to some extent on the size of the food piece.     

Freeze-drying of raw beef I.—Influence of some raw material variables

The effect of raw material variables, such as breed, sex, age, fat content of muscle and ageing time, upon the quality of the reconstituted product were studied in multifactorial experiments of a preliminary nature. The effects observed were in fair agreement with general experience with unprocessed raw beef and with the limited published data for reconstituted freeze-dried raw beef.     

Freeze-drying of raw beef. II. Influence of some freezing and dehydration variables

The effect of pre-treatment variables such as freezing rate and method of cutting the meat, and of product surface temperature during freeze-drying as well as dehydration per se on quality were studied in multifactorial experiments. Very high freezing rate, using liquid nitrogen spray, resulted in lowered reconstitution ability after freezedrying and marked paleness of both dried and reconstituted meat, compared with freezing rates in the range spanned by conventional freezing methods. Cutting the meat before freezing impaired reconstitution ability compared with slicing it in the semifrozen state or sawing it in the frozen state before freeze-drying, but improved colour of the dry product. Comparisons between freeze-dried and frozen beef indicated a quality loss inherent in the dehydration method itself, while drying in the range of surface temperatures from +20° to +60° resulted in no appreciable difference in quality with temperature. The results obtained are in fair agreement with published data where they have been available.     

Freeze-drying of raw beef. 3. Influence of some packaging and some storage variables

Minimisation of oxygen exposure of freeze-dried raw beef before packaging and during storage was studied in multifactorial experiments for its effect on the quality of the product, as was the effect of storage time and temperature, intramuscular fat content, and different head-space gas compositions. Exposure of the freeze-dried meat to the air before packaging under nitrogen, which is commercial practice today, clearly shortened storage life, particularly of meat with a high fat content. The effect could be counteracted by the application of an oxygen scavenging system, which proved effective at initial head-space oxygen levels as high as 4%. The use of carbon dioxide in the head-space reduced product quality compared with nitrogen or the hydrogen-nitrogen mixture employed in the oxygen scavenging system. The red meat colour as well as quality as a whole was much better preserved by cold storage and particularly frozen storage, by packaging without pre-exposure to the atmosphere or by using an oxygen scavenging system based on the reaction of oxygen with hydrogen over a palladium catalyst enclosed in the package. Of these methods, refrigerated storage is probably not a realistic solution under present commercial conditions.     

Water-soluble precursors of beef flavour: I. Effect of diet and breed

The effects of diet and breed on the concentration of water-soluble flavour precursors, namely sugars, free amino acids, ribonucleotides, creatinine, carnosine and creatine, were studied in beef longissimus lumborum muscle. Diet had a significant effect on the concentration of free amino acids, with animals fed on grass silage having higher free amino acid levels than animals fed on a concentrate diet, whereas animals fed concentrates had a higher total reducing sugar content. Differences between a beef breed (Aberdeen Angus×Holstein-Friesian) and a dairy breed (Holstein-Friesian) were generally small.     

有机溶剂的冷冻干燥研究

介绍了非水有机溶剂的特点及其在冷冻干燥中的应用,重点论述了叔丁醇的冷冻干燥特性。     

Growth rates of Salmonella and Escherichia coli O157:H7 in irradiated beef.

Ground beef was irradiated to 0, 2, or 4 kGy and then inoculated with a mixed culture of four serotypes of salmonellae or five strains of Escherichia coli O157:H7. The ground beef was stored at either 15 or 25 degrees C, and the growth of the inoculated bacteria was monitored over time. Growth parameters were determined for both the salmonellae and the E. coli O157:H7 using the Gompertz equation. There was no significant difference in lag phase duration or generation time, irrespective of the dose to which the ground beef had previously been exposed. Furthermore, the lag phase durations and generation times determined in this study did not differ significantly from previously published values. This suggests that, although irradiation eliminates a significant portion of the spoilage microflora in ground beef, the absence of this microflora provides no competitive advantage to the growth of salmonellae or E. coli O157:H7 in ground beef.     

