FOOD ENGINEERING

Lesson 2. RHEOLOGY OF PROCESSED FOODS

Module 1. Rheology of foods

Lesson 2

RHEOLOGY OF PROCESSED FOODS

2.1 Introduction

Rheology of process food is very important in the dairy products as it controls the body and texture of typical dairy products like cream, plastic cream, processed cheeses, traditional Indian dairy products (peda, burfi, halwasan, thabadi, sandesh, chhana podo etc.). Control of rheological properties is very much required in the development of new functional and health dairy products like low fat and low sugar ice cream, fat mimic products to avoid defects related to body and texture. Study of rheology is also important in the other food processing industries, like meat industries, fruits and vegetables processing, snack foods, bakery and confectionaries.

2.2 Examples of Application of Rheological Study in the Food Industry

• Meat products: To evaluate type of breed; its growth rate (tenderness); to evaluate effect of pickling, chilling, aging, preservation, etc. on rheological property of meat; for measurement of toughness and compactness of meat and meat products; establishment of quality grade for marketing and export.
• Fruits and vegetables: To evaluate variety of crop; for predicting the effect of storage and ripening period on process; prediction of storage and ripening period; in prediction of stage of harvesting and stage of maturing; used for sorting; measurement of\ textural variation, gives us an idea about growing practice; method of harvesting.
• Jams and jellies: helps to decide variety of blending ingredients, esp. pectin; deciding jelling quality of pectin as well as integrity of gel structure, helps in deciding ingredients.
• Snack foods: To evaluate formula for dough making and paste, particularly for extrusion; for measurement and adjustment of solids content; for measurement of textural properties like crispiness, hardness, softness and other properties to decide packaging and packing material; helps in predicting shelf-life of product under given storage conditions and history of product (method of harvesting, storage conditions, pre-treatments and processing unit operations).
• Confectioneries: To evaluate the quality of raw material; to optimize the processing parameters; to decide the ingredient varieties to be used; for measuring properties like thickness of coating, chewiness, elasticity, brittleness and shelf life of product.
• Paste: (Tomato paste, spreads, relishes, puddings, gels, jams, jellies, etc.) – used to evaluate consistency of mixture used for measured viscometric parameters at different stages of processing; deciding the pectin retention and prediction of consistency of final products.
• Bakery: To evaluate dough consistency; to estimate floor time and rise time; effect of additives; prediction of shelf life.
• Dairy products: To evaluate the effect of ingredients i.e. creaming in fat-free dairy products, fat mimic products by using micro-fluidization of whey protein concentrate, desired quality of mozzarella.

2.3 Texture and Structure of Heat and Acid Coagulated Indigenous Milk Products

Characterization of various food products on the basis of their rheology and microstructure forms the backbone of the scientific approach to product process development and of quality assurance in modern industrial practices. The current trends round the globe favour such studies to facilitate product description/specification for promoting process control and for international trade. Furthermore, the interest of researchers and manufacturers in the texture and structure of various milk products has been growing, as it is recognized that there are definite correlationship between the structure and other physical properties of the products. The physical manifestation of food materials is due to its chemical make-up and a micro structural study may yield the true insight into their textual attributes. Evaluation of geometrical properties of foods are important for their characterization; these properties refer to the arrangement of constituents of food including the size, shape and orientation of the particles. Electron microscopy is useful to study surface topology and to develop correlation between the structure of various food material and then physico –chemical properties .

At a juncture when the need for modernizing the manufacturing and marketing of traditional milk products is being emphasized in India, such rheological and electron microscopic studies would be sine qua non to obtain much needed information for product/process development. Further, the Bureau of Indian Standards (BIS) is actively considering the views of defining/describing the food products based on their structure. It is worthwhile to mention here that BIS has already made a headway in this direction in respect of some of the food products such as roasted chicory and coffee powder. In the past few years, some work has been directed to study the rheology of selected indigenous dairy products such as paneer, khoa, rasogoIla and sandesh. However, the area encompassing the micro structural studies has not received much scientific inputs so far in our country. Since rheology is determined by micro structure studies, study of rheological parameters would help us later to establish the relationship between microstructure and rheological properties. Keeping this in view, an attempt is made in this lecture to put forth the textural and structural aspects of some of the heat and acid coagulated indigenous milk products such as paneer, chhana and rasogolla.

2.3.1 Textural properties of paneer

Paneer is widely used in all vegetable dishes as well as for preparation of special foods, which requires to have rheological properties. The control of processing parameters during manufacture of paneer like temperature, pressure of press, control of pH, chilling and freezing during storage etc. are critical parameters, which requires study of its effect on the textural properties of paneer. The data on the objective textural properties of raw and fried and cooked paneers made from cow and buffalo milks has been shown in Table 2.1 It is evident from the table that primary parameters such as hardness and springiness differed significantly between cow and buffalo milk paneers. Cohesiveness, on the other hand, did not differ much between these two paneers. Since secondary parameters such as gumminess and chewiness are dependent on primary parameters, buffalo milk paneer revealed considerably higher vales for gumminess and chewiness compared to those recorded for cow milk paneer.

Table 2.1 Instron texture profile analysis of paneer made from cow and buffalo milks

Attributes	Cow milk panner	Buffalo milk panner
Raw	Fried & cooked	Raw	Fried & cooked
Hardness, mN	25.59	8.66	40.72	9.31
Cohesiveness	0.67	0.70	0.64	0.70
Springiness, mm	7.50	9.38	7.70	9.59
Gumminess, mN	17.04	6.12	25.19	6.46
Chewiness, mN. Mm	131.27	54.27	206.36	63.32

Frying in oil and cooking- in salt water remarkably reduced the hardness, gumminess and chewiness and increased the cohesiveness and springiness of both the paneer.

