现代食品加工技术

《现代食品工程技术》

1.Non-thermal Processing

Defintion

Non thermal technologies are preservation treatments that are effective at ambient or sub-lethal temperatures, thereby minimising negative thermal effects on food nutritional and quality parameters.

History，Background and Development

To date, the application of heat is the most common method for processing food, because of its ability to inactivate microorganisms and spoilage enzymes (PPO, PME, etc.). The use of certain thermal processing techniques (e.g. water immersion, hot air drying) has been favored by the important technological developments experienced over the last few years together with the easier management of the equipment used. However, heat processing particularly under severe conditions may induce several chemical and physical changes that impair the organoleptic properties and may reduce the content or bioavailability of some bioactive compounds. Therefore, there is a demand for mild processing technologies such as high pressure processing, irradiation, pulsed electricfields, power ultrasound, ozone and oscillating magneticfields etc. Recent interests in these technologies are not only to obtain high-quality food with “fresh-like” characteristics, but also to provide food with improved functionalities. As a result in the past decade novel non thermal technologies such as high hydrostatic pressure treatment and high intensity pulse electricfield treatment have become established. High pressure processed products are now available world wide while pulsed electricfield technologies are on the verge of commercialization.

Technologies and Application

Non-thermal processing technologies include the application of high-voltage pulsed electric fields (PEF), high hydrostatic pressure, ultraviolet light (UV), high intensity light pulses (HILP) and manothermosonication (MTS). Combinations of these have been recently exploited for microbial and enzymatic inactivation in fruit juices and milk.

(1)Pulsed electric field:

Pulsed electricfield is a technology that has been extensively investigated in recent years for its applications in food processing. By the mechanism of electropermeabilization, pulsed electrical fields have proved a valid technology for the production of safe beverage products and shown a positive influence in the texture of solid plant foods, leading to enhanced yields of extraction of metabolites, as well as increased juice yields.

(2)High pressure processing:

High-pressure (HP) processing, also sometimes known as high hydrostatic pressure (HHP), or ultra high pressure (UHP) processing, is a relatively new non-thermal food processing method that subjects liquid or solid foods, with or without packaging, to pressures between 50 and 1000 MPa. HPP treatment at ambient temperature is reported to have minimal effect on the bioactive content of various fruits and vegetables but fewer studies have been conducted on exotic fruits.

(3)High Intensity Pulsed Light Technology

Pulsed light technology (PLT) is an innovative method of purification and sterilization of food items by using very high-power and very short-duration pulses of light emitted by inert gas flash lamps. Such a technology, known since the 1980s, was approved by the Food and drug administration (FDA) in 1996 and recently has been widely investigated in view of possible commercial applications.

(4)Ultrasound:

Power ultrasound has shown promise as alternative technology to thermal treatment for food processing and

has been identified as a potential technology to meet the US Food and Drug Administration (USFDA) requirement of a 5 log reduction of Escherichia coli in fruit juices. It has been reported to be effective against foodborne pathogens found in orange juice, guava juice.

(5)Irradiation

Irradiation (cold pasteurisation) of food is achieved by exposing the product to a source of ionizing energy. It is a physical means of food processing that involves exposing pre-packaged or bulk foodstuffs to gamma rays (Cobalt-60), X-rays, or electrons.Depending upon the radiation dose, foods may be pasteurised to reduce or eliminate foodborne pathogens.Application of gamma radiation to pomegranate juice, carrot and kale juice and UV radiation to orange, guava-and-pineapple juice has been reported for the inactivation of microorganisms. (6)Ozone processing

The interest in ozone as a preservation technology is based on its high biocidal efficacy and wide antimicrobial spectrum. Within the food industry, ozone has been used routinely for washing and storage of fruits and vegetables by gaseous treatment. With the recent FDA approval of ozone as a direct additive to food, the potential of ozonation in liquid food applications has begun to be exploited.

Advantage and Disadvantage

Advantage: In addition to their possible beneficial effects on nutritional and bioactive content many of these novel technologies are more cost-efficient and environment friendly for obtaining premium quality foods which have led to their revival and commercialization.

Disadvantage: In general, longer exposure PEF treatment times may induce reduction in the product retention of vitamin C. Longer exposure time may also generate free radicals which may speed up vitamin C degradation. Ozone is not universally beneficial and in some cases may promote oxidative spoilage in foods. Surface oxidation, discoloration or development of undesirable odors may occur in substrates from excessive use of ozone. Irradiation of food products causes minimal modification in the flavor, color, nutrients, taste, and other quality attributes of food.

Future trend

The combination of various non thermal technologies has been shown to be more effective for inactivating resistant microbes and enzymes. There is need for focused studies on the stability of bioactive compounds using a combined approach. The applications of natural antimicrobial agents are likely to grow steadily in the future because of greater consumer demands for minimally processed foods and those containing naturally derived preservation ingredients. More complex considerations arise for combinations of technologies, particularly with respect to optimisation of practical applications. Intelligent selection of appropriate systems based on detailed sequential studies is necessary. The impact of product formulation, extrinsic storage parameters and intrinsic product parameters on the efficacy of novel applications of combined non thermal systems requires further study.

2．Integration with PEF and Freeze Concentration in Processing Orange Juice

PEF processing has been successful in a variety of fruit juices with low viscosity and electrical conductivity such as orange, apple, and cranberry juice. Recent studies reported more than a 3-10g reduction in orange juice and apple juice. Freeze concentration is a processing technique used to extract water from a sample without losing valuable aromatics. One very common use of this method is in the handling of fruit juices to produce concentrates with an intense flavor.