芭蕉片的真空冷冻干燥工艺

采用真空冷冻干燥方法冻干芭蕉,电阻法测定芭蕉的共晶点和共熔点,绘制了冻干芭蕉的干燥曲线,通过优化确定了最佳冻干程序。试验分析了预冻方式、不同成熟度、切片厚度、浸泡方式对冻干芭蕉品质的影响。结果表明:最佳程序为:第一段-20℃,5.6 h;第二段-10℃,6.1 h;第三段,20℃,5.6 h;第四段,30℃,6.3 h。进一步试验表明2种预冻方式对冻干产品影响不大,成熟芭蕉,切片厚度为3~5 mm,直接干燥得到产品品质好。与市售香蕉脆片比较,品质更佳。     

The bacteriological quality of British beef 2. Frozen minced beef

Frozen minced beef was obtained from five commercial wholesale producers, and the aerobic plate count, and counts of coliforms, Escherichia coli type 1 and Staphylococcus aureus, determined. No E. coli O157:H7 or salmonellas were detected. The results were evaluated in relation to: (1) two earlier surveys involving fresh mince purchased from retail butchers and supermarkets in the UK (Roberts et al., 1980; Hudson et al., 1986); and (2) the standards set out in Annex II of the Minced Meat Directive 94/65/EC. In respect to the latter, none of the 99 samples of mince examined was unsatisfactory.     

方便米饭的真空冷冻干燥工艺

采用真空冷冻干燥方法确定冻干米饭的最佳工艺参数,并分析比较快速冻结和慢速冻结两种方法对冻干米饭的影响。结果表明：在冻结温度－ 50℃、时间1.5h;一次干燥温度－ 20℃、时间8h,二次干燥温度50℃、时间5.5h;真空度维持在10Pa 左右的条件下工艺最佳,并绘制了冻干曲线。而且从综合品质和营养角度上分析,快速冻结的产品具有更高的品质,附加值高。     

冻干技术的发展及冻干食品的市场前景

阐明了冻干技术的概念和冻干食品的特点，强调了发展我国冻干设备的必要性和成绩，指明了我国冻干食品产业的发展前景。     

Death rates of Salmonellae in fishmeals with different water activities. I. During storage

Fishmeals with different moisture contents, having water activities (a_w) ranging from 0.34 to 0.71, were infected with strains of Salmonella oranienburg or S. senftenberg and were stored at 15, 20 and 30°c, packed in nitrogen or air. The initial death rates (calculated over storage periods of 70 days) were not appreciably influenced by reduction of a_w from 0.71 to 0.54. Further reduction of a_w to 0.34 caused a decrease of the initial death rate, in particular in meals stored at 15 or 20°c. As compared with fishmeals packed in air, meals packed in nitrogen showed a decrease of initial death rates, especially in meals with the lowest a_w and stored at 15 and 20°c. Death rates calculated over longer storage periods were smaller than initial death rates, in particular in meals with a_w 0.54 stored at 15 and 20°c and meals with a_w 0.34 stored at 15, 20 and 30°c. Pelletisation of meals with a_w 0.72-0.74 caused a 10²-10³-fold reduction of initial Salmonella counts. The death rates of salmonellae which survived the pelletisation treatments were of about the same order as found for the initial death rates of salmonellae in non-pelletised meals with about the same a_w.     

常见浆果的真空冷冻干燥特性研究

浆果不耐贮藏、易腐烂变质,需进行及时的采后处理。真空冷冻干燥技术可有效保存物料中的营养成分和感官特征,尤其适合高附加值水果的加工。试验以香蕉、葡萄、猕猴桃和蓝莓4种常见浆果为原料进行冻干加工,分别记录动态干燥过程,并对冻干产品特性进行分析。试验表明:香蕉、猕猴桃和蓝莓均能保持新鲜物料的形态,疏松多孔、质地酥脆,葡萄出现体积收缩、表面褶皱;香蕉、猕猴桃、蓝莓在升华干燥中出现了恒速干燥阶段,而葡萄无明显的恒速干燥阶段;冻干香蕉、猕猴桃和蓝莓3种浆果复水性分别为72.34%、76.23%和55.88%,维生素C保存率分别为88.47%、96.67%和78.11%,而葡萄复水性和维生素C仅为33.53%和22.67%。含糖量较高的浆果冻干效果不太理想,需继续进行干燥工艺研究。试验为浆果冻干提供了适宜性评价依据,在货架期延长、方便快捷运输等方面也具有一定的参考价值。     