2.3.1.1 Microstructure of paneer

Scanning electron microscopy (SEM) reveals that in the raw state, both cow, and buffalo paneers possessed uniformly aggregated protein particles and fat globules are evenly distributed in the protein net work. Transmission Electron microscopy: (TEM) confirmed the existence of granular structure in paneer and also exhibited the internal structure of the protein particles. Raw cow milk paneer has uniformly packed small protein particles and resembled cottage cheese, while in raw buffalo milk paneer protein particles were more densely packed and fused. Core-and- lining structure, which is characteristic of curds obtained by coagulation of hot milk at pH 5.5 is well developed in both the paneers. The development of core-and-lining structure is influenced by the temperature and pH of coagulation.

Frying of paneer in oil severely changed its structure, resulting into compaction suppressing the smooth granularity of the protein matrix in cow milk paneer. The granularity totally vanished in the buffalo milk paneer. The compaction is more clearly evident in TEM ultragraphs. The compaction also caused the fat globules to acquire sharp and pointed outlines unlike their globular shape in raw paneer. Cooking of fried paneer in salt water restored both the granular structure and core-and- lining structure of the protein bodies. This restoration was more in case of cow milk paneer as compared to buffalo milk paneer.

2.3.2 Textural properties of chhana

Instron textural attributes of chhana made from cow and buffalo milks are given in table 2.2. It is evident that all the textural values were less for cow milk chhana compared to that of buffalo milk chhana. The secondary parameters such as gumminess and chewiness for buffalo milk chhana were more than two times to those values for cow milk chhana. However there was not much difference between cow milk and buffalo milk chhana as for as the adhesiveness was concerned.

Table 2.2 Instron texture profile properties of chhana

Attributes	Cow milk chhana	Buffalo milk chhana
Hardness, mN	11.60	19.50
Cohesiveness	0.59	0.67
Springiness, mm	3.60	5.00
Gumminess, mN	6.48	13.06
Chewiness, mN. mm	24.64	65.32
Adhesiveness mN	0.35	0.38

2.3.2.1 Microstructure of chhana

SEM of a defatted cow milk chhana reveals conglomerated and compact protein material (casein and whey protein complexes with numerous small uniformly distributed pores of irregular shape. The protein particles coalesced and fused densely during coagulation and lost their natural identity of subunit' sizes as seen in milk. The coalesced, smooth protein bodies were joined with thick bridges. SEM of defatted buffalo milk chhana also shows a similar compact, coalesced protein net work with numerous globular and irregular voids throughout the matrix, but slightly more uneven as compared to cow milk chhana. The globular void spaces indicate that the casein-whey protein complexes are closely interspersed with numerous fat globules due to the usage of whole milk. Cow and buffalo milk chhana has been shown to contain fat globules embedded in coalesced casein micelles with some whey-filled spaces at the edge. The agglomerated large protein particles form continuous thick strands joined together forming somewhat uneven matrix with numerous void spaces in between. The fat globules are strongly cemented in these thick protein strands. The overall structure is more or less similar to that of cream cheese, in which the fat globules are found cemented together with the coalesced protein particles as seen in chhana.

2.3.3 Textural properties of rasogolla

Instron textural attributes of rasogolla are shown in Table 2.3. It is clear from the table that cow milk rasogolla has significantly lower hardness, springiness, gumminess and chewiness than that of buffalo milk rasogolla. The hardness of buffalo milk rasogolla in 2-3 times higher than that of cow milk rasogolla. Springiness of buffalo milk rasogolla (6.4 mm) is markedly higher than that of cow milk rasogolla (4.8 mm). Cohesiveness varied from 0.61 (cow milk rasogolla) - 0.70 (buffalo milk rasogolla). As the consequence of higher hardness and springiness in buffalo milk rasogolla, their gumminess and chewiness values also increased remarkably than that of cow milk rasogollas. No adhesive force, however, has been recorded for either of the rasogollas.

Table 2.3 Instron texture profile properties of rasogolla

Attributes	Cow milk rasogolla	Buffalo milk rasogolla
Hardness, mN	5.85	16.82
Cohesiveness	0.61	0.70
Springiness, mm	4.80	6.40
Gumminess, mN	3.57	12.17
Chewiness, mN. mm	17.15	77.88

2.3.3.1 Microstructure of rasogolla

Cooking of chhana in sugar syrup (for 15 min.) severely altered the structure of both the fat and the protein phases. The microstructure of rasogolla exhibits a distinctly different protein net work from, chhana at low magnification, a ragged and cracked protein matrix can be seen obscured with fat and several void spaces interspersed throughout. Higher magnification revealed that the fat globules are shrunken and ruptured, finally coalescing to a large mass and losing their natural identity as globular with a smooth surface as is found in chhana.

A defatted rasogolla sample showed a ragged porous, loose protein matrix with a folded thread-like structure. The clumped protein particles formed a corrugated edge around the void space. Higher magnification showed that the folded protein particles were interlinked with thick protein bridges forming a core type structure with numerous voids.

Similarly, the fat globule structure in buffalo milk rasogolla revealed drastic shrinkage of the fat globule membrane and globules partly detached from the protein bodies. The defatted protein matrix in buffalo milk rasogolla was more compact and ragged with lesser voids as compared to cow milk rasogolla.

2.3.4 Interrelationships between texture and microstructure of chhana and rasogolla

The denser protein network present in chhana reduced the mean free path of the coalesced casein micelles which reduced the capacity of the fat and protein phases to move in relation to each other. Where as in rasogolla the large voids between the coalesced protein gave the free access of the protein bodies to move freely during the instron testing, resulting its lower hardness but higher springiness. This higher springiness in rasogollas may be attributed to its loose, porous and ragged protein matrix.