The Manufacturing Process

Technological process: fresh fruit→Cleaning/Grading→Extraction →PEF treatments →Freeze Concentration →Packaging/filling

Cleaning/Grading

The fruit must be inspected and graded before it can be used. The certified fruit is then transported along a conveyor belt where it is washed with a detergent as it passes over roller brushes. This process removes debris and dirt and reduces the number of microbes. The fruit is rinsed and dried. Graders remove bad fruit as it passes over the rollers and the remaining quality pieces are automatically segregated by size prior to extraction. Proper size is critical for the extraction process.

Extraction

Proper juice extraction is important to optimize the efficiency of the juice production process as well as the quality of the finished drink. Some of the peel oil may be removed prior to extraction by needles which prick the skin, thereby releasing the oil which is washed away. Modern extraction equipment can slice, ream, and eject a peel in about 3 seconds. The extracted juice is filtered through a stainless steel screen before it is ready for the next stage.

PEF treatments

Comparable to pasteurization, yet without the thermal component, PEF has the potential to pasteurize several foods via exposure to high voltage short pulses maintained at temperatures below 30-40°C. The basic definition of PEF technology relies on the use of high intensity pulsed electric fields (l0-80 kV/cm) for cell membrane disruption where induced electric fields perforate microbial membranes by electroporation, a biotechnology process used to promote bacterial DNA interchange.

PEF technology is based on a pulsing power delivered to the product placed between a set of electrodes confining the treatment gap of the PEF chamber. The equipment consists of a high voltage pulse generator and a treatment chamber with a suitable fluid handling system and necessary monitoring and controlling devices (Fig.

1.). Food product is placed in the treatment chamber, either in a static or continuous design, where two electrodes are connected together with a nonconductive material to avoid electrical flow from one to the other. Generated high voltage electrical pulses are applied to the electrodes, which then conduct the high intensity electrical pulse to the product placed between the two electrodes. The food product experiences a force per unit charge, the so-called electric field, which is responsible for the irreversible cell membrane breakdown in microorganisms.

Figure 1. Flow chart of a PEF food processing system with basic component.

Freeze Concentration

Concentration is useful because it extends the shelf life of the juice and makes storage and shipping more economical.Freeze concentration of fruit juices is a method of removing water from juice without heating and changing juice flavor.Freeze concentration is effective because a solute in solution has a lower melting point than pure ice, and can be separated from the water component (ice) as the temperature approaches the melting point of pure ice. Water is partially frozen to produce an ice crystal slurry in concentrated product. Separation of ice crystals is then accomplished using some washing technique.

Reconstitution

When the juice processor is ready to prepare a commercial package for retail sale, concentrate is pulled from several storage batches and blended with water to achieve the desired sugar to acid ratio, color, and flavor. Packaging/filling

出师表

两汉：诸葛亮

先帝创业未半而中道崩殂，今天下三分，益州疲弊，此诚危急存亡之秋也。然侍卫之臣不懈于内，忠志之士忘身于外者，盖追先帝之殊遇，欲报之于陛下也。诚宜开张圣听，以光先帝遗德，恢弘志士之气，不宜妄自菲薄，引喻失义，以塞忠谏之路也。

宫中府中，俱为一体；陟罚臧否，不宜异同。若有作奸犯科及为忠善者，宜付有司论其刑赏，以昭陛下平明之理；不宜偏私，使内外异法也。

侍中、侍郎郭攸之、费祎、董允等，此皆良实，志虑忠纯，是以先帝简拔以遗陛下：愚以为宫中之事，事无大小，悉以咨之，然后施行，必能裨补阙漏，有所广益。

将军向宠，性行淑均，晓畅军事，试用于昔日，先帝称之曰“能”，是以众议举宠为督：愚以为营中之事，悉以咨之，必能使行阵和睦，优劣得所。

亲贤臣，远小人，此先汉所以兴隆也；亲小人，远贤臣，此后汉所以倾颓也。先帝在时，每与臣论此事，未尝不叹息痛恨于桓、灵也。侍中、尚书、长史、参军，此悉贞良死节之臣，愿陛下亲之、信之，则汉室之隆，可计日而待也。

臣本布衣，躬耕于南阳，苟全性命于乱世，不求闻达于诸侯。先帝不以臣卑鄙，猥自枉屈，三顾臣于草庐之中，咨臣以当世之事，由是感激，遂许先帝以驱驰。后值倾覆，受任于败军之际，奉命于危难之间，尔来二十有一年矣。

先帝知臣谨慎，故临崩寄臣以大事也。受命以来，夙夜忧叹，恐托付不效，以伤先帝之明；故五月渡泸，深入不毛。今南方已定，兵甲已足，当奖率三军，北定中原，庶竭驽钝，攘除奸凶，兴复汉室，还于旧都。此臣所以报先帝而忠陛下之职分也。至于斟酌损益，进尽忠言，则攸之、祎、允之任也。

愿陛下托臣以讨贼兴复之效，不效，则治臣之罪，以告先帝之灵。若无兴德之言，则责攸之、祎、允等之慢，以彰其咎；陛下亦宜自谋，以咨诹善道，察纳雅言，深追先帝遗诏。臣不胜受恩感激。

今当远离，临表涕零，不知所言。