Theoretical analysis of two hot water cabinet systems for decontamination of sides of beef

The theory underlying the use of hot water to reduce the numbers of microbial contaminants of public health significance on freshly slaughtered animals was investigated. An analysis of steady-state predictions of microbial reductions and accompanying evaporative heat and mass losses in a hot water cabinet for the decontamination of beef sides is developed, based on published correlations. This is more comprehensive than previously undertaken and is used to compare two methods of contacting hot water with the sides of beef: conventional pressure sprays and an alternative, overhead distributor or 'deluge' system. Predictions indicate that a distributor cabinet does not need doors or thermal insulation, and highlight the potential economy and practical advantages over conventional spraying.     

Composition and retention of lipid nutrients in cooked ground beef relative to heat-transfer rates

This study documents the effects of different heat transfer kinetics on lipid nutrient composition in regular ground beef patties cooked using high (HHT) and low (LHT) heat transfer frying surfaces. Rate of heating from frying surfaces to ground beef patties was measured using iron-constantan thermocouples and related to the retention of beef pattie color, texture, total crude lipid content, fatty acid and cholesterol composition. Ground beef patties cooked for 1, 3 and 6 min using an HHT frying surface reached a higher (P⩽ 0.05) internal temperature than on the LHT frying surface. This observation was associated with higher (P⩽ 0.05) cook value and cook loss, and greater (P⩽0.05) shear force, L* color value and ΔE values. The fatty acid composition of the HHT cooked patties reflected a greater (P⩽ 0.05) loss in monounsaturated fatty acids, than with patties cooked on the LHT surface. The two frying methods produced little effect on beef a* and b* color values, total crude lipid, or polyunsaturated fatty acid content. Cholesterol content of HHT patties was reduced (P⩽ 0.05) relative to both uncooked patties and patties cooked with a LHT surface. These results indicate that heat conductivity of the cooking surface has an impact on the lipid composition of cooked ground beef patties.     

真空冷冻干燥食品的品质控制

讨了影响冻干食品复水性、色泽及微生物污染的因素,并分析如何选择工艺条件参数以控制冻干食品之品质。     

大蒜冷冻干燥工艺研究

研究了冷冻干燥技术加工大蒜制品的生产工艺。采用电阻法测定出大蒜共晶点为- 1 8℃。通过单因素试验 ,探讨了不同加热温度对大蒜冻干时间的影响。经过研究冷冻干燥的最佳工艺为 :操作压力为 1 3 3～ 40Pa ,升华干燥温度控制在 - 2 0℃左右 ,隔板加热温度控制在 40℃。     

冷冻干燥对长双歧杆菌损伤作用的研究

冷冻干燥是乳酸菌和益生菌干燥的主要方式,冷冻和干燥2个过程都会影响菌体细胞的存活率,探讨冷冻和干燥过程对长双歧杆菌的损伤作用有利于指导高存活性益生菌粉的生产。采用透射电镜观察菌体在冷冻干燥过程中的微结构,通过溢出胞外的β-半乳糖苷酶活性评价冷冻和干燥后细胞膜通透性的变化,通过测定冷冻和干燥后胞内Na~+和K~+浓度的变化研究离子效应的影响。研究结果表明,长双歧杆菌在冷冻干燥过程中的形态、细胞膜通透性和胞内离子浓度变化程度随冻结时间的延长而加剧;冷冻干燥过程使菌体出现细胞质浓缩、质壁分离、菌体皱缩以及破损现象;冷冻干燥使细胞膜通透性增大,使胞内Na~+、K~+浓度显著下降,并伴随比例失调。     

冷冻干燥工艺生产银杏粉

研究了以银杏为原料生产冻干银杏粉的工艺及设备选用。银杏经脱壳、磨浆、冻干、包装等工序得到最后产品，银杏粉产品具有色泽淡黄、质地均匀、冲调性好等特